SE515974C2 - Method of biasing a joint containing two or more threaded fasteners - Google Patents

Abstract

A method for pretensioning a joint to a predetermined clamping load level, which joint is apt to settlement during pretensioning and comprises two or more threaded fasteners (15 a-d) to be tightened by a simultaneous engagement of individually rotated and controlled torque delivering tools (19 a-d), comprising the steps of: I) applying simultaneously a tightening torque on each one of the fasteners (15 a-d), II) measuring during the torque application the instantaneous axial load (F) in each one of the fasteners (15 a-d), III) interrupting the torque application on each one of the fasteners as the axial load (F) on the respective fastener (15 a-d) has reached a predetermined axial load (FT) which corresponds to the intended clamping load level in the joint (10), IV) reapplying a tightening torque one or more times on that fastener or those fasteners (15 a-d) on which the axial load (F) has decreased due to settlement of the joint (10) during one or more relaxation time intervals so as to regain the predetermined axial load level (FT), and V) terminating the pretensioning process of the joint (10) as the axial load (F) has remained substantially unchanged at the predetermined axial load level (FT) for a final time interval in all of the fasteners (15 a-d), thereby providing the intended clamping load level in the joint (10).

Description

515 9314 noc us en oun- nu 1 o: een u non: uooo: can cnn 0 IQ u Oc o 1 none nauo alo- o Q nu 1 0 1 ua: Q 2) a dependence on the significant difference in static and dynamic frictional resistance present in the threaded fasteners, which also results in an undesired spread of the bias level in the joint, and 3) a continuous torque application of the tightening tool, even at standstill, which means that it is difficult to use electric tightening tools due to overheating problems in tooling problems.

As for the friction dependence of the torque-related tightening method, it is important to note that the bulk, sometimes about 90% of the torque delivered, is absorbed by frictional resistance in the threaded fasteners, and since the friction typically varies by 25-30%, it is obvious that the measured torque is a very poor reflection of the clamping force achieved by the fastening elements.

The main object of the invention is to solve the above-mentioned problems and to substantially improve the accuracy of the obtained clamping force by reference to the actual axial force in each fastening element during the entire prestressing process.

According to a preferred embodiment of the invention, the tightening method is based on the determination of the variations in axial force on each of the fastening elements via the determination of variations in the length of the fastening elements by measuring variations in flight time for supersonic waves induced in the fastening elements.

Further features and advantages of the invention will become apparent from the following description in which an example is described with reference to the accompanying drawings.

List of drawings: 515 974 Fig. 1 shows a joint including four threaded fastening elements and a tightening device for simultaneous and individual tightening of the fastening elements.

Fig. 2 shows a longitudinal section through a torque-emitting spindle with a nut sleeve and a sound-path-inducing electrode for co-operation with a threaded fastening element.

Fig. 3 is a diagram illustrating the tightening of one of the fasteners of a joint during tightening in accordance with the invention.

Fig. 4 shows a diagram similar to Fig. 3 and illustrates tightening of a second fastening element included in the same joint.

Fig. 5 shows a diagram illustrating tightening of one of the fastening elements of a joint in accordance with a two-stage prestressing method. The prestressing method according to the invention will be described in connection with a joint comprising at least two threaded fastening elements. The example shown in Fig. 1 comprises a dressing 10 formed by a main part 11 and a connected upper part 12. The dressing 10 comprises a gasket 13 for providing a tight connection between the two parts 10, 11. A common application of this type of joints are the mounting of cylinder heads on engine blocks of internal combustion engines.

However, the invention is equally well applicable to dressings without gaskets but comprising one or more parts which make the dressing prone to settle under the pretension.

The main thing is that the joint has such properties that the clamping force in the joint tends to decrease during an initial tightening stage, provided that no specific measures are taken.

The joint illustrated in Fig. 1 comprises four threaded fasteners 15d in the form of head screws inserted through the upper part 12 and threaded into the main part 11, or in the form of 4% of pin bolts fixedly mounted in the main part 11 and nuts threaded on the upper part of such studs.

Fig. 1 also shows a multiple nutrunner 18 having four individually rotatable and controlled output shafts 19a-d which are intended to simultaneously cooperate with the threaded fastening elements 15a-d and to exert a tightening torque thereon.

As shown in Fig. 2, each of the nut puller spindles 19 has a splined end 22 for supporting a sleeve 23 for co-operation with the fastener.

Each spindle 19 is axially displaceable and spring-loaded by a spring 24 towards the engaging position with respective fastening elements 15. Each of the spindles 19 is also provided with a means 25 for generating and detecting ultrasonic waves by abutting interaction with the end surface 26 of the fastening element 15.

In this example, the means 25 for generating and detecting ultrasonic waves comprises an electrode 27 arranged concentrically in the spindle 19, and a piezoelectric sensor 28 which is inseparably mounted on the rear end surface 26 of the fastening element 15. The electrode 27 is connected to a control unit (not shown) located in or spaced apart from the multiple nutrunner 18, and the end of the electrode is axially spring loaded by a spring (not illustrated) into contact with the sensor 28.

