CA2325557A1 - Apparatus and method for attaching a load indicating device to a fastener - Google Patents
Apparatus and method for attaching a load indicating device to a fastener Download PDFInfo
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- CA2325557A1 CA2325557A1 CA002325557A CA2325557A CA2325557A1 CA 2325557 A1 CA2325557 A1 CA 2325557A1 CA 002325557 A CA002325557 A CA 002325557A CA 2325557 A CA2325557 A CA 2325557A CA 2325557 A1 CA2325557 A1 CA 2325557A1
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- fastener
- indicating device
- load indicating
- housing
- load
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
- G01L5/24—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes for determining value of torque or twisting moment for tightening a nut or other member which is similarly stressed
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Abstract
A method and apparatus for attaching a load-indicating device to a fastener so that the accuracy of the load-indicating device is not affected by yielding and non-uniform elastic deformation of the fastener. The apparatus is attached to the fastener in a region of the fastener which is not likely to yield or elastic deform in a non-uniform manner when the fastener is loaded. The region may be determined using finite element analysis, physical experimentation or other means such as by mathematical computation. By securing the load-indicating device in such a manner, the device is not affected by yielding or non-uniform deformation of the fastener, cyclic loading, shock or thermal expansion.
Description
APPARATUS AND METHOD FOR ATTACHING A LOAD INDICATING
DEVICE TO A FASTENER
RELATED APPLICATIONS
This application claims the benefit of Provisional Application Serial Number 60/079,460, filed March 26, 1998.
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to load indicating apparatuses, and more particularly, to apparatus and methods of attaching load-indicating apparatuses to a fastener.
BACKGROUND OF THE INVENTION
For safety considerations, it is often important that bolts and studs maintain a precise clamping load on specific locations of an assembly. Sufficiently attaining and maintaining this correct clamping load are problems which industry has tried to overcome with limited success.
Several common methods are used to control the initial clamping load during assembly. Of these methods, torque control is the simplest and most common.
This method relies on the complex relationship between torque and tension. The method is typically not accurate because there are many variables that effect the coefficient of friction in the threads and under the head of the bolts. These variables include the type of lubrication (if any), type of thread, condition of the threads, plating, dirt, hardness of parts, finishes, and speed of tightening. In general, most of the energy input through torque is lost to friction. Since so many variables influence the coefficient of friction, in most cases it is very difficult to accurately know how much torque energy is lost to friction, and thus it is very difficult to accurately know the tension produced from a particular torque value.
Another problem faced with torque and other conventional tightening methods is that they provide little information on the loss of tension after the assembly process.
For example, fasteners typically experience a loss of tension when put into service for a variety of reasons including local yielding and relaxation, vibration loosening, gasket creeping out of the joint, and thermal expansion. This unmonitored loss of tension can lead to serious problems such as joint slippage, premature wear, and joint failure.
Thus, several types of apparatuses have been developed in attempts to solve both the problems of accurate assembly tension and in-service monitoring of tension.
For example, many of these apparatuses measure the strain (elongation) of the fastener in order to determine the clamping load. Various methods are used to measure the strain including electrical, mechanical, optical-mechanical, ultrasonic and the like.
Since the relationship between stress and strain in most bolt materials is well known, strain measuring methods can potentially achieve better clamping force accuracy than torque methods.
Some strain measuring apparatuses are substantially external to the fastener while other apparatuses are substantially or entirely internal to the fastener. For example strain measuring apparatuses which are external to the fastener are disclosed in U.S. Pat. No. 3,954,004 issued May 4, 1976 to Orner, U.S. Pat. No.
4,676,109 issued June 30, 1987 to Wallace, U.S. Pat. No. 4,899,591 issued Feb. 13, 1990 to Kibblewhite, U.S. Pat. No. 4,823,606 issued Apr. 25, 1989 to Malicki, U.S.
Pat. No.
4,114,428 issued Sep. 19, 1978 to Popenoe, and U.S. Pat. No. 2,600,029 issued June 10, 1952 to Stone, all of which are hereby incorporated by reference.
Generally, these are delicate instruments which are attached to the bolt during measurement and removed when the bolt is put into service.
While there are various types of strain measuring apparatuses which are substantially or entirely located internally within the fastener, an important group includes those strain measuring apparatuses that have the ability to continually respond to changes in fastener elongation and continuously display the strain or tension. In most cases, the apparatus fits into a bore drilled partially into the axis of the fastener. In addition to containing the apparatus, the hole establishes the section of the fastener in which the strain measurement takes place. Examples of such WO 99/49289 PCT/(JS99/06612 apparatus are found in U.S. Pat. No. 3,964,299 issued June 22, 1976 to Johnson, U.S.
Pat. No. 3,987,668 issued Oct. 26,1976 to Popenoe, U.S. Pat. No. 5,668,323 issued Sep. 16,1997 to Waxman, and UK Pat. No. GB 2-265-954-B Published May 31, 1995 to Ceney, all of which are hereby incorporated by reference.
These devices have unique advantages over many other apparatuses in that typically no external equipment is needed, no physical interaction with the device is needed, and an inexperienced operator may be able to monitor the tension by simply making a visual observation of the apparatus. Additionally, in some cases, remote sensing technology can continually record the in-service fastener tension.
These devices are typically attached to the fastener at two points which are used as reference points in the strain measuring process. The distance between these two points is called the gauge length. As shown in Figures 1 a,b the reference point A is approximately at the end of the fastener. Typically the strain gauge is attached to the fastener at or near this point A. The second reference point, B, is located near the bottom of the hole drilled through the axis.
The process of making a strain measurement involves comparing the changing distance between the two reference points to the original distance, i.e., the original gauge length. As shown in Figure 1 (the effect is exaggerated), when the bolt is tightened, the reaction forces from the joint and nut cause the bolt to stretch elastically (in most applications the body of the bolt will experience stresses in the elastic region, safely below the yield point). This measured elongation can be converted to stress levels and bolt clamping force by using the known elastic modulus of the material and basic engineering equations. The amount of the elongation is typically very small, for example, a 1" grade 5 bolt tightened to the ASTM specified maximum safe load (proof load) will experience only a few thousandths of an inch elongation per inch.
This elongation causes several potential problems, including yielding (permanent deformation) of bolt heads and fastener ends, non-uniform stretching which occurs at the end sections of the fastener and where the nut engages a stud, and the changing position of the nut for applications involving studs.
Strain gauged bolts often become inaccurate after an initial assembly or while in service. Often, after the bolt is removed and inspected, the strain gauge will indicate tension in the fastener even though no tension is present. The false indications are likely related to a slight yielding in the head or end of the fastener.
Yielding commonly occurs in bolt heads even when the clamping force is well below the specified capability of the bolt. That is, the head often experiences permanent deformation even when the body of the bolt is at stress levels well below the yield point of the material.
One common condition that leads to such yielding is when the head of the fastener sits on a surface that is not perpendicular to the axis of the bolt.
As shown in Figures 2a-c, as the bolt is tightened, one part of the head experiences a high reaction force, creating a prying effect (also known as a wedge effect). These uneven forces can cause yielding and permanent deformation.
Another condition which can produce yielding in the head is shown in Figures 3a-c and occurs when the hole upon which the bolt head lies is too large. In this situation, the contact force between the head of the bolt and the mating surface is concentrated in a small area. Again, this produces high stress levels and local yielding in the head.
Additionally, because they typically contain a bore, fasteners with internal strain gages tend to have weakened ends. Since they are weakened, they have an increased susceptibility to both of these types of yielding.
Although yielding in the head is generally not a problem in non-strain-gauged-bolts, in strain-gauged-bolts, it can cause many problems and lead to significant inaccuracies. These problems arise because of the nature of the strain measuring process. As described earlier, the strain measurement is made by comparing the gauge length of the unstrained bolt to the gauge length of the strained bolt.
The difference between these two lengths is the elongation. Elongation is generally very small, i.e., on the order of only a few thousands of an inch per inch.
As shown in Figures 2 and 3, the gauge length of the unstressed bolt can be changed due to the yielding. Any change in the gauge length of the unstressed bolt must be accounted for when determining the elongation. Since one of the reference points is at the end of the fastener, yielding in this end can result in a changed unloaded distance between the two reference points that will change the gauge length of the unstressed bolt.
DEVICE TO A FASTENER
RELATED APPLICATIONS
This application claims the benefit of Provisional Application Serial Number 60/079,460, filed March 26, 1998.
