METHOD FOR PRETENSIONING A JOINT CONTAINING TWO OR MORE THREADED FASTENERS

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: Method for pretensioning a joint containing two or more threaded fasteners. The invention relates to a method for pretensioning a settling joint containing two or more threaded fasteners to an intended clamping load by applying simultaneously a torque delivering tool on each fastener. In particular, the invention concerns a method for pretensioning a joint which contains two or more threaded fasteners and which is apt to settle during an initial stage of the axial load application, thereby causing a relaxation of one or more of the threaded fasteners and a loss of axial clamping load obtained in the joint. A previously known method for pretensioning a joint of the above described type is based on a torque related tightening of the fasteners. This means that the actual torque applied on each one of the fasteners is measured continuously during the entire tightening process, and that occurring settlements in the joint at the intended pretension level is met by a continuous maintenance of the predetermined torque level applied on the fasteners for a certain time interval, meaning rotation as well as stand still condition in the fasteners as be required to maintain the predetermined torque level . This known method suffers from two serious drawbacks, namely: 1) a dependency on the inevitable variations in frictional resistance in the threaded fasteners, which results in an undesirable scattering of the clamping load obtained in the joint, 2) a dependency on the considerable difference between static and dynamic resistance in the threaded fasteners, which also results in an undesirable scattering of the joint pretensioning level, and 3) a continuous torque application by the tightening tool, also at still stand conditions, which means that the use of electrically powered tightening tools is difficult due to over-heating problems related to the tightening tool motors . As to the friction dependency of the torque related tightening method, it is important to note that a major part, sometimes about 90%, of the applied torque is absorbed by the frictional resistance in the threaded fasteners, and since the friction typically varies by 25 - 30 %, it is evident that the measured torque is a very poor reflection of the clamping load obtained by the fasteners. The main object of the invention is to solve the above mentioned problems and to substantially improve the resulted joint clamping load accuracy by referring to the actual axial load in each fastener during the entire pretensioning process . According to a preferred embodiment of the invention, the joint pretensioning process is based on the determination of the axial load variations on each fastener via a determination of the variations in the fasteners length by measuring the variations in transit time for ultra-sonic waves induced in the fasteners . Further characteristics and advantages of the invention will appear from the following specification in which is described an example with reference to the accompanying drawing . On the drawing : Fig. 1 shows a joint including four threaded fasteners, and a tightening device for simultaneously and individually tightening the fasteners. Fig. 2 shows a longitudinal section through a torque delivering spindle with a nut socket and a sound wave inducing electrode for engagement with a threaded fastener. Fig. 3 shows a diagram illustrating tightening one of the fasteners of a joint to be pretensioned in accordance with the invention.. Fig. 4 shows a diagram similar to Fig. 2 and illustrating tightening of a second fastener comprised in the same joint . Fig. 5 shows a diagram illustrating tightening of one of the fasteners of a joint in a two-step pretensioning process . The pretensioning process according to the invention will be described in connection with a joint comprising at least two fasteners. The example shown in Fig. 1 comprises a joint 10 formed by a main structure 11 and an attached upper part 12. The joint 10 comprises a gasket 13 for accomplishing a tight connection between the two parts 10,11. A common application of this type of joint is the attachment of the cylinder head on an engine block of a combustion engine. However, the invention is as well applicable on joints without gaskets but including one or more parts making the joint apt to settle during pretensioning. The main thing is that the joint has such properties that the clamping load in the joint tends to decrease under an initial tightening stage, provided no specific measures are being taken. The joint illustrated in Fig. 1, comprises four threaded fasteners 15 a - d in the form of head screws entered through the upper part 12 and threaded into the main structure 11 or in the form of studs secured in the main structure 11 and nuts engaging the upper ends of such studs . In Fig. 1, there is also illustrated a multiple nutrunner 18 having four individually rotated and controlled output spindles 19 a - d which are intended to engage simultaneously the threaded fasteners 15 a - d and to apply a tightening torque thereon. As illustrated in Fig. 2, each one of the nutrunner spindles 19 has a splined end 22 for carrying a fastener engaging nut socket 23. Each spindle 19 is axially displaceable and biassed into engagement with the respective fastener 15 by a spring 24. Each one of the spindles 19 are also provided with ultra-sonic wave generating and detecting means 25 for abutting engagement with the