GB1577341A

GB1577341A - Shear pin load cell load measuring equipment

Info

Publication number: GB1577341A
Application number: GB819377A
Authority: GB
Original assignee: British Hovercraft Corp Ltd
Current assignee: British Hovercraft Corp Ltd
Priority date: 1978-02-20
Filing date: 1978-02-20
Publication date: 1980-10-22

Description

(54) SHEAR PIN LOAD CELL LOAD MEASURING EQUIPMENT (71) We, BRITISH HOVERCRAFT CORPORATION LIMITED, of Yeovil, in the County of Somerset, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to load measuring equipment, and particularly to electrical strain measurement in pins used in machine junctions where the pin is subjected to shear stress.

In machinery such as cranes, hoists, conveyor rollers, stub axles, etc., and in other applications, for example, cable mooring points, there are necessarily employed various junctures in which operational forces are transmitted or supported by pivotal forces bolts or local axle members. In many such installations, certain pivotal joints are subjected to stresses which vary in magnitude and frequently also in direction, and it is often desirable to measure them under actual working conditions.

It has been proposed, for example in U.S.

Patent Specifications 3,695,096 and 3,754,610, to measure the loads transmitted by such pins or bolts by substituting a specially constructed shear pin load cell in place of the original pin or bolt.

Since such prior art load cells are sensitive to the direction of the applied load, it has been necessary to retain a fixed spatial relationship between load sensing elements in the load pin and the angle of the applied load. In, for example, a clevis and yoke combination, this has necessitated the provision of locking means to lock the load pin to either the yoke or the clevis, so that it retains a known angular relationship with the direction of an applied load.

Several forms of locking means are disclosed in the aforementioned U.S. patent specifications. These include a special boss welded onto the fitting to contact a shaped portion provided on the pin, a key and slot arrangement and a threaded locking pin.

Since it is one of the features of the apparatus that the load cells are replaceable with an existing pin or bolt then it follows that, whatever form of locking means is provided, some modification is required to the existing equipment.

Apart from the fact that it may be impractical to modify certain forms of existing equipment, thus precluding use of the load cells, in cases where it is possible it represents an undesirable complication and added expense. Furthermore, in some installations, such as in cable mooring points, the required modifications may represent a very hazardous operation.

U.S. Patent Specification 3,695,096 discloses an arrangement for measuring a load applied to the load cell from an unknown radial direction, however, in this arrangement it is also necessary to fix the load cell in a known position relative to X and Y axes, so that it is again necessary to provide means to lock the load cell to one of the parts of the apparatus.

In its broadest aspect, the present invention provides load measuring equipment including a shear pin load cell coupling a pivotal joint subject during operation to loads varying both in magnitude and in radial direction, said shear pin load cell having strain-sensitive elements arranged to provide signals significant respectively of load components in two radial directions in the shear pin; and electrical processing circuitry connected to said elements and responsive to said load componentsignificant signals to compute the magnitude of a load, in any radial direction, giving rise to said signals.

Except in those applications in which it is desired to compute both the magnitude and the direction, relative to one of the joint components, of an applied force, the shear pin load cell may be installed in the joint without restraint against rotation.

Preferably the said strain-sensitive elements are arranged to provide signals significant of load components in radial directions inclined at 90" to each other. In one embodiment, diametrically opposed pairs of strain-sensitive elements are arranged with the sensitive plane of one pair inclined at 90" to that of the other.

In another aspect, the invention provides load measuring equipment including a shear pin load cell coupling a pivotal joint subject during operation to loads varying both in magnitude and in radial direction, said load cell being substantially rotationally free in said pivotal joint and having two diametrically opposed pairs of strain-sensitive elements arranged with their sensitive planes radial to the pin, that of one pair being at 90 degrees to that of the other, each said pair of strain-sensitive elements being connected into an individual electrical bridge circuit to provide an electrical signal proportional to the component of an applied load in the sensitive plane of that pair, and electrical processing circuitry connected to said bridge circuits and arranged to square and summate said signals to provide a resultant output electrical signal directly proportional to the magnitude of said applied load regardless of the radial direction of said applied load relative said load cell.