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to load indicating apparatuses, and more particularly, to apparatus and methods of attaching load-indicating apparatuses to a fastener.
BACKGROUND OF THE INVENTION
For safety considerations, it is often important that bolts and studs maintain a precise clamping load on specific locations of an assembly. Sufficiently attaining and maintaining this correct clamping load are problems which industry has tried to overcome with limited success.
Several common methods are used to control the initial clamping load during assembly. Of these methods, torque control is the simplest and most common.
This method relies on the complex relationship between torque and tension. The method is typically not accurate because there are many variables that effect the coefficient of friction in the threads and under the head of the bolts. These variables include the type of lubrication (if any), type of thread, condition of the threads, plating, dirt, hardness of parts, finishes, and speed of tightening. In general, most of the energy input through torque is lost to friction. Since so many variables influence the coefficient of friction, in most cases it is very difficult to accurately know how much torque energy is lost to friction, and thus it is very difficult to accurately know the tension produced from a particular torque value.
Another problem faced with torque and other conventional tightening methods is that they provide little information on the loss of tension after the assembly process.
For example, fasteners typically experience a loss of tension when put into service for a variety of reasons including local yielding and relaxation, vibration loosening, gasket creeping out of the joint, and thermal expansion. This unmonitored loss of tension can lead to serious problems such as joint slippage, premature wear, and joint failure.
Thus, several types of apparatuses have been developed in attempts to solve both the problems of accurate assembly tension and in-service monitoring of tension.
For example, many of these apparatuses measure the strain (elongation) of the fastener in order to determine the clamping load. Various methods are used to measure the strain including electrical, mechanical, optical-mechanical, ultrasonic and the like.
Since the relationship between stress and strain in most bolt materials is well known, strain measuring methods can potentially achieve better clamping force accuracy than torque methods.
Some strain measuring apparatuses are substantially external to the fastener while other apparatuses are substantially or entirely internal to the fastener. For example strain measuring apparatuses which are external to the fastener are disclosed in U.S. Pat. No. 3,954,004 issued May 4, 1976 to Orner, U.S. Pat. No.
4,676,109 issued June 30, 1987 to Wallace, U.S. Pat. No. 4,899,591 issued Feb. 13, 1990 to Kibblewhite, U.S. Pat. No. 4,823,606 issued Apr. 25, 1989 to Malicki, U.S.
Pat. No.
4,114,428 issued Sep. 19, 1978 to Popenoe, and U.S. Pat. No. 2,600,029 issued June 10, 1952 to Stone, all of which are hereby incorporated by reference.
Generally, these are delicate instruments which are attached to the bolt during measurement and removed when the bolt is put into service.
While there are various types of strain measuring apparatuses which are substantially or entirely located internally within the fastener, an important group includes those strain measuring apparatuses that have the ability to continually respond to changes in fastener elongation and continuously display the strain or tension. In most cases, the apparatus fits into a bore drilled partially into the axis of the fastener. In addition to containing the apparatus, the hole establishes the section of the fastener in which the strain measurement takes place. Examples of such WO 99/49289 PCT/(JS99/06612 apparatus are found in U.S. Pat. No. 3,964,299 issued June 22, 1976 to Johnson, U.S.
Pat. No. 3,987,668 issued Oct. 26,1976 to Popenoe, U.S. Pat. No. 5,668,323 issued Sep. 16,1997 to Waxman, and UK Pat. No. GB 2-265-954-B Published May 31, 1995 to Ceney, all of which are hereby incorporated by reference.
These devices have unique advantages over many other apparatuses in that typically no external equipment is needed, no physical interaction with the device is needed, and an inexperienced operator may be able to monitor the tension by simply making a visual observation of the apparatus. Additionally, in some cases, remote sensing technology can continually record the in-service fastener tension.
These devices are typically attached to the fastener at two points which are used as reference points in the strain measuring process. The distance between these two points is called the gauge length. As shown in Figures 1 a,b the reference point A is approximately at the end of the fastener. Typically the strain gauge is attached to the fastener at or near this point A. The second reference point, B, is located near the bottom of the hole drilled through the axis.
The process of making a strain measurement involves comparing the changing distance between the two reference points to the original distance, i.e., the original gauge length. As shown in Figure 1 (the effect is exaggerated), when the bolt is tightened, the reaction forces from the joint and nut cause the bolt to stretch elastically (in most applications the body of the bolt will experience stresses in the elastic region, safely below the yield point). This measured elongation can be converted to stress levels and bolt clamping force by using the known elastic modulus of the material and basic engineering equations. The amount of the elongation is typically very small, for example, a 1" grade 5 bolt tightened to the ASTM specified maximum safe load (proof load) will experience only a few thousandths of an inch elongation per inch.
This elongation causes several potential problems, including yielding (permanent deformation) of bolt heads and fastener ends, non-uniform stretching which occurs at the end sections of the fastener and where the nut engages a stud, and the changing position of the nut for applications involving studs.
Strain gauged bolts often become inaccurate after an initial assembly or while in service. Often, after the bolt is removed and inspected, the strain gauge will indicate tension in the fastener even though no tension is present. The false indications are likely related to a slight yielding in the head or end of the fastener.
Yielding commonly occurs in bolt heads even when the clamping force is well below the specified capability of the bolt. That is, the head often experiences permanent deformation even when the body of the bolt is at stress levels well below the yield point of the material.
One common condition that leads to such yielding is when the head of the fastener sits on a surface that is not perpendicular to the axis of the bolt.
As shown in Figures 2a-c, as the bolt is tightened, one part of the head experiences a high reaction force, creating a prying effect (also known as a wedge effect). These uneven forces can cause yielding and permanent deformation.
Another condition which can produce yielding in the head is shown in Figures 3a-c and occurs when the hole upon which the bolt head lies is too large. In this situation, the contact force between the head of the bolt and the mating surface is concentrated in a small area. Again, this produces high stress levels and local yielding in the head.
Additionally, because they typically contain a bore, fasteners with internal strain gages tend to have weakened ends. Since they are weakened, they have an increased susceptibility to both of these types of yielding.
Although yielding in the head is generally not a problem in non-strain-gauged-bolts, in strain-gauged-bolts, it can cause many problems and lead to significant inaccuracies. These problems arise because of the nature of the strain measuring process. As described earlier, the strain measurement is made by comparing the gauge length of the unstrained bolt to the gauge length of the strained bolt.
The difference between these two lengths is the elongation. Elongation is generally very small, i.e., on the order of only a few thousands of an inch per inch.
As shown in Figures 2 and 3, the gauge length of the unstressed bolt can be changed due to the yielding. Any change in the gauge length of the unstressed bolt must be accounted for when determining the elongation. Since one of the reference points is at the end of the fastener, yielding in this end can result in a changed unloaded distance between the two reference points that will change the gauge length of the unstressed bolt.
If the change in the gauge length of the unstressed bolt is not accounted for, errors in the displayed tension can occur. For example, the bolt could self loosen to zero tension, while the strain gauge still measures the residual strain (permanent elongation due to yielding) and falsely convert this to a stress or tension.
In general, if the changed reference length is not accounted for, then all of the calculated stress levels will be inaccurate.
Many commonly used strain measuring devices offer no method to account for a changed reference length and thus can not be relied upon after the initial assembly.
Further, in cases where there is a method to determine or adjust for this problem, the procedure for doing so is generally inconvenient. In order to make this adjustment, it is typically necessary to remove the tension from the fastener. However, removing the tension is typically either not possible or prohibitively burdensome as it usually involves taking the machinery out of service.
Another common fastener problem arises when attaching a strain gauge to the 1 S end of a stud (a non-headed fastener). In particular, the final position of the nut on the stud after installation will directly affect the length of the strained region. To understand this, consider Figures 4a and 4b. The Figures show a stud with nuts in two positions. Since the region between point A and B is unstrained, the strained region must be located between points B and D. However, the exact region subjected to strain is very unclear and relies upon at least two variables: ( 1 ) the respective distances between A and D and between B and D, and (2) the cumulative effect of non-uniform forces on the stud by the nut.
The distance between A, the upper surface of the stud, and B, the upper surface of the nut, is called the nut standoff. In most bolting applications, little attention is paid to precisely controlling and/or maintaining this distance.
Many variables will effect the nut standoff as shown in Figures Sa and Sb, such as, for example, the overall length of the fastener, the amount of thread engagement of the nuts, the height of the nuts, variances in the dimensions of the joint and gasket, and the dimensions of any washers.