end surface 26 of the fastener 15. In this example, the ultra-sonic wave generating and detecting means 25 comprises an electrode 27 disposed concentrically in the spindle 19 and a piezo-electric transducer 28 inseparably attached to the rear end surface 26 of the fastener 15. The electrode 27 is connected to a control unit (not shown) located in the multiple nutrunner 18 or at a location remote therefrom, and the tip of the electrode is axially biassed by a non-illustrated spring into contact with the transducer 28. This type of ultra-sonic wave generating and detecting means is previously known per se in, for instance, US Patent No. 4,846,001. In order to illustrate the pretensioning method according to one embodiment of the invention, it is referred to the two diagrams shown in Figs. 3 and 4, which illustrate the clamping load F and the rotation angle Φ in relation to time for two of the fasteners of the joint 10. The tightening response of the other two non-illustrated fasteners are about the same, but for clarity reasons the description is limited to just two of the fasteners. It should be noted that the described and illustrated pretensioning process is just one example of method according to the invention in which the intended final clamping load level F τ in the joint is accomplished directly, i. e. this process does not involve any intermediate steps where the joint is pretensioned to one or more lower load levels. However, the invention is not at all limited to the described process but may as well include intermediate steps ending at one or more load levels below the intended final clamping load level F τ . The joint 10 as a whole as well as each one of the fasteners 15 a - d are intended to be pretensioned to a predetermined clamping load target level F τ . This is obtained in that the spindles 19 a - d of the nutrunner 18 are brought into engagement with the fasteners 15 a - d, and a tightening torque is applied simultaneously on all of the fasteners 15 a - d until the intended target load is reached. Then, this nutrunner spindle is stopped. Accordingly, the axial load F in the fastener, which is illustrated by a continuous line in the diagram in Figs. 3 and 4, increases directly from zero to the target level F τ . This is reached at a point t 17 where the fastener has been rotated to an angular position Φ x . The angular position of the fastener is illustrated by the dash line in the diagram, and when the rotation ceases the dash line becomes horizontal. After a short while, at point t 2 , a settlement has occurred in the joint large enough to initiate a restart of that particular output spindle of the nutrunner 18 to rotate the fastener to a new position Φ 2 where the axial load in the fastener has regained the intended target value F τ . This occurs at point t 3 , where the spindle is again stopped. After another time interval and a further settlement of the joint 10, the nutrunner spindle is restarted at point t 4 , moving the fastener to the position Φ 3 which is reached at t 5 . The target load F τ is regained. After still another rest period, the joint 10 has settled further, and the nutrunner spindle is restarted a third time at point t 6 to re-establish the intended axial load level F τ in the fastener. This is obtained at point t 7 . After this, the axial load remains constant, i.e. no further settlement influencing this fastener occurs. At a point t τ where the detected axial load F in all of the fasteners have been constant for a certain time interval, the pretensioning process is interrupted. In parallel with the tightening of the fastener illustrated in the diagram in Fig. 3, the other three fasteners of the joint 10 are tightened in a similar way. 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 pretensioning to the target load level F τ at a point t x . At this point, the fastener has reached an angular position Φ 1# and after four additional re- tightenings at the points t 2 , t 3 , t 4 and t 5 , adding rotational angles to the fastener from Φ λ to Φ 2 , Φ 3 , Φ 4 and Φ 5 . The tightening process is interrupted at t τ where the axial load F has remained unchanged at the target load level F τ for a certain time interval. As mentioned above, when the axial load has remained constant in all of the fasteners for a certain time interval, i. e. no further settlement is detected in the joint, the entire process is interrupted. This is indicated by the point t τ in the diagram in Fig. 3. Dependent on different individual characteristics of the fasteners 15 a - d , dependent i. a. on their different locations on the joint, the tightening restarting points occur at different occasions in the two fasteners, i.e. no one of the points t x - t 7 of the first illustrated fastener has to coincide with any one of the points t 1 - t 5 of the second fastener. The final process interruption point t τ is common to both spindles, though. As the target load level F τ remain constant in all of the fasteners 15 a - d for a certain time interval, i. e. without any further settlement has occurred in the joint, the pretensioning process is complete and the nutrunner 18 is finally shut down. An alternative embodiment of the invention is illustrated in Fig. 5, namely a two-step pretensioning process, wherein the joint is initially pretensioned to a preliminary clamping load level corresponding to an axial load F p in each one of the fasteners. As this preliminary load level is safely maintained in all of the fasteners, the process is continued by pretensioning the fasteners to