Two sets of diametrically opposed pairs of elements may be respectively disposed at axially spaced-apart locations on the shear pin load cell.

The strain-sensitive elements may be located at regions of reduced cross section on the load cell.

Conveniently, electrical connecting leads may be routed to the strain-sensitive elements through an axial bore and through radial bores in the load cell. The electrical processing circuitry may be disposed externally and connected to the strain-sensitive elements by means of these leads, or such circuitry may be located within the axial bore. When the processing circuitry is to be situated at a remote location, together with suitable display means, it may be connected to said shear pin load cell either by electrical cables or through radio telemetry techniques. In the latter case. suitable encoder modules and a transmitter may be incorporated in the shear pin for transmitting an encoded signal to a telemetry receiver at a remote display centre.

A removable cover at one end of the load cell may conveniently include means providing for external connection to the electrical connecting leads or to the processing circuitry, as the case may be.

Each strain-sensitive element or each diametrically opposed pair of strainsensitive elements may be connected into an electrical bridge circuit so as to provide an electrical signal proportional to the sensed component of an applied load. The processing circuitry may include electronic conditioning and linearising circuits and means for treating the load component-significant signals or the computed load magnitude with a calibration constant.

An embodiment of the inventon will now be described by way of example only, and with reference to the accompanying drawings, in which: Figure I is a perspective part sectioned exploded view of a shear pin load cell according to the present invention, Figure 2 is a sectioned view taken along lines X-X of Figure 1, Figure 3 is an electrical circuit diagram, Figure 4 is a perspective exploded view of a cable mooring hook including the shear pin load cell of the present invention, Figure 5 is a fragmentary side view in the direction of arrow Y or Figure 4, and Figure 6 is a diagrammatic illustration of a section of the shear pin load cell showing load paths effective during operation.

Referring to Figures 1 and 2, a shear pin load cell 10 comprises a generally tubular pin 11 provided with two regions 12 and 13 of reduced cross-section diameter. The surface of each reduced section region is provided with four axially aligned flats arranged at 90 degrees to each other, and strain-sensitive elements comprising diametrically opposed pairs A and B are secured to the flats. Each member of each of the pairs of strain-sensitive elements comprises two parallel strain gauges so that at region 12 pair A comprises gauges 20, 21 and 24, 25, and at region 13 comprises gauges 22, 23, and 26, 27 whereas at region 12 pair B comprises gauges 28, 29, and 32, 33, and at region 13 comprises gauges 30, 31 and 34, 35.

An axial bore 14 extends through the load cell 10 and is connected by radial bores 15 to a position adjacent each flat in both reduced section regions 12 and 13 to provide access for electrical leads (not shown) to each of the strain gauges.

A cover plate 16 is provided at each end of the load cell 10. Conveniently, one of said cover plates may be provided with a gland or socket arrangement (not shown) for connecting an input supply voltage to the circuits and for connecting an electrical output from the circuits to processing circuitry.

The reduction in diameter of the pin at the regions 12 and 13 prevent wear and degradation of the accurately sized cross-section of the shear faces in these regions, and also serves to eliminate the application of direct compressive stresses to these faces and to the strain gauges mounted thereon.

The reduced cross-sectional regions 12 and 13 of the pin 11 will not generally weaken the pin sufficiently to make it unsuitable for satisfactorily carrying the intended loads and, in any case, this may readily be compensated for by using a higher grade material for the load cell 10 than that used in the manufacture of the pin which it is designed to replace in a particular application.

Strain gauges 20 to 35 inclusive are interconnected in the form of two separate Wheatstone bridge circuits A and B, as shown in Figure 3. Connecting leads generally indicated at 17 are routed through the axial bore 14 and the radial bores 15 in the pin 11, and are connected to an electrical power supply and to signal conditioning and linearising circuits as hereinafter more fully explained.

The arrangement of four pairs of strain gauges in each of the Wheatstone bridge circuits A and B minimises the effects of bending and axial stresses in the pin 11 on the required signal output. The gauged areas and cavities in the pin 11 are protected from the effects of the environment by any suitable means, for example a potting compound (not shown).