A second variable effecting the length of the strained region involves the non-uniform forces exerted on the stud by the nut. In general, the greatest contact forces occur in the lower threads, and the contact forces decrease to approximately zero at the uppermost thread (point B). The exact distribution varies for each individual stud and nut combination and depends upon the exact dimensions, the exact thread pitch, the hardness of the parts, and the tension in the fastener.
Furthermore, for an individual nut and stud, the force distribution will change over time. As the first few threads experience the highest load, in service they will often yield. Thus, nearby threads may exert greater contact forces on the stud over time or upon re-assembly. Such unknown and changing force distributions makes it .
impossible to know exactly where the yield should be located between points B' and C' in Figures 4 and 5. Since this end of the stretched section is not known, the exact length of the stretched region is not known.
The combined effect of these two variables, and particularly the variations in the standoff can lead to significant problems in the strain monitoring process.
Although elongation can still be measured, in order to convert this elongation to a stress level it is necessary to know the length of the strained region. To address this problem, the user can attempt to control the standoff in the field. However, this can be a prohibitively time consuming process that may require the removal of the nut and the insertion or removal of spacing washers, or the adjusting of thread engagement of the nuts which may involve at least a partial dismantling of the joint.
Further, the retraining of already experienced operators to pay close attention to nut standoff is a problem. In most bolt assembly operations, the emphasis is on speed, and there is a great resistance to more complicated assembly procedures.
Sensitivity to shock, external forces and extended cyclic loading are additional problems with current fastener and load indicating device designs. For example, bolts and studs are often dropped, pounded out of fixtures, subjected to vibration and generally exposed to rugged conditions. When the fastener is subjected to such shocks and external forces, inaccuracy in the calibration of the load indicating device can be introduced. Thus, re-zeroing or re-calibration of the load indicating device is necessary.
Some load indicating devices have incorporated an "on-off ' mechanism in attempts to allow re-calibration of the load indicator. Generally, the mechanism consists of two dowels, the first pressed into the load indicating device, the second pressed into the fastener. When the fastener and load indicating device are assembled, WO 99/49289 PCT/(TS99/06612 another dowel is placed in the fastener end such that when the load indicating device is rotated, the dowels engage one another such that the ring is prevented from further rotation in that direction. The dowels are generally accurately positioned in order that the Load indicating device can be rotated into an "on" orientation which will engage the load indicating device to a specific position for making the strain measurement.
After the measurement is performed, the load indicating device may be rotated to an "off " position. In the "off position, the load indicating device is disengaged from the fastener (or gauge pin).
However, with present calibration designs, there is typically no accurate way to adjust the dowel placement if the calibration of the load indicating devices changes.
Generally, this is because the dowels are permanently affixed to the ring and fastener.
Should error in the "zero" of the load indicating device occur, the dowel placement cannot easily be adjusted to re-zero or re-calibrate the load indicating device. The fastener and load indicating device must typically be returned to the manufacturer in order to re-adjust the dowel placement. Additionally, the machining involved with properly placing and inserting the towels increases the time and costs associated with manufacturing the load indicating and calibrations devices.
The device disclosed in U.S. Patent No. 5,668,323, attempted to solve many of the aforementioned problems and one embodiment is shown in Figures 6-9.
However, the technology of the '323 patent still leaves problems relating to ruggedness, manufacturability, performance at elevated temperatures, and performance under cyclic loading applications. To date there has been only one available embodiment of the device on the market. As shown in Figure 8, the body of the device comprises two main sections, a head 1 and a body 2, which are spot welded together. The head 1 is a turned and threaded piece with a rectangular slot punched through the bottom.
The body 2 is a sheet metal bracket. The manufacturing of the bracket is generally very time consuming and involves many operations, including wire EDM cutting, a series of bending, spot welding, fine tuning and straightening, drilling, pressing, and hand chamfering.
Unfortunately, long assembly times are another problem of the design. There are different methods for locking the device into position. With reference to Figure 7, one of these methods includes inserting a series of shims 5 or a locking nut 6 so that the device is properly tightened when the indicating lever points to zero (this process is called zeroing the device.) However, when doing this, the assembler must be careful not to over-tighten the device as it will tend to pull the bushing 7 from the bolt.
S Once the device is assembled into the bolt and zeroed, the bolt is usually loaded to its proof load at least three times. This lengthy loading process is in part to calibrate the device and to work harden the head of the bolt to reduce the effects of yielding. The complexities of manufacturing the base of the device and the lengthy process of installation and preloading the bolts three times are merely exemplary of several manufacturing complications which lead to prohibitively high retail prices for .
most applications. For example, currently, the insertion of the load indicator will usually add $90 to $200 or more to the price of the fastener.
Aside from the cumbersome and complicated manufacturing procedures, there are other inherent problems with the '323 patent design. First, for most applications, the '323 patent device will not adequately perform at higher temperatures.
This is because the coefficient of thermal expansion of the bracket will usually be different from the coefficient of expansion of the alloys used for high temperature fasteners. In order for the device to work properly at elevated temperatures, the device would have to expand similarly to the material of the fastener. That is, the device and the bolt would need to have the same coefficient of thermal expansion.
However, matching these coefficients is usually not possible due to other constraints of the bracket material. One of these constraints is that the '323 patent device is formed from a material that can withstand the strain of the bending without fracturing, particularly at point E in Figure 9. Another drawback is that the material is generally only available in sheets of thickness approximately 0.020 inch. Yet another aspect of this problem is that the material must be weldable to the material of the head 2 as shown in Figure 9. Finally, there must not be a corrosion problem that can occur when dissimilar metals are connected.
To date, generally, all of these constraints have only been found to be satisfied with one type of material, 1095 sheet metal which has a coefficient of thermal expansion different than that of most fasteners. Although other materials having different coefficients of thermal expansion may exist, they are typically rare, and may _g_ involve raw stock that would need to be specially manufactured. Thus, in practice, it is only possible to match the coefficient of thermal expansion for a select few materials.
Another inherent problem of the current embodiment of the '323 patent device S involves the interface between the body 2 and a pin 4 as shown in Figure l0a and l Ob.
The pin 4 has a round hole 9 drilled in it. However, the cross section of the body 2 is generally rectangular. Thus, a precise fit of the two pieces is typically impossible and the device may slip positions, thereby leading to inaccuracies when the device is subjected to shock, vibration, or cyclic loading. Additionally, the inaccuracies lead to wearing of the corners of the body 2, further increasing accuracy problems.
A third and significant problem with the disclosed design is the inherently weak nature of the design. Since the strain in forming the bend E in Figure 9 is significant, a relatively soft material must be used. However, under shock this material readily yields, and even a few tenths of a thousandth of an inch can significantly affect a measurement which may only be a few thousandths of an inch.
Further, the rectangular nature of the cross-section renders the bend weaker in some directions than in other directions.
The final problem described here regarding the current design is related to a general inability to properly lock the cartridge into the indicating position without affecting the accuracy of the device. It has been noted that when the device is rotated from the "on" position to the "off ' position, and then rotated and locked back to the "on" position, the device will often lose accuracy. This loss of accuracy is related to the nature of the locking methods. Since thread tolerances of the cartridge can allow play between the cartridge and the bolt, it is difficult to repeat an exact distance between the cartridge top and the gauge pin 4. Thus, shims S and locking nut 6 are used to tighten the device.
However, for both of these methods, the actual "tightness" of the locked position is typically dependent upon the person tightening the fastener. Since different people will have different opinions of what is properly "tight", the device will not be locked into the "on" position in the same manner each time. This inconsistency combined with the thread tolerances in the cartridge again can cause significant inaccuracies.
Accordingly, methods and apparatus for attaching a load-indicating device to a fastener which do not suffer from these problems is desirable.
SUMMARY OF THE INVENTION
An apparatus for attaching an internal strain gauge to a fastener according to various aspects of the present invention presents a housing which secures the strain gauge to the fastener in such a manner that the strain gauge readings are less affected by strains at the end of the fastener as well as external factors on the fastener such as non-uniform elastic deformation, yielding, shock, cyclic loading, temperature changes and the like.
In accordance with the present invention, the housing is secured internally to the fastener sufficiently far enough from a region susceptible to yielding and non-uniform deformation that the gauge length measured by the strain gauge is unaffected by such yielding and non-uniform elastic deformation of the fastener.