the target load level F τ corresponding to the desired clamping level in the joint. As illustrated in the diagram in Fig. 5, the two-step pretensioning process is carried out by initially tighten each one of the fasteners 15 a-d to the preliminary load level F p . This is obtained at a point of time t x , wherein the fastener has been rotated to an angular position Φ 1 . After some time, there has been a settlement in the joint 10, which calls for a re-application of the tightening torque at point t 2 . The preliminary load F p in the fastener is regained at Φ 2 . After another settlement in the joint, the tightening torque is again re-applied on the fastener at a point t 3 to again maintain the axial load in the fastener at F p . This is obtained at the angular position Φ 3 . After two more settlements in the joint 10, tightening torque is re-applied at the fastener at t 4 and t 5 , bringing the fastener to the angular positions Φ 4 and Φ 5 . After the joint has been pretensioned and maintained at the preliminary clamping load level, the second pretensioning stage is commenced at point t 6 . This means that each fastener is tightened to the target load F τ which is reached in the position Φ 6 . After still another settlement of the joint 10, a tightening torque is re-applied on each fastener at point t 7 , and when, within a certain final time interval, no further settlements are detected, the process is completed and the power nutrunner 18 is shut off. Then, the illustrated fastener has reached its final angular position Φ 7 . It is to be noted that the invention is not limited to the above described examples but may be further varied within the scope of the claims. Accordingly, the pretensioning process may be carried out in more than two stages. In particular, when applied on larger joints containing a substantial number of fasteners. An uneven settling of such a joint may cause different degrees of relaxation of the fasteners and also require tightening to more intermediate levels to obtain an even final pretension of the joint.

Claims: Claims. 1. Method for pretensioning a settling joint (10) containing two more threaded fasteners (15a-d) to an intended clamping load level by applying simultaneously a torque delivering tightening tool (19 a-d) on each fastener (15a-d) , comprising the steps of: I) applying simultaneously a tightening torque on each one of said fasteners (15a-d) , II) measuring during said torque application the instantaneous axial load (F) in each one of said fasteners (15a-d) , III) interrupting said torque application on each of said fasteners (15a-d) as the axial load on the respective fastener (15a-d) has reached a predetermined axial load level (F τ ) which corresponds to said intended clamping load level, IV) reapplying a tightening torque one or more times on that fastener or those fasteners (15a-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 said predetermined axial load level (F τ ) , and V) terminating the pretensioning of the joint (10) as the axial load (F) has remained substantially unchanged at said predetermined axial load level (F τ ) for a final time interval in all of said fasteners (15a-d) . 2. Method according to claim 1, wherein said predetermined axial load level is an intermediate joint relaxation level provided below the final clamping load level, and comprising the additional steps of: reapplying simultaneously a tightening torque on all of said fasteners (15 a-d) until said predetermined axial load level (F τ ) is reached in all fasteners (15 a-d) , interrupting said torque application on all of said fasteners (15 a-d) , and reapplying after a certain time interval a tightening torque on that fastener or those fasteners on which the axial load (F) has decreased due to settlement of the joint (10) until said predetermined axial load level (F τ ) is reached in all of said fasteners (15 a-d) . 3. Method according to claim 1 or 2 , wherein a tightening torque is reapplied on that fastener or those fasteners only on which the axial load (F) has decreased more than 10% of said predetermined axial load (F τ ) . 4. Method according to claim 1, wherein said predetermined axial load level (F τ ) corresponds said intended final clamping load level. 5. Method according to anyone of claims 1- 4, wherein the axial load (F) in each one of said fasteners (15 a-d) is determined as a function of the variations in the fastener length determined by measuring the variations in travel time of ultra-sonic waves induced in the fastener. 6. Method according to claim 5, wherein said ultra-sonic waves are induced in the fasteners (15 a-d) by a sound wave generating means rigidly affixed to said each fastener and energized by an electric current applied via said tightening tool. 7. Method for pretensioning a joint (10) containing two or more threaded fasteners (15a-d) to an intended clamping load level by applying simultaneously a torque delivering tightening tool (19a-d) on each fastener (15a-d) , comprising the steps of : I) applying simultaneously a tightening torque on each one of said fasteners (15) II) measuring during said torque application the instantaneous axial load (F) in each fastener (15) , III) interrupting said torque application on each of said fasteners (15) as the axial load (F) on the respective fastener (15) has reached a preliminary axial load level (F p ) which corresponds to a clamping load level in the joint (10) that is lower than said intended clamping load level, IV) re-applying individually a tightening torque one or more times