This type of device for generating and detecting ultrasonic waves is previously known per se, for example by U.S. Patent 4,846,001.

To illustrate the biasing method according to an embodiment of the invention, reference is made to the two diagrams shown in Figs. 3 and 4 which illustrate the axial force F and the angle of rotation @ in relation to the time of two of the fasteners in the joint 10. The tightening response of the other two not illustrated the fasteners are approximately the same but for the sake of clarity the description is limited to only two of the fasteners. It should be noted that the described and illustrated biasing method is only an example of the method according to the invention in which the intended final clamping force level FT in the joint is achieved directly, i.e. this method does not involve any intermediate steps where the joint is tightened to one or more clamping force levels.

However, the invention is not at all limited to the described method but may equally include intermediate steps terminating at one or more clamping force levels below the intended final clamping force level FT. predetermined measuring level P3 of the axial force. This is achieved by applying the spindles 19a - d of the nutrunner 18 to the fastening elements 15a-d and a tightening torque is applied simultaneously to all the fastening elements 15a-d until the intended clamping force level FT is reached. Then the nutrunner spindles stop. The axial force F in the fastening elements, which is illustrated by a solid line in the diagrams in Figs. 3 and 4, thus rises directly from zero to the target level FT. This is achieved at the point t1 where the fastening element has been rotated to an angular position G5. The angular position of the fastener is illustrated by the dashed line in the diagram and when the rotation ceases, the dashed line becomes horizontal.

After a short moment, at point t2, a settlement has occurred in the joint which is large enough to initiate a restart of that particular spindle of the nutrunner 18 to rotate the fastener to a new angular position G5 where the axial force in the fastener has regained the intended target level FT _ This occurs at the point ts where the spindle is stopped again. After a further time interval and a further setting of the joint 10, the nutrunner spindle is restarted at point t4, to move the fastener to a position G5 which is reached at point ts. The target load level FT has been reached again.

After a further pause, the joint 10 has settled further and the nutrunner spindle is restarted a third time at point ts to re-establish the intended axial force level FT in the fastener. This is achieved at point t7. After this, the axial force remains constant, ie. no further settlements affecting the fastener occur.

At a point tT where the measured axial force F in all the fastening elements has been constant for a certain time interval, the tightening process is interrupted.

In parallel with the tightening of the fastening element as illustrated in the diagram in Fig. 3, the other three fastening elements in the joint 10 are tightened in a similar manner. One of these is illustrated in the diagram in Fig. 4. As in the previously described example, the initial rotation of this fastener results in a direct bias to the target load level FT at a point t1. At this point, the fastener has reached an angular position G5, and after four further retractions at the points tg, ty t¿ and ty, additional angles of rotation are added to the fastener from there to ®2, G5, GQ and Qg. The tightening process is interrupted in tT where the axial force F has been unchanged at the target load level Eg for a certain time interval.

As mentioned above, the whole process is interrupted when the axial force has been constant in all the fastening elements for a certain time interval, ie. no further settlements have taken place in the 515,974 unit. This is indicated at point tT in the diagram in Fig. 3.

Due to different individual characteristics of the fastening elements 15 a-d, depending i.a. at their different locations in the joint, the tightening restart occurs at different times for different fasteners, ie. none of the points tl-tv of the first illustrated fastener need to coincide with any of the points tl-ts of the second fastener. However, the final point tT for terminating the process is common to both spiders.

When the target load level FT remains constant in all the fastening elements 15 a-d for a certain time interval, i.e. without any further settling having occurred in the joint, the tightening process is complete and the nutrunner 18 is finally shut off.

An alternative embodiment of the invention is illustrated in Fig. 5, namely a two-stage prestressing process where the joint is initially prestressed to a preliminary clamping force level corresponding to an axial force Fpi each of the fastening elements. Since this preliminary load level is safely maintained in all the fasteners, the process continues by biasing the fastener to the target load level FT which corresponds to the desired clamping force level in the joint.

As illustrated in the diagram in Fig. 5, the two-stage biasing process is performed by an initial tightening of each of the fasteners 15a-d to a preliminary load level FP. This is achieved at a time t1, whereby the fastening element has been rotated to an angular position GQ.

After a certain time there has been a settling in the joint 10 which requires a tightening torque again 515 974 øøoøov I 0 caucus n 0 0 n se oo rann A no urin nano 0 Q o 0 nan ': leo nn 0 0 ecco no 0 0 coca a 0 n 0 0 0 O nu 0 0: duo n 0 1 nen: U of 0 0 u 0 is applied to the joint at point tz. The preliminary load FP in the fastening element is regained at G5 “After a further setting in the joint, the tightening torque on the fastening element is resumed at point t3 in order to once again achieve the axial force FP in the fastening element.