Additionally, the housing is provided with a calibration ring for "zeroing"
the 1 S strain gage when no load is applied to the fastener and for re-calibrating or removing the load on the strain gauge as desired.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional aspects of the present invention will become evident upon reviewing the non-limiting embodiments described in the specification and the claims taken in conjunction with the accompanying figures, wherein like numerals designate like elements, and:
Figure la is a side view of an untightened bolt with an internal strain gauge bore;
Figure lb is a side view of a tightened bolt with an internal strain gauge bore;
Figure 2a is a side view of an unyielded bolt on a surface which is not perpendicular to the bolt's axis;
Figure 2b is a side view of a loaded bolt on a surface which is not perpendicular to the bolt's axis;
Figure 2c is a side view of a yielded bolt after removal from a surface which is not perpendicular to the bolt's axis;
Figure 3a is a side view of an unloaded bolt in a bore too large for the bolt head;
Figure 3b is a side view of a loaded bolt in a bore too large for the bolt head;
Figure 3c is a side view of a yielded bolt after removal from a bore too large for the bolt head;
Figure 4a is a side view of a stud with a first distance between two nuts;
Figure 4b is a side view of a stud with a second distance between two nuts;
Figure Sa is a side view of a stud with an internal strain gauge using washers;
Figure Sb is a side view of a stud with an internal strain gauge without washers;
Figure 6 is a perspective view showing insertion of an internal strain gauge into a bolt head;
Figure 7 is a cross-sectional side view of a bolt with an internal strain gauge;
Figure 8 is a cross-sectional side view of an internal strain gauge;
Figure 9 is a front view of the body of the Waxman device;
Figure l0a is a side view of the interface between the head and the pin;
Figure 1 Ob is a top view of the interface between the head and the pin;
Figure 11 a is a cross-sectional side view of a preferred embodiment of an internal strain gauge and housing in a bolt;
Figure 1 lb is a top view of a preferred ernbodirnent of an internal strain gauge and housing;
Figure 12 is an isometric drawing of a bolt head containing internal strain gauge showing an indicator arrow pointed to a calibrated scale;
Figure 13 is a cross-sectional side view of an internal strain gauge and housing retrofitted into a stud;
Figure 14 is a cross-sectional side view of a housing used with the strain gauge of U.S. Patent No. 4,571,133;
Figure 15 is a cross-sectional view of one embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 16 is a cross-sectional view of an alternative embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
WO 99/49289 PCT/US99/Obbl2 Figure 17 is a cross-sectional view of yet another embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 18 is a cross-sectional side view of a simplified embodiment of the housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 19 is a cross-sectional side view of an alternative embodiment of a housing according to the present invention.
Figure 20a is a cross-sectional side view of the calibration device attached to the load indicator;
Figure 20b is a top view of the calibration device attached to the load indicator;
Figure 21 is a top view of an alternative embodiment of the calibration device attached to the load indicator; and Figure 22 is a top view of yet another alternative embodiment of the calibration device attached to the load indicator The following descriptions are of preferred exemplary embodiments only, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing a preferred embodiment of the invention. Various changes may be made in the function and arrangement of elements described in the preferred embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.
In general, according to various aspects of the present invention provides a method and apparatus for attaching a load-indicating device to a fastener such that the load-indicating device is capable of continually monitoring the load on the fastener, and such that the load indicating device is not subject to external factors such as non-uniform elastic deformation, yielding, shock, cyclic loading, temperature and the like.
Further, though the various embodiments described below will be directed to a bolt or stud fastener, various other fasteners subject to external loads including pins; dowels, screws and the like may similarly incorporate the present invention.
Thus, with reference to Figures 11-14, in accordance with the present invention, a bolt 10 suitably includes a housing 20, a load indicator 30 for displaying the load status of bolt 10 and a calibration ring 50.
In accordance with the present exemplary embodiment of the present invention, bolt 10 is suitably configured with an internal bore 12 for accommodating housing 20. Internal bore 12 is suitably configured with a set of internal threads 14.
As described in more detail herein, when housing 20 is inserted into internal bore 12 and securely fastened to bolt 10, a gauge length 40 is measured in a section of bolt 10 which does not yield nor elastically deform in a non-uniform manner.
In accordance with various embodiments of the present invention, load indicator 30 is suitably comprised of any internal device for indicating a load, such as, for example, devices disclosed in U.S. Patent No. 4,571,133, U.K. Patent No.
265-954-B, U.S. Patent No. 4,525,114, and the like. In a preferred embodiment of the present invention, load indicator 30 is of a type disclosed in the U.S. Patent No.
5,668,323. Thus, in general, load indicator 25 is suitably comprised of a slotted plug 31, a return spring 32, a conical washer 33, a gauge pin 34, O-rings 35, a transparent disk 36, snap rings 29 and lever 38.
In accordance with a preferred embodiment of the present invention, gauge pin 34 suitably establishes a reference point B. Slotted plug 31 is suitably pressed into a lower end 21 of housing 20. Plug 31 suitably incorporates pivot pin 39 about which lever 38 rotates. As bolt 10 is loaded, the strain in bolt 10 causes gauge pin 34 to move away from housing 20. As gauge pin 34 moves away from housing 20, slotted plug 31 suitably moves a corresponding amount, thus changing gauge length 40.
As gauge length 40 changes, lever 39 rotates about pivot pin 34. As shown in Figure 12, indicator end 39 opposite pivot pin 39 moves across a display 41, thereby displaying the load on bolt 10.
In accordance with one aspect of the present exemplary embodiment, conical washer 33 sits between housing 20 and fastener 10 and applies continual pressure at both an "off ' and an "on" position.
In accordance with another aspect of the present exemplary embodiment, O-rings 35, along with transparent disk 10 and snap ring 11 suitably secure a head 45 of load indicator 30 and prevent moisture and dirt penetration into load indicator 30 and bolt 10.
In accordance with still another aspect of the present exemplary embodiment, load indicator 30 can be set in an "on" and an "off" position. With reference to S Figures 11 a,b, housing 20 is suitably rotated so that a dowel 42 is aligned with either the "on" or "off' markings. The maneuver of rotating housing 20 to the off position through a counter clockwise revolution suitably lifts lever 38 off pin 34, thereby taking the load off indicator 30, placing housing 20 and load indicator 30 in the "off' position.
Now, with reference to Figure I I, in accordance with a preferred embodiment of the present invention, housing 20 is suitably comprised of a hollow cylinder of the same material as bolt 10, though housing 20 may suitably be comprised of varying other shapes and sizes and materials. Preferably housing 20 is configured of a material with the same coefficient of thermal expansion as bolt I 0 such as low carbon 4140 steel, 4340 steel, B-7 steel and the like. Housing 20 is suitably configured to contain load indicator 30 with the hollow of housing 20.
As mentioned briefly above, in the case where fastener I O is a bolt, the influence of the end yielding and non-uniform elastic deformation decreases as the upper attachment point of housing 20 is moved away from an upper surface I S
of a head I I of bolt 10. In accordance with the present invention, if housing 20 is suitably secured beneath head 11 or, alternatively, closer to a bottom surface 17 of head 11 (i.e., where head 11 meets body 1 b) the adverse effect of such yielding and elastic deformation on the load monitoring process can be entirely eliminated.
Thus, housing 20 is suitably configured to be rigidly secured to bolt 10 in an area on bolt 10 outside the region susceptible to yielding and non-uniform elastic deformation. In the present exemplary embodiment, housing 20 is configured with a set of external threads 31 designed to engage a set of internal threads 22 in bore 12.
However, in accordance with various alternative embodiments of the present invention, housing 20 may be secured in bore 12 by press fit (see Figure 17), set screws, shear pins and the like. Additionally, in accordance with an alternative embodiment of the present invention, housing 20 and load indicator 30 may suitably be integrated as one unit. That is, rather than assembling housing 20 and load indicator 30 in separate steps, housing 20 and load indicator 30 are manufactured as a self contained assembly.
Thus, in accordance with a preferred embodiment of the present invention, and with reference to Figures 11 and 13, securing housing 20 to bolt 10 establishes a reference point A. For bolt 10, the proper positioning is suitably somewhere below a head 14 of bolt 10 or where head 14 meets a body 16 of bolt 10.
In accordance with one aspect of the preferred embodiment of the present invention, the preferable position of reference point A is found by first determining the region of bolt 10 most susceptible to yielding. For example, determination of the region is suitably done through finite element analysis, physical experiment, mathematical calculation and the like.