Figure 4 is an exploded view of a typical cable mooring hook assembly, it being noted that the previously described shear pin load cell 10 of the present invention is inserted in place of an existing pin to pivotally connect a base portion 36 to a trunnion 37. The trunnion 37 is connected to a hook 38 via side plates 39 and two further pins 40 and 41. The side plates 39 are located by spacing bars 42 retained by nuts (not shown), and an inner end of the hook 38 is retained by a conventional quick release mechanism (not shown).

It will be apparent that in such an installation, since all of the pins transmit the total tension of a mooring line in double shear, the load cell 10 could be used in place of either of the pins 40 or 41 as well as the actual location shown in the drawing.

In the described arrangement, the reduced cross-sectional areas 12 and 13 on the shear pin load cell 10, and hence the position of the strain-sensitive elements are located so as to lie between the central part of the pin 11 which is coupled to one side of the load through the trunnion 37 and the two outer parts of the pin 11 which are coupled to the other side of the load through the base 36.

Excessive end float of the shear pin load cell 10 is undesirable. However, since the equipment in which the pin is to be fitted will normally include some means for retaining an existing pin, then the same means can be utilised to retain the load cell 10 in position. Alternatively, end float may be prevented by any standard engineering practice.

Figure 6 is a diagrammatic illustration of a section of the load cell 10 showing the principle of operation of the strain measuring equipment of the invention.

The pairs of strain-sensitive elements A and B are arranged at 90 degrees to each other so that their sensitive planes are also at 90 degrees to each other, and are connected into two separate Wheatstone bridge circuits A and B as hereinbefore explained.

Let us suppose that a force F is applied to the load cell 10 as shown at an unknown angle 0 from an axle through the pair of sensing elements A. Since the pairs of sensing elements A and B are arranged perpendicularly, each pair of sensing elements A and B will be affected not in proportion to the total force F, but rather in proportion to its horizontal and vertical components Fa and Fb respectively. Thus, by employing the usual rectangular coordinate convention, Fa = F sin 0 and Fb = F cos 0. The force F is thus resolved into two components, F sin O and F cos 0, and each of the two bridge circuits A and B will register one component or vector. Circuit A will register a signal proportional to F sin 0 and circuit B will register a signal proportional to F cos 0.

The signals from each bridge circuit are fed into suitable electronic processing circuitry to operate mathematically on the signals. The circuitry is arranged to square and summate the two signals and to provide a resultant signal that, because of the trigonometrical relationship of cos20 + sin20 = 1, is directly proportional to the applied load F. Preferably means are also included in the circuitry to square root the resultant signal so that the relationship between the signal and the applied load is of a linear form.

It will be understood that this computation remains constant regardless of the actual amount of the angle 0, so that in the equipment of the present invention it is not necessary to control or know the radial position of the load cell 10 relative to the angle 0 of the applied load. A principal advantage of the present equipment, therefore, resides in the fact that it is capable of measuring applied loads from any direction without having to lock the load cell 10 in any particular part of the coupling through which loads are to be measured. The load cell 10 can thus be made precisely in the exterior form of a coupling pin which it is designed to replace and can be readily substituted therefore without the need to provide an additional locking device to retain the load cell 10 in a precise spatial relationship with the direction of an applied load, as is the case in prior equipment.

The signal resulting from the previously mentioned computation is displayed on a suitable display unit, for example, a millivoltmeter, that may be arranged to display the signal in millivolts for further manual or automatic computation utilising a known calibration constant of the load cell 10 to convert the signal to the actual load. Alternatively, the millivoltmeter may be calibrated so as to provide a direct load read-out, or the processing circuitry may include means to multiply either the separate electrical signals or the resultant signal by the calibration constant.

The processing circuitry includes electronic conditioning and linearising circuits, and may be located either adjacent to the load cell or remote therefrom.

For example, certain applications of the equipment may involve the use of long cable runs to connect the load cell 10 to the processing circuitry and display equipment.