This is achieved at the angular position G5. After two further insertions in the joint 10, the torque application on the fastening element at res.

After the joint has been prestressed and kept at the preliminary clamping force level Eg, the second prestressing step begins at point ts. This means that each of the fastening elements is tightened to the target load FT which is achieved in the position ®6. After a further setting of the joint 10, a tightening torque is re-applied to each of the fastening elements at point t, and when after a certain time interval no further settling has been indicated, the process ends and the power nutrunner 18 is switched off. The illustrated fastener has then reached its final angular position ®7.

It should be noted that the invention is not limited to the example described above but can be further varied within the scope of the claims. The tightening process can thus be carried out in more than two steps, especially when it is applied to larger joints comprising a large number of fastening elements. An uneven setting of such joints can cause varying degrees of stress reduction in the fasteners and may also require tightening to more intermediate levels to achieve an even final prestressing level in the joint.

It should also be pointed out that the method according to the invention is based on a direct determination of the instantaneous ono 0 515 971% pineapple a 0 0 o on masturbation: uu 0 o nu ou our: n I 0 our: some 0 0 Q 0 coca 1000 nn 0: con 0 n Q 0 coca Icon 0 n 0 0: can n on 0 0 0 n now the axial load in the joint fastener. However, the method is not limited to using the method described above for determining the axial load via measuring the individual elongation of each fastener.

Instead of using piezoelectric ultrasonic wall generators on the fasteners, other equivalent methods can be used to determine the elongation of the fastener and thereby also the axial force in the fasteners. For example, ultrasonic waves can be induced in the fasteners by electrodynamic sound generators arranged on the spindles of the nutrunner.

Claims (4)

515 97¿ ïäm.í ¥ fë. $. {Ï3 10 Eatentkrayi A method of biasing prone screw joints (10) comprising two or more threaded fasteners (15a-d) to an intended final clamping force level (P3) by simultaneously applying a torque-releasing tightening tool (19a-d) to each fastener (15a-d) , comprising the following steps: I) simultaneous application of a tightening torque on all fasteners (l5a-d), II) determination of the instantaneous axial force (F) during current torque application in each of the fasteners (l5a-d) by measuring the variationswiw. ,, ....,. @. f .... »_.... w ~ -w- *“ the flight time of ultrasonic waves in fl induced in each of the fasteners ~ (l5a ~ d) y hlwnwwwim III) interruption of said torque application on each of the fastening elements (l5a-d) when the instantaneous axial force (F) on the respective fastening element (l5a-d) has reached said intended final clamping force levellïgwqkx IV) reattachment of a tightening torque one or more times on the fastening element or elements ( 15a-d) on which the axial force (F) has decreased as a result of placing the joint (10) during one or more relaxation intervals to regain the said intended final clamping force level (FT), and V) terminating the tightening of the joint (10) when the axial force (F) has remained substantially unchanged at a level corresponding to said intended final clamping force level (FT) during a final time interval in all fasteners (l5a-d). Method according to claim 1, characterized in that said ultrasonic waves are induced in the fastening elements (15a-d) by a sound wave generating means fixedly mounted on each of the fastening elements and activated by an electrical voltage applied via said tightening tool. 11 A method of biasing a joint (10) containing two or more threaded fasteners (15a-d) to an intended clamping force level by simultaneously applying a torque-releasing tightening tool (19a-d) to each of the fasteners (15a-d), comprising the following steps: I) simultaneously applying a tightening torque to each of said fastening elements (15), II) determining during current torque application the instantaneous axial force (F) in each of the fastening elements (15a-d) by measuring the variations in flight time of ultrasonic waves induced in each of the fasteners (15a ~ d), III) interruption of said torque application on each of the fasteners (15) when the axial force (F) on each fastener (15) has reached a preliminary level (FP) corresponding to a clamping force level in the joint (10) which is lower than said intended final clamping force level (FQ, IV) individual reattachment of tightening torque one or more times on the fastening element (15) on which axial ften (F) has dropped depending on the setting of the joint (10) during one or more fixed time intervals to thereby recover said preliminary clamping force level (FP) in each fastening element, V) simultaneous reapplication of a tightening torque on all fastening elements (15) to guide them to a target clamping force level (FT) corresponding to said intended final clamping force level in the joint (10), and VI) individual reapplication of a tightening torque one or more times on the fastening element (15) on which the axial force (F) has decreased as a result of placing the joint (10) for one or more time intervals to thereby recover said target clamping force level (FT) in the respective fastening element, and VII) terminating the tightening of the joint (10) when the axial force (F) has remained substantially constant at said target clamping force level (FT) during a certain time interval in all fasteners (15). Method according to claim 3, characterized in that said simultaneous torque application on all fastening elements (15) is interrupted and said individual re-application of torque is also repeated at one or more intermediate clamping force levels in said fastening element (15), between said preliminary clamping force level (FP) and said final clamping force level (FT), in order to achieve an even settlement and clamping force build-up in the joint (10).