Thus, reference points A and B, and corresponding gauge length 40, are suitably outside the region subject to yielding or non-uniform elastic deformation. In this manner, though yielding and non-uniform elastic deformation at one end of stud 10 or bolt 10 can still occur, such yielding and deformation will not influence gauge length 40.
Now, with reference to Figure 13, in accordance with another embodiment of the present invention, housing 20 can be retrofitted into stud 10. However, the proper position of reference point A may vary depending upon the particular stud 10 and the particular application of stud 10. For example, in accordance with the present exemplary embodiment, in the case where fastener I O is a stud, it should first be determined what the maximum nut standoff will be after assembly when stud 10 is in service. Once the standoff is determined, the proper position for reference point A
will suitably be either below a nut 50 or toward the bottom of nut 50 when at this maximum standoff position.
In accordance with a preferred embodiment of the present invention, housing 20 may be suitably provided with calibration device 60. With reference to Figures 20a,b, calibration device 60 is suitably adjustable. Calibration device 60 is suitably configured with a clip 61 and a set screw bore 62 suitably configured to accept a set screw 63.
In the present exemplary embodiment, clip 61 is suitably configured as a ring which surrounds load indicating device head 45. However, with momentary reference to Figure 21, according to various alternative aspects of the present exemplary embodiment, clip 61 may suitably configured as a arcuate segment which does not entirely surround load indicating device head 45.
In accordance with the present exemplary embodiment and with reference to Figures 20 and 21, set screw bore 62 and set screw 63 are suitably oriented perpendicular to a tangent of ring 61. However, in accordance with various alternative aspects of the present embodiment and with reference to Figure 22, set screw bore 62 and set screw 63 may be suitably oriented tangentially to ring 61, through a set of brackets 64 surrounding an opening 65, such that set screws closes opening 65, thereby tightening ring 61. Additionally, in accordance with various other alternative aspects of the present exemplary embodiment, set screw 63 may be replaced with other suitable locking means, such as, for example, shear pins and the Like.
During assembly of load indicating device 30 and fastener 10, load indicating device 30 is suitably secured to fastener 10 such that load indicating device 30 is at a "zero" load position. Ring 61 is rotated about load indicating head 45 until set screw 63 suitably engages dowel 42, thus setting the zero load point of load indicating device 30: Set screw 63 is then suitably tightened to rigidly secure ring 61 to load indicating device head 45.
Thus, in accordance with the present exemplary embodiment, when ring 61 is rotated, load indicating device 30 is similarly rotated, thus turning load indicating device 30 "on" and "off '. Should the calibration of load indicating device 30 become inaccurate, set screw 63 is simply loosened, unloaded load indicator 30 is rotated until display 41 indicates "zero" load, ring 61 is rotated until set screw 63 engages dowel 42 and set screw 63 is re-tightened, thus re-calibrating load indicating device 30.
The various embodiments described above therefore have several distinct advantages. First of all, the embodiments suitably allow reference points A
and B, and thus gauge length 40, to be established outside the region where problems can arise from end yielding, non-uniform elastic deformation and nut locations.
Housing 20 can easily be lengthened or shortened depending upon the design of fastener 10 and the application of fastener 10. In terms of manufacturability, housing 20 and pivot pin 39 can be made on conventional lathes or screw machines, and no welding or complicated forming processes are necessary. However, alternatively, housing may also be manufactured from sheet metal forming processes or other suitable manufacturing or machining processes. Additionally, the machining of bolt 10 is simplified, and internal threads 14 can be easily tapped rather than using an internal single point threading operation or bushing 7 (as shown in Figure 7).
Further, assembling bolt 10 with housing 20 as shown in Figure 14 is significantly quicker than that of the current designs. Housing 20 does not necessarily need to be locked into position with shims S or locking nut 6. Rather, housing 20 is merely suitably rotated into position. Thus, checking the zero or changing load indicator 30 from off to on or vice versa can be much quicker and can be performed by an operator in the field. Finally, bolt 10 does not necessarily need to be during assembly in order to work-harden or "set" head 11 as end yielding will not adversely effect the performance of load indicator 30.
Another improvement is that the housing 20 is symmetrical and can be comprised of hardened materials such as low carbon 4140 steel, 4340 steel, B-7 steel and the like. Such materials result in improved shock resistance superior to that of current designs.
For elevated temperature applications, it is a simple matter to find stock materials to which the coefficient of thermal expansion is suitably similar to the material of bolt 10. In many cases, it is possible to simply use the same material that is used for bolt 10 to make housing 20.
Conical washer 33 may suitably establish a consistent locking pressure in the "on" position and remove the variables and inaccuracies associated with the operator when determining the appropriate locking tightness. Conical washer 33, as shown in Figures 11, 13, and 14, additionally suitably supplies a constant force to take out "back lash" caused by internal and external threads 14, 22. Washer 33 suitably pulls housing 20 snugly against the upper thread faces of thread 14, 22, but does not lock them into position as retaining shims 5 or lock nut 6 do. Conical washer 33 also suitably acts as a shock absorber between the housing 20 and bolt 10.
Additionally, the precision of the fit between plug 2 and gauge pin 34 can be increased, thus reducing inaccuracies that arise due to the "looseness" of current designs.
While the principles of the invention have been described in illustrative embodiments, many modifications of structure, arrangement, proportions, the elements, materials and components, used in the practice of the invention and not specifically described may be varied and particularly adapted for a specific environment and operating requirement without departing from those principles.
In general, if the changed reference length is not accounted for, then all of the calculated stress levels will be inaccurate.
Many commonly used strain measuring devices offer no method to account for a changed reference length and thus can not be relied upon after the initial assembly.
Further, in cases where there is a method to determine or adjust for this problem, the procedure for doing so is generally inconvenient. In order to make this adjustment, it is typically necessary to remove the tension from the fastener. However, removing the tension is typically either not possible or prohibitively burdensome as it usually involves taking the machinery out of service.
Another common fastener problem arises when attaching a strain gauge to the 1 S end of a stud (a non-headed fastener). In particular, the final position of the nut on the stud after installation will directly affect the length of the strained region. To understand this, consider Figures 4a and 4b. The Figures show a stud with nuts in two positions. Since the region between point A and B is unstrained, the strained region must be located between points B and D. However, the exact region subjected to strain is very unclear and relies upon at least two variables: ( 1 ) the respective distances between A and D and between B and D, and (2) the cumulative effect of non-uniform forces on the stud by the nut.
The distance between A, the upper surface of the stud, and B, the upper surface of the nut, is called the nut standoff. In most bolting applications, little attention is paid to precisely controlling and/or maintaining this distance.
Many variables will effect the nut standoff as shown in Figures Sa and Sb, such as, for example, the overall length of the fastener, the amount of thread engagement of the nuts, the height of the nuts, variances in the dimensions of the joint and gasket, and the dimensions of any washers.
A second variable effecting the length of the strained region involves the non-uniform forces exerted on the stud by the nut. In general, the greatest contact forces occur in the lower threads, and the contact forces decrease to approximately zero at the uppermost thread (point B). The exact distribution varies for each individual stud and nut combination and depends upon the exact dimensions, the exact thread pitch, the hardness of the parts, and the tension in the fastener.
Furthermore, for an individual nut and stud, the force distribution will change over time. As the first few threads experience the highest load, in service they will often yield. Thus, nearby threads may exert greater contact forces on the stud over time or upon re-assembly. Such unknown and changing force distributions makes it .
impossible to know exactly where the yield should be located between points B' and C' in Figures 4 and 5. Since this end of the stretched section is not known, the exact length of the stretched region is not known.
The combined effect of these two variables, and particularly the variations in the standoff can lead to significant problems in the strain monitoring process.
Although elongation can still be measured, in order to convert this elongation to a stress level it is necessary to know the length of the strained region. To address this problem, the user can attempt to control the standoff in the field. However, this can be a prohibitively time consuming process that may require the removal of the nut and the insertion or removal of spacing washers, or the adjusting of thread engagement of the nuts which may involve at least a partial dismantling of the joint.
Further, the retraining of already experienced operators to pay close attention to nut standoff is a problem. In most bolt assembly operations, the emphasis is on speed, and there is a great resistance to more complicated assembly procedures.
Sensitivity to shock, external forces and extended cyclic loading are additional problems with current fastener and load indicating device designs. For example, bolts and studs are often dropped, pounded out of fixtures, subjected to vibration and generally exposed to rugged conditions. When the fastener is subjected to such shocks and external forces, inaccuracy in the calibration of the load indicating device can be introduced. Thus, re-zeroing or re-calibration of the load indicating device is necessary.