To prevent degradation of the electrical supply in such an application, the Wheatstone bridges of the strain sensitive elements are fed from a remote four terminal power supply through a multi-core cable, the sense leads ensuring a stable bridge excitation voltage at the device. The output from each bridge circuit is converted to a current directly proportional to the output of that bridge by two-wire process transmitters, mounted adjacent the load cell, for retransmission to the processing means and display equipment over cores of the abovementioned cable. This system prevents undue degradation of the signal from the bridge circuits due to cable resistance and electrical interference from adjacent cable and local R.F. sources and, when used with suitable approved transmitters and safety barriers, allows measurement of loads to be obtained from within certified hazardous areas.

In other applications, where the signals from the Wheatstone bridges may have to be transmitted several miles to the display equipment, radio telemetry techniques may be employed. In such an installation the conditioning and linearising circuits may be fitted, together with encoder modules, either within the bore of the load cell 10, or adjacent the load cell 10 in a suitable enclosure. The encoded signal is fed to a transmitter and received by a telemetry receiver at a display centre where it is decoded and fed to suitable display equipment.

Alternatively. in such an application, the encoder modules may be mounted either within or adjacent the load cell 10 and the conditioning and linearising circuits may be located at the remote display centre.

Whilst one embodiment has been described and illustrated, it is to be understood that many modifications may be made without departing from the scope of the invention as defined in the following claims. The shear pin load cell may be of other suitable configurations. For example, the strain sensitive elements may be mounted internally in the bore of the shear pin load cell directly underlying the reduced shear cross-sectional regions between the load coupling regions.

Alternatively, the groove forming the reduced shear cross-sectional region may be replaced by segments machined from the surface of the pin, on which segments the strain sensitive elements are mounted. The load cell may be of a cantilever or stub axle configuration utilising a single shear sensitive region only, as opposed to the double shear configuration hereinbefore described.

In a double shear configuration, the strain sensitive elements at each end of the load cell may be connected into separate Wheatstone bridge circuits. In such an arrangement, the strain sensitive elements at each end of the load cell may be misaligned axially and the processing circuitry may be arranged to compare the resultant signals from the circuits at each end of the load cell, and to provide an averaged resultant signal.

WHAT WE CLAIM IS: 1. Load measuring equipment including a shear pin load cell coupling a pivotal joint subject during operation to loads varying both in magnitude and in radial direction, said shear pin load cell having strainsensitive elements arranged to provide signals significant respectively of load components in two radial directions in the shear pin; and electrical processing circuitry connected to said elements and responsive to said load component-significant signals to compute the magnitude of a load, in any radial direction, giving rise to said signals.

2. Load measuring equipment as claimed in Claim 1, wherein said strainsensitive elements are arranged to provide signals significant of load components in radial directions inclined at 90 degrees to each other.

3. Load measuring equipment as claimed in Claim 2, comprising two diametrically opposed pairs of strain sensitive elements arranged with the sensitive plane of one pair inclined at 90 degrees to that of the other.