on that fastener or those fasteners (15) on which the axial load (F) has decreased due to settlement of the joint (10) during one or more certain time intervals, thereby regaining said preliminary load level (F p ) in the respective fastener or fasteners, V) re-applying simultaneously a tightening torque on all of said fasteners (15) to bring them to a target load level (F τ ) which corresponds to said intended clamping load level in the joint (10) , and VI) re-applying individually a tightening torque one or more times on that fastener or those fasteners (15) on which the axial load (F) has decreased due to settlement of the joint (10) during one or more time intervals, thereby regaining said target load level (F τ ) in the respective fastener or fasteners, and VII) terminating the pretensioning of the joint (10) as the axial load (F) has remained substantially unchanged at said target load level (F τ ) for a certain time interval in all of said fasteners (15) . 8. Method according to claim 7, wherein said simultaneous torque application on all of said fasteners (15) is interrupted and said individual re-application of torque is repeated also at one or more intermediate load levels in said fasteners (15) , between said preliminary load level (F p ) and said target load level (F τ ) , so as to provide for an even settlement and clamping load build-up in the joint (10) . AMENDED CLAIMS [received by the International Bureau on 30 May 2000 (30.05.00.); original claims 1 - 8 replaced by new claims 1 - 4 (3 pages)] 1. Method for pretensioning a settling joint (10) containing two more threaded fasteners (15a-d) to an intended final clamping load level (F τ ) by applying simultaneously a torque delivering tightening tool (19 a-d) on each fastener (15a-d) , comprising the steps of: I) applying simultaneously a tightening torque on all of said fasteners (15a-d) , II) determining during said torque application the instantaneous axial load (F) in each one of said fasteners (15a-d) as a function of variations in fastener length determined by measuring the variation in travel time of ultra-sonic waves induced in each one of said fasteners (15a-d) , III) interrupting said torque application on each one of said fasteners (15a-d) as the instantaneous axial load (F) on the respective fastener (15a-d) has reached said intended final clamping load level (F τ ) , IV) reapplying a tightening torque one or more times on that fastener or those fasteners (15a-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 said intended final clamping load level (F τ ) , and V) terminating the pretensioning of the joint (10) as the axial load (F) has remained substantially unchanged at said intended final clamping load level (F τ ) for a final time interval in all of said fasteners (15a-d) . 2. Method according to claim 1, wherein said ultra-sonic waves are induced in said fasteners (15 a-d) by a sound wave generating means rigidly affixed to said fastener and energized by an electric current applied via said tightening tool. 3. Method for pretensioning a joint (10) containing two or more threaded fasteners (15a-d) to an intended final target load level (F τ ) by applying simultaneously a torque delivering tightening tool (19a-d) on each fastener (15a- d) , comprising the steps of : I) applying simultaneously a tightening torque on each one of said fasteners (15a-d) II) measuring during said torque application the instantaneous axial load (F) in each fastener (15a-d) , as a function of variations in fastener length determined by measuring the variation in travel time of ultra-sonic waves induced in each one of said fasteners (15a-d) , III) interrupting said torque application on each of said fasteners (15) as the axial load (F) on the respective fastener (15) has reached a preliminary axial load level (F p ) which corresponds to a clamping load level in the joint (10) that is lower than said intended final target load level (F τ ) , IN) re-applying individually a tightening torque one or more times on that fastener or those fasteners (15) on which the axial load (F) has decreased due to settlement of the joint (10) during one or more certain time intervals, thereby regaining said preliminary load level (F p ) in the respective fastener or fasteners, N) re-applying simultaneously a tightening torque on all of said fasteners (15) to bring them to said target load level (F τ ) , and VI) re-applying individually a tightening torque one or more times on that fastener or those fasteners (15) on which the axial load (F) has decreased due to further settlement of the joint (10) during one or more time intervals, thereby regaining said target load level (F τ ) in the respective fastener or fasteners, and VII) terminating the pretensioning of the joint (10) as the axial load (F) has remained substantially unchanged at said target load level (F τ ) for a certain time interval in all of said fasteners (15) . 4. Method according to claim 3 , wherein said simultaneous torque application on all of said fasteners (15) is interrupted and said individual re-application of torque is repeated also at one or more intermediate load levels in said fasteners (15) , between said preliminary load level (F p ) and said target load level (F τ ) , so as to provide for an even settlement and clamping load build-up in the joint (10) .

Inventors: LINDBAECK TOBIAS AXEL, LINDBAECK, TOBIAS, AXEL
Applicants: ATLAS COPCO TOOLS AB, LINDBAECK TOBIAS AXEL, LINDBAECK, TOBIAS, AXEL
Application year: 1999
Main classification: B23P19
IPC codes: B23P19, B25B23