Some load indicating devices have incorporated an "on-off ' mechanism in attempts to allow re-calibration of the load indicator. Generally, the mechanism consists of two dowels, the first pressed into the load indicating device, the second pressed into the fastener. When the fastener and load indicating device are assembled, WO 99/49289 PCT/(TS99/06612 another dowel is placed in the fastener end such that when the load indicating device is rotated, the dowels engage one another such that the ring is prevented from further rotation in that direction. The dowels are generally accurately positioned in order that the Load indicating device can be rotated into an "on" orientation which will engage the load indicating device to a specific position for making the strain measurement.
After the measurement is performed, the load indicating device may be rotated to an "off " position. In the "off position, the load indicating device is disengaged from the fastener (or gauge pin).
However, with present calibration designs, there is typically no accurate way to adjust the dowel placement if the calibration of the load indicating devices changes.
Generally, this is because the dowels are permanently affixed to the ring and fastener.
Should error in the "zero" of the load indicating device occur, the dowel placement cannot easily be adjusted to re-zero or re-calibrate the load indicating device. The fastener and load indicating device must typically be returned to the manufacturer in order to re-adjust the dowel placement. Additionally, the machining involved with properly placing and inserting the towels increases the time and costs associated with manufacturing the load indicating and calibrations devices.
The device disclosed in U.S. Patent No. 5,668,323, attempted to solve many of the aforementioned problems and one embodiment is shown in Figures 6-9.
However, the technology of the '323 patent still leaves problems relating to ruggedness, manufacturability, performance at elevated temperatures, and performance under cyclic loading applications. To date there has been only one available embodiment of the device on the market. As shown in Figure 8, the body of the device comprises two main sections, a head 1 and a body 2, which are spot welded together. The head 1 is a turned and threaded piece with a rectangular slot punched through the bottom.
The body 2 is a sheet metal bracket. The manufacturing of the bracket is generally very time consuming and involves many operations, including wire EDM cutting, a series of bending, spot welding, fine tuning and straightening, drilling, pressing, and hand chamfering.
Unfortunately, long assembly times are another problem of the design. There are different methods for locking the device into position. With reference to Figure 7, one of these methods includes inserting a series of shims 5 or a locking nut 6 so that the device is properly tightened when the indicating lever points to zero (this process is called zeroing the device.) However, when doing this, the assembler must be careful not to over-tighten the device as it will tend to pull the bushing 7 from the bolt.
S Once the device is assembled into the bolt and zeroed, the bolt is usually loaded to its proof load at least three times. This lengthy loading process is in part to calibrate the device and to work harden the head of the bolt to reduce the effects of yielding. The complexities of manufacturing the base of the device and the lengthy process of installation and preloading the bolts three times are merely exemplary of several manufacturing complications which lead to prohibitively high retail prices for .
most applications. For example, currently, the insertion of the load indicator will usually add $90 to $200 or more to the price of the fastener.
Aside from the cumbersome and complicated manufacturing procedures, there are other inherent problems with the '323 patent design. First, for most applications, the '323 patent device will not adequately perform at higher temperatures.
This is because the coefficient of thermal expansion of the bracket will usually be different from the coefficient of expansion of the alloys used for high temperature fasteners. In order for the device to work properly at elevated temperatures, the device would have to expand similarly to the material of the fastener. That is, the device and the bolt would need to have the same coefficient of thermal expansion.
However, matching these coefficients is usually not possible due to other constraints of the bracket material. One of these constraints is that the '323 patent device is formed from a material that can withstand the strain of the bending without fracturing, particularly at point E in Figure 9. Another drawback is that the material is generally only available in sheets of thickness approximately 0.020 inch. Yet another aspect of this problem is that the material must be weldable to the material of the head 2 as shown in Figure 9. Finally, there must not be a corrosion problem that can occur when dissimilar metals are connected.
To date, generally, all of these constraints have only been found to be satisfied with one type of material, 1095 sheet metal which has a coefficient of thermal expansion different than that of most fasteners. Although other materials having different coefficients of thermal expansion may exist, they are typically rare, and may _g_ involve raw stock that would need to be specially manufactured. Thus, in practice, it is only possible to match the coefficient of thermal expansion for a select few materials.
Another inherent problem of the current embodiment of the '323 patent device S involves the interface between the body 2 and a pin 4 as shown in Figure l0a and l Ob.
The pin 4 has a round hole 9 drilled in it. However, the cross section of the body 2 is generally rectangular. Thus, a precise fit of the two pieces is typically impossible and the device may slip positions, thereby leading to inaccuracies when the device is subjected to shock, vibration, or cyclic loading. Additionally, the inaccuracies lead to wearing of the corners of the body 2, further increasing accuracy problems.
A third and significant problem with the disclosed design is the inherently weak nature of the design. Since the strain in forming the bend E in Figure 9 is significant, a relatively soft material must be used. However, under shock this material readily yields, and even a few tenths of a thousandth of an inch can significantly affect a measurement which may only be a few thousandths of an inch.
Further, the rectangular nature of the cross-section renders the bend weaker in some directions than in other directions.
The final problem described here regarding the current design is related to a general inability to properly lock the cartridge into the indicating position without affecting the accuracy of the device. It has been noted that when the device is rotated from the "on" position to the "off ' position, and then rotated and locked back to the "on" position, the device will often lose accuracy. This loss of accuracy is related to the nature of the locking methods. Since thread tolerances of the cartridge can allow play between the cartridge and the bolt, it is difficult to repeat an exact distance between the cartridge top and the gauge pin 4. Thus, shims S and locking nut 6 are used to tighten the device.
However, for both of these methods, the actual "tightness" of the locked position is typically dependent upon the person tightening the fastener. Since different people will have different opinions of what is properly "tight", the device will not be locked into the "on" position in the same manner each time. This inconsistency combined with the thread tolerances in the cartridge again can cause significant inaccuracies.
Accordingly, methods and apparatus for attaching a load-indicating device to a fastener which do not suffer from these problems is desirable.
SUMMARY OF THE INVENTION
An apparatus for attaching an internal strain gauge to a fastener according to various aspects of the present invention presents a housing which secures the strain gauge to the fastener in such a manner that the strain gauge readings are less affected by strains at the end of the fastener as well as external factors on the fastener such as non-uniform elastic deformation, yielding, shock, cyclic loading, temperature changes and the like.
In accordance with the present invention, the housing is secured internally to the fastener sufficiently far enough from a region susceptible to yielding and non-uniform deformation that the gauge length measured by the strain gauge is unaffected by such yielding and non-uniform elastic deformation of the fastener.
Additionally, the housing is provided with a calibration ring for "zeroing"
the 1 S strain gage when no load is applied to the fastener and for re-calibrating or removing the load on the strain gauge as desired.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional aspects of the present invention will become evident upon reviewing the non-limiting embodiments described in the specification and the claims taken in conjunction with the accompanying figures, wherein like numerals designate like elements, and:
Figure la is a side view of an untightened bolt with an internal strain gauge bore;
Figure lb is a side view of a tightened bolt with an internal strain gauge bore;
Figure 2a is a side view of an unyielded bolt on a surface which is not perpendicular to the bolt's axis;
Figure 2b is a side view of a loaded bolt on a surface which is not perpendicular to the bolt's axis;
Figure 2c is a side view of a yielded bolt after removal from a surface which is not perpendicular to the bolt's axis;
Figure 3a is a side view of an unloaded bolt in a bore too large for the bolt head;
Figure 3b is a side view of a loaded bolt in a bore too large for the bolt head;
Figure 3c is a side view of a yielded bolt after removal from a bore too large for the bolt head;
Figure 4a is a side view of a stud with a first distance between two nuts;
Figure 4b is a side view of a stud with a second distance between two nuts;
Figure Sa is a side view of a stud with an internal strain gauge using washers;
Figure Sb is a side view of a stud with an internal strain gauge without washers;
Figure 6 is a perspective view showing insertion of an internal strain gauge into a bolt head;
Figure 7 is a cross-sectional side view of a bolt with an internal strain gauge;
Figure 8 is a cross-sectional side view of an internal strain gauge;
Figure 9 is a front view of the body of the Waxman device;
Figure l0a is a side view of the interface between the head and the pin;
Figure 1 Ob is a top view of the interface between the head and the pin;
Figure 11 a is a cross-sectional side view of a preferred embodiment of an internal strain gauge and housing in a bolt;
Figure 1 lb is a top view of a preferred ernbodirnent of an internal strain gauge and housing;
Figure 12 is an isometric drawing of a bolt head containing internal strain gauge showing an indicator arrow pointed to a calibrated scale;
Figure 13 is a cross-sectional side view of an internal strain gauge and housing retrofitted into a stud;
Figure 14 is a cross-sectional side view of a housing used with the strain gauge of U.S. Patent No. 4,571,133;
Figure 15 is a cross-sectional view of one embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 16 is a cross-sectional view of an alternative embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
WO 99/49289 PCT/US99/Obbl2 Figure 17 is a cross-sectional view of yet another embodiment of housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 18 is a cross-sectional side view of a simplified embodiment of the housing used with the strain gauge of U.K. Patent No. GB 2-265-954-B;
Figure 19 is a cross-sectional side view of an alternative embodiment of a housing according to the present invention.