4. Load measuring equipment comprising a shear pin load cell coupling a pivotal joint subject during operation to loads varying both in magnitude and in radial direction, said load cell being substantially rotationally free in said pivotal joint and having two diametrically opposed pairs of strain-sensitive elements arranged with their sensitive planes radial to the pin, that of one pair being at 90 degrees to that of the other,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. relationship with the direction of an applied load, as is the case in prior equipment. The signal resulting from the previously mentioned computation is displayed on a suitable display unit, for example, a millivoltmeter, that may be arranged to display the signal in millivolts for further manual or automatic computation utilising a known calibration constant of the load cell 10 to convert the signal to the actual load. Alternatively, the millivoltmeter may be calibrated so as to provide a direct load read-out, or the processing circuitry may include means to multiply either the separate electrical signals or the resultant signal by the calibration constant. The processing circuitry includes electronic conditioning and linearising circuits, and may be located either adjacent to the load cell or remote therefrom. For example, certain applications of the equipment may involve the use of long cable runs to connect the load cell 10 to the processing circuitry and display equipment. To prevent degradation of the electrical supply in such an application, the Wheatstone bridges of the strain sensitive elements are fed from a remote four terminal power supply through a multi-core cable, the sense leads ensuring a stable bridge excitation voltage at the device. The output from each bridge circuit is converted to a current directly proportional to the output of that bridge by two-wire process transmitters, mounted adjacent the load cell, for retransmission to the processing means and display equipment over cores of the abovementioned cable. This system prevents undue degradation of the signal from the bridge circuits due to cable resistance and electrical interference from adjacent cable and local R.F. sources and, when used with suitable approved transmitters and safety barriers, allows measurement of loads to be obtained from within certified hazardous areas. In other applications, where the signals from the Wheatstone bridges may have to be transmitted several miles to the display equipment, radio telemetry techniques may be employed. In such an installation the conditioning and linearising circuits may be fitted, together with encoder modules, either within the bore of the load cell 10, or adjacent the load cell 10 in a suitable enclosure. The encoded signal is fed to a transmitter and received by a telemetry receiver at a display centre where it is decoded and fed to suitable display equipment. Alternatively. in such an application, the encoder modules may be mounted either within or adjacent the load cell 10 and the conditioning and linearising circuits may be located at the remote display centre. Whilst one embodiment has been described and illustrated, it is to be understood that many modifications may be made without departing from the scope of the invention as defined in the following claims. The shear pin load cell may be of other suitable configurations. For example, the strain sensitive elements may be mounted internally in the bore of the shear pin load cell directly underlying the reduced shear cross-sectional regions between the load coupling regions. Alternatively, the groove forming the reduced shear cross-sectional region may be replaced by segments machined from the surface of the pin, on which segments the strain sensitive elements are mounted. The load cell may be of a cantilever or stub axle configuration utilising a single shear sensitive region only, as opposed to the double shear configuration hereinbefore described. In a double shear configuration, the strain sensitive elements at each end of the load cell may be connected into separate Wheatstone bridge circuits. In such an arrangement, the strain sensitive elements at each end of the load cell may be misaligned axially and the processing circuitry may be arranged to compare the resultant signals from the circuits at each end of the load cell, and to provide an averaged resultant signal. WHAT WE CLAIM IS:

1. Load measuring equipment including a shear pin load cell coupling a pivotal joint subject during operation to loads varying both in magnitude and in radial direction, said shear pin load cell having strainsensitive elements arranged to provide signals significant respectively of load components in two radial directions in the shear pin; and electrical processing circuitry connected to said elements and responsive to said load component-significant signals to compute the magnitude of a load, in any radial direction, giving rise to said signals.

each said pair of strain-sensitive elements being connected into an individual electrical bridge circuit to provide an electrical signal proportional to the component of an applied load in the sensitive plane of that pair, and electrical processing circuitry connected to said bridge circuits and arranged to square and summate said signals to provide a resultant output electrical signal directly proportional to the magnitude of said applied load regardless of the radial direction of said applied load relative said load cell.

5. Load measuring equipment as claimed in Claim 3 or 4, having two sets of diametrically opposed pairs of strainsensitive elements respectively disposed at axially spaced-apart locations on the shear pin load cell.

6. Load measuring equipment as claimed in any preceding Claim, wherein said strain-sensitive elements are located on the surface of regions of reduced crosssection on the shear pin load cell.

7. Load measuring equipment as claimed in any preceding Claim, wherein electrical connecting leads are routed to the strain-sensitive elements through an axial bore and through radial bores in the shear pin load cell.

8. Load measuring equipment as claimed in any preceding claim, wherein said electrical processing circuitry is located within the axial bore of the shear pin load cell.

9. Load measuring equipment as claimed in Claim 7 or 8, including a removable cover at one end of the load cell, said cover having means providing for external connection to the electrical connecting leads or to said processing circuitry.

10. Load measuring equipment as claimed in any preceding Claim, wherein said electrical processing circuitry includes electronic conditioning and linearising circuits and means for treating said load component-significant signals or said computed load magnitude with a calibration constant.

11. Load measuring equipment substantially as described with reference to and as shown in the accompanying drawings.