Figure 20a is a cross-sectional side view of the calibration device attached to the load indicator;
Figure 20b is a top view of the calibration device attached to the load indicator;
Figure 21 is a top view of an alternative embodiment of the calibration device attached to the load indicator; and Figure 22 is a top view of yet another alternative embodiment of the calibration device attached to the load indicator The following descriptions are of preferred exemplary embodiments only, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing a preferred embodiment of the invention. Various changes may be made in the function and arrangement of elements described in the preferred embodiments without departing from the spirit and scope of the invention as set forth in the appended claims.
In general, according to various aspects of the present invention provides a method and apparatus for attaching a load-indicating device to a fastener such that the load-indicating device is capable of continually monitoring the load on the fastener, and such that the load indicating device is not subject to external factors such as non-uniform elastic deformation, yielding, shock, cyclic loading, temperature and the like.
Further, though the various embodiments described below will be directed to a bolt or stud fastener, various other fasteners subject to external loads including pins; dowels, screws and the like may similarly incorporate the present invention.
Thus, with reference to Figures 11-14, in accordance with the present invention, a bolt 10 suitably includes a housing 20, a load indicator 30 for displaying the load status of bolt 10 and a calibration ring 50.
In accordance with the present exemplary embodiment of the present invention, bolt 10 is suitably configured with an internal bore 12 for accommodating housing 20. Internal bore 12 is suitably configured with a set of internal threads 14.
As described in more detail herein, when housing 20 is inserted into internal bore 12 and securely fastened to bolt 10, a gauge length 40 is measured in a section of bolt 10 which does not yield nor elastically deform in a non-uniform manner.
In accordance with various embodiments of the present invention, load indicator 30 is suitably comprised of any internal device for indicating a load, such as, for example, devices disclosed in U.S. Patent No. 4,571,133, U.K. Patent No.
265-954-B, U.S. Patent No. 4,525,114, and the like. In a preferred embodiment of the present invention, load indicator 30 is of a type disclosed in the U.S. Patent No.
5,668,323. Thus, in general, load indicator 25 is suitably comprised of a slotted plug 31, a return spring 32, a conical washer 33, a gauge pin 34, O-rings 35, a transparent disk 36, snap rings 29 and lever 38.
In accordance with a preferred embodiment of the present invention, gauge pin 34 suitably establishes a reference point B. Slotted plug 31 is suitably pressed into a lower end 21 of housing 20. Plug 31 suitably incorporates pivot pin 39 about which lever 38 rotates. As bolt 10 is loaded, the strain in bolt 10 causes gauge pin 34 to move away from housing 20. As gauge pin 34 moves away from housing 20, slotted plug 31 suitably moves a corresponding amount, thus changing gauge length 40.
As gauge length 40 changes, lever 39 rotates about pivot pin 34. As shown in Figure 12, indicator end 39 opposite pivot pin 39 moves across a display 41, thereby displaying the load on bolt 10.
In accordance with one aspect of the present exemplary embodiment, conical washer 33 sits between housing 20 and fastener 10 and applies continual pressure at both an "off ' and an "on" position.
In accordance with another aspect of the present exemplary embodiment, O-rings 35, along with transparent disk 10 and snap ring 11 suitably secure a head 45 of load indicator 30 and prevent moisture and dirt penetration into load indicator 30 and bolt 10.
In accordance with still another aspect of the present exemplary embodiment, load indicator 30 can be set in an "on" and an "off" position. With reference to S Figures 11 a,b, housing 20 is suitably rotated so that a dowel 42 is aligned with either the "on" or "off' markings. The maneuver of rotating housing 20 to the off position through a counter clockwise revolution suitably lifts lever 38 off pin 34, thereby taking the load off indicator 30, placing housing 20 and load indicator 30 in the "off' position.
Now, with reference to Figure I I, in accordance with a preferred embodiment of the present invention, housing 20 is suitably comprised of a hollow cylinder of the same material as bolt 10, though housing 20 may suitably be comprised of varying other shapes and sizes and materials. Preferably housing 20 is configured of a material with the same coefficient of thermal expansion as bolt I 0 such as low carbon 4140 steel, 4340 steel, B-7 steel and the like. Housing 20 is suitably configured to contain load indicator 30 with the hollow of housing 20.
As mentioned briefly above, in the case where fastener I O is a bolt, the influence of the end yielding and non-uniform elastic deformation decreases as the upper attachment point of housing 20 is moved away from an upper surface I S
of a head I I of bolt 10. In accordance with the present invention, if housing 20 is suitably secured beneath head 11 or, alternatively, closer to a bottom surface 17 of head 11 (i.e., where head 11 meets body 1 b) the adverse effect of such yielding and elastic deformation on the load monitoring process can be entirely eliminated.
Thus, housing 20 is suitably configured to be rigidly secured to bolt 10 in an area on bolt 10 outside the region susceptible to yielding and non-uniform elastic deformation. In the present exemplary embodiment, housing 20 is configured with a set of external threads 31 designed to engage a set of internal threads 22 in bore 12.
However, in accordance with various alternative embodiments of the present invention, housing 20 may be secured in bore 12 by press fit (see Figure 17), set screws, shear pins and the like. Additionally, in accordance with an alternative embodiment of the present invention, housing 20 and load indicator 30 may suitably be integrated as one unit. That is, rather than assembling housing 20 and load indicator 30 in separate steps, housing 20 and load indicator 30 are manufactured as a self contained assembly.
Thus, in accordance with a preferred embodiment of the present invention, and with reference to Figures 11 and 13, securing housing 20 to bolt 10 establishes a reference point A. For bolt 10, the proper positioning is suitably somewhere below a head 14 of bolt 10 or where head 14 meets a body 16 of bolt 10.
In accordance with one aspect of the preferred embodiment of the present invention, the preferable position of reference point A is found by first determining the region of bolt 10 most susceptible to yielding. For example, determination of the region is suitably done through finite element analysis, physical experiment, mathematical calculation and the like.
Thus, reference points A and B, and corresponding gauge length 40, are suitably outside the region subject to yielding or non-uniform elastic deformation. In this manner, though yielding and non-uniform elastic deformation at one end of stud 10 or bolt 10 can still occur, such yielding and deformation will not influence gauge length 40.
Now, with reference to Figure 13, in accordance with another embodiment of the present invention, housing 20 can be retrofitted into stud 10. However, the proper position of reference point A may vary depending upon the particular stud 10 and the particular application of stud 10. For example, in accordance with the present exemplary embodiment, in the case where fastener I O is a stud, it should first be determined what the maximum nut standoff will be after assembly when stud 10 is in service. Once the standoff is determined, the proper position for reference point A
will suitably be either below a nut 50 or toward the bottom of nut 50 when at this maximum standoff position.
In accordance with a preferred embodiment of the present invention, housing 20 may be suitably provided with calibration device 60. With reference to Figures 20a,b, calibration device 60 is suitably adjustable. Calibration device 60 is suitably configured with a clip 61 and a set screw bore 62 suitably configured to accept a set screw 63.
In the present exemplary embodiment, clip 61 is suitably configured as a ring which surrounds load indicating device head 45. However, with momentary reference to Figure 21, according to various alternative aspects of the present exemplary embodiment, clip 61 may suitably configured as a arcuate segment which does not entirely surround load indicating device head 45.
In accordance with the present exemplary embodiment and with reference to Figures 20 and 21, set screw bore 62 and set screw 63 are suitably oriented perpendicular to a tangent of ring 61. However, in accordance with various alternative aspects of the present embodiment and with reference to Figure 22, set screw bore 62 and set screw 63 may be suitably oriented tangentially to ring 61, through a set of brackets 64 surrounding an opening 65, such that set screws closes opening 65, thereby tightening ring 61. Additionally, in accordance with various other alternative aspects of the present exemplary embodiment, set screw 63 may be replaced with other suitable locking means, such as, for example, shear pins and the Like.
During assembly of load indicating device 30 and fastener 10, load indicating device 30 is suitably secured to fastener 10 such that load indicating device 30 is at a "zero" load position. Ring 61 is rotated about load indicating head 45 until set screw 63 suitably engages dowel 42, thus setting the zero load point of load indicating device 30: Set screw 63 is then suitably tightened to rigidly secure ring 61 to load indicating device head 45.
Thus, in accordance with the present exemplary embodiment, when ring 61 is rotated, load indicating device 30 is similarly rotated, thus turning load indicating device 30 "on" and "off '. Should the calibration of load indicating device 30 become inaccurate, set screw 63 is simply loosened, unloaded load indicator 30 is rotated until display 41 indicates "zero" load, ring 61 is rotated until set screw 63 engages dowel 42 and set screw 63 is re-tightened, thus re-calibrating load indicating device 30.
The various embodiments described above therefore have several distinct advantages. First of all, the embodiments suitably allow reference points A
and B, and thus gauge length 40, to be established outside the region where problems can arise from end yielding, non-uniform elastic deformation and nut locations.
Housing 20 can easily be lengthened or shortened depending upon the design of fastener 10 and the application of fastener 10. In terms of manufacturability, housing 20 and pivot pin 39 can be made on conventional lathes or screw machines, and no welding or complicated forming processes are necessary. However, alternatively, housing may also be manufactured from sheet metal forming processes or other suitable manufacturing or machining processes. Additionally, the machining of bolt 10 is simplified, and internal threads 14 can be easily tapped rather than using an internal single point threading operation or bushing 7 (as shown in Figure 7).
Further, assembling bolt 10 with housing 20 as shown in Figure 14 is significantly quicker than that of the current designs. Housing 20 does not necessarily need to be locked into position with shims S or locking nut 6. Rather, housing 20 is merely suitably rotated into position. Thus, checking the zero or changing load indicator 30 from off to on or vice versa can be much quicker and can be performed by an operator in the field. Finally, bolt 10 does not necessarily need to be during assembly in order to work-harden or "set" head 11 as end yielding will not adversely effect the performance of load indicator 30.
Another improvement is that the housing 20 is symmetrical and can be comprised of hardened materials such as low carbon 4140 steel, 4340 steel, B-7 steel and the like. Such materials result in improved shock resistance superior to that of current designs.
For elevated temperature applications, it is a simple matter to find stock materials to which the coefficient of thermal expansion is suitably similar to the material of bolt 10. In many cases, it is possible to simply use the same material that is used for bolt 10 to make housing 20.
Conical washer 33 may suitably establish a consistent locking pressure in the "on" position and remove the variables and inaccuracies associated with the operator when determining the appropriate locking tightness. Conical washer 33, as shown in Figures 11, 13, and 14, additionally suitably supplies a constant force to take out "back lash" caused by internal and external threads 14, 22. Washer 33 suitably pulls housing 20 snugly against the upper thread faces of thread 14, 22, but does not lock them into position as retaining shims 5 or lock nut 6 do. Conical washer 33 also suitably acts as a shock absorber between the housing 20 and bolt 10.
Additionally, the precision of the fit between plug 2 and gauge pin 34 can be increased, thus reducing inaccuracies that arise due to the "looseness" of current designs.
While the principles of the invention have been described in illustrative embodiments, many modifications of structure, arrangement, proportions, the elements, materials and components, used in the practice of the invention and not specifically described may be varied and particularly adapted for a specific environment and operating requirement without departing from those principles.
Claims (15)
1. An apparatus configured for attaching a load indicating device to a fastener, comprising a housing wherein said housing is secured to said fastener in a non-yielding location.
2. An apparatus configured for attaching a load indicating device to a fastener, according to claim 1, wherein said load indicating device can continuously monitor the load exerted on said fastener.
3. An apparatus configured for attaching a load indicating device to a fastener, according to claim 1, wherein said load indicating device is substantially internal to said fastener.
4. An apparatus configured for attaching a load indicating device to a fastener, according to claim 1, wherein said housing is comprised of the same material as said fastener.
5. An apparatus configured for attaching a load indicating device to a fastener according to claim l, wherein said housing is comprised of a material of substantially the same coefficient of thermal expansion as said fastener.
6. An apparatus configured for attaching a load indicating device to a fastener according to claim 1, wherein said housing is fastened internally to said fastener.
7. An apparatus configured for attaching a load indicating device to a fastener according to claim 1, wherein said housing is configured to be threaded into a bore of said fastener.
8. An apparatus configured for attaching a load indicating device to a fastener according to claim 1, wherein said housing is configured to be press fit into a bore of said fastener.
9. A method for attaching a load indicating device to a fastener, comprising the steps of:
providing a housing having a load indicating device;
determining a non-yielding region of said fastener; and securing said housing in said non-yielding region of said fastener.
providing a housing having a load indicating device;
determining a non-yielding region of said fastener; and securing said housing in said non-yielding region of said fastener.
10. A method for attaching a load indicating device to a fastener according to claim 9, wherein said non-yielding region determining step is performed by finite element analysis.
11. A method for attaching a load indicating device to a fastener according to claim 9, wherein said non-yielding region determining step is performed by physical experimentation.
12. An apparatus configured for attaching a load indicating device to a stud, comprising a housing wherein said housing is secured to said stud such that a gauge length measured by said load indicating device is independent of a nut standoff of said stud.
13. An apparatus configured for attaching a load indicating device to a fastener, according to claim 12, wherein said load indicating device can continuously monitor the load exerted on said stud.
14. An apparatus configured for attaching a load indicating device to a fastener, according to claim 12, wherein said load indicating device is substantially internal to said stud.
15. An apparatus configured for calibrating a load indicating device, comprising:
a calibration clip;
a set screw bore; and a set screw.
a calibration clip;
a set screw bore; and a set screw.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US7946098P | 1998-03-26 | 1998-03-26 | |
| US60/079,460 | 1998-03-26 | ||
| PCT/US1999/006612 WO1999049289A1 (en) | 1998-03-26 | 1999-03-25 | Apparatus and method for attaching a load indicating device to a fastener |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2325557A1 true CA2325557A1 (en) | 1999-09-30 |
Family
ID=22150704
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002325557A Abandoned CA2325557A1 (en) | 1998-03-26 | 1999-03-25 | Apparatus and method for attaching a load indicating device to a fastener |
Country Status (3)
| Country | Link |
|---|---|
| AU (1) | AU3116299A (en) |
| CA (1) | CA2325557A1 (en) |
| WO (1) | WO1999049289A1 (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US20090206705A1 (en) * | 2008-02-19 | 2009-08-20 | Nies Jacob J | Fasteners with welded ultrasonic stress transducers |
| DE102017102360A1 (en) | 2017-02-07 | 2018-08-09 | Juko Technik Gmbh | Device for determining a preload force |
Family Cites Families (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4292835A (en) * | 1980-02-25 | 1981-10-06 | Raymond Engineering, Inc. | Calibration apparatus and method for strain measuring instruments |
| US4525114A (en) * | 1980-10-08 | 1985-06-25 | Rotabolt Limited | Fastener device |
| US4429579A (en) * | 1981-10-26 | 1984-02-07 | Helm Instrument Co., Inc. | Tie rod tension sensor |
| US5211061A (en) * | 1991-07-16 | 1993-05-18 | Goodwin Jerry J | Bolt clamping force sensor and clamping force validation method |
| GB9207880D0 (en) * | 1992-04-10 | 1992-05-27 | Ceney Stanley | Load indicating fasteners |
| US5668323A (en) * | 1996-07-12 | 1997-09-16 | Waxman; Cory S. | Method and apparatus for indicating a load |
-
1999
- 1999-03-25 CA CA002325557A patent/CA2325557A1/en not_active Abandoned
- 1999-03-25 WO PCT/US1999/006612 patent/WO1999049289A1/en active Application Filing
- 1999-03-25 AU AU31162/99A patent/AU3116299A/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| WO1999049289A1 (en) | 1999-09-30 |
| AU3116299A (en) | 1999-10-18 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |