GB2035171A - Tightening a threaded fastener - Google Patents

Abstract

In tightening a threaded fastener, a check is made to determine whether a tightened fastener has been turned through the correct angle within limits to achieve the desired tension in the fastener and that the torque used to tighten the fastener to this position is correct within limits. The angle is measured by an angle encoder 18 which emits pulses corresponding to the tool rotation, and the torque is sensed by a torque sensor 11. Angle signals are compared with preset angle values, and are passed to an acceptance logic circuit 36. Torque signals are also passed to this circuit 36, an output therefrom indicating whether the fastened joint is acceptable or is to be rejected. The tool is automatically switched off when a preset angle is reached, by action of a comparator 26, an OR gate 29 and a stop command logic circuit 9. Alternatively, if an upper preset torque limit is reached before the preset angle limit, then the tool is again switched off, and a visual signal of flashing lights indicates this condition. Multiple tools with respective display units may be coupled to a master display unit. <IMAGE>

Description

SPECIFICATION Tightening a threaded fastener This invention relates to a method of tightening a threaded fastener, and to apparatus for performing such a method.

The object in tightening a fastener, e.g. a bolt, is to produce a tension in the bolt. This tension provides a clamping force on the joint in which the bolt is used.

It is important that the tension achieved should be correct, because if the bolt is taken to too high a tension, it may snap, and if it is at too low a tension, the clamping force will be insufficient.

It is accepted that applying torque to a bolt is an indirect way of arriving at bolt tension because of the mechanical advantage of the screw thread. This thread is known to have an uncertainty of torque to tension conversion of about 30%. This uncertainty is due to friction between the flanks of the thread and under the abutment shoulder of the bolt head. The friction is likely to be of greater magnitude during the final stages of the tensioning process.

If a datum torque is selected after initial settlement of the joint, it can be argued that the elastic stretch of the bolt will have a known relationship with torque.

However, at higher levels of load both bolt and clamping surfaces deteriorate causing a major change in the uncertainty of the torque-tension relationship.

A bolt tensioned to datum torque can be given a predictable further increase in tension by applying a specified angle of rotation to it. This method takes advantage of the "micrometer" effect of the thread pitch as a means of bolt tensioning independent of torque, and is frequently known as the "turn of nut" method.

This method offers the advantage of tensioning the bolt by measured and controlled rotational movement as compared with torque, the latter having no restraint when the bolt material has reached its elastic limit. Consistently high tension at or near the elastic limit can therefore be applied to fasteners.

Systems are available for counting the angle a fastener is turned through, and for shutting down the tool after a preset angle - see for example U.S.

Patent Specification 4083 270. However it is possible for a fastener to be turned through the correct angle without correctly clamping the joint.

If a substandard fastener, e.g. a mild steel bolt amongst high-tensile bolts, finds its way into a joint, it will be possible for it to be turned through the correct angle without producing the correct clamping force, and the torque applied to it at yield, for example, would be lower than for the high-tensile bolts tightened within the elastic limit. Similar problems arise if, for example, a tapped hole for a bolt is too short and the bolt bottoms, or if a bolt or nut has insufficient thread.

It is therefore desirable to have information as to the torque as well as the angle turned through, in deciding whether a joint is acceptable.

According to the invention, there is provided a method of tightening a threaded fastener using a pneumatic tool, wherein the tool is automatically stopped when the fastener has been tightened, and signals are produced when the fastener has been tightened to indicate the angle through which the fastener has been turned beyond a datum point, and the torque which has been reached, the signals indicating whether the tightened fastener is accepted and has acceptable torque andangle values, or-is rejected for having an unsuitable value of torque, or angle, or both.

Preferably the tool is switched off when a predetermined angle value is reached, and the fastener is accepted or rejected depending on whether the torque reached lies within or outside preset torque limits.

The invention also provides apparatus for tightening a threaded fastener to produce a clamped joint, comprising a rotary pneumatic tool for turning the fastener, means for sensing the torque applied by said tool to the fastener, counting means for counting the number of degrees of angle through which the tool turns the fastener, and electronic circuitry connecting the tool and the said means and adapted to switch on the counting means when a first preset torque has been reached, to switch off the tool when the tool has turned the fastener through a preset number of degrees, to indicate acceptance of the joint only if the counting means indicates that the angle turned through lies between two present limits and the torque sensing means indicates that the maximum torque reached also lies between two present limits, and to indicate rejection of the joint if these conditions are not satisfied.

When a number of motors are used as one multiple unit the first, preset datum torque level can be used to arrest the tightening of individual spindles until every other motor has reached the datum torque, whereupon all the motors start again to complete the tightening of their respective fasteners.

The motors used will normally have a certain amount of inertia and will continue to rotate through a small angle after having been switched off.

However, this angle information is retained in the memory of the electronic module appertaining to the individual spindles and is included in the total count when the motors are again started to complete the tightening function.

Also, the inertia of the motors will require the final switching off signal to be normally below the lower of the present angle limits.

An arrangement such as that described in our copending British Patent Application 5220/78 may be used to count the revolutions of the motor of the tool, which will preferably be operated by compressed air. Since there will normally be step-down gearing between the motor and the tool output shaft, the counting resolution can be high.

The tool is preferably switched off by the torque sensor if the torque applied reaches the upper preset torque limit without having turned through the preset number of degrees.

The electronic circuitry is preferably connected to a display panel which indicates the state of the apparatus. The display panel includes a digital angle display which starts to run when counting is started and three status lights indicating that the angle turned through once the tool has stopped is either below the lower limit (LO), between the limits (OK) or above the upper limit (HI). If the angle is within limits (OK) but the torque is not within the preset torque limits, then all three lights are out, and if the tool has been switched off at the upper preset torque limit, the digital angle display flashes. An ACCEPT light indicates an accepted spindle; any other light or no lights indicate a rejected spindle.

A number of spindles may be connected up as one unit, with a master display panel carrying displays relating to the status (i.e. accepted or rejected) of each spindle. In addition,the electronic master panel has unit "ACCEPT" or "REJECT" lights, so that it can be seen immediately whether a the fasteners of the unit have been correctly tightened.

It can be possible to stop the tool or tools before the switch-off point is reached, and then to restart them. The action of restarting would normally reset the system and apply a full angle count in addition to any already applied to the bolts. The system is therefore, fitted with a reset inhibit time delay initiated by the first spindle to arrive at datum torque. This facility allows the individual spindles to hold angle data in their memory modules so that later spindle rotation will include a total of all angle movement from datum torque. The duration of the time delay is intended to cover the expected time interval for a normal assembly operation plus the movement of the assembly away from station.

Since the tool to be used in this apparatus is known, the invention extends to apparatus as described above but omitting the torque motor or tool itself.

It will be apparent that the system described can produce a memory record of the data relating to each and every fastening which is tightened. This facilitates quality control, and can also provide a check-back on the assembly of particular joints.

All the systems used in a particular factory can be connected to a master computer, whereby the operations of all tools can be monitored. Maintenance work can thereby be facilitated, because a statistical record can be computed to compare against known performance criteria, and a prediction of maintenance requirements can be signalled before the tools start to produce inconsistent results.

Digital circuitry makes the system relatively simple and well suited to a factory environment. Also, the memory data is in a form directly acceptable to computer or printer systems.

The tension in a fastener tightened in accordance with the invention is accurately known after tightening, and this can therefore be taken into the yield region for that particular bolt. This produces a very high clamping force from a particular bolt.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram showing apparatus according to the invention; Figure2 is a graph showing the relationship between torque and bolt rotation in degrees, when a bolt is tightened to below the permanent set limit; and Figure 3 is a graph showing the relationship between torque and bolt rotation in degrees, when a bolt is tightened to above the permanent set limit.

The apparatus shown diagrammatically in Figure 1 can be used in conjunction with any suitable torqueapplying tool. The tool shown is a pneumatic tool which includes an arrangement for counting the revolutions of the motor and atorquesensor 11 for emitting signals at three predetermined torque levels.

Referring now to the block diagram of Figure 1, the tool has a torque motor 1 driven by compressed air from a supply 2 via a pressure regulator 3. The air supply to the motor 1 is controlled by a solenoid valve 4.

To start the apparatus, a "START CYCLE" control 5 on the master board is operated. This sends a signal via the line 6 to a START/RESET logic circuit 7 which in turn passes a signal along the line 8to a STOP/START COMMAND logic circuit 9. A signal passes from circuit 9 along line 10 to the solenoid valve 4which opens to admit airto the motor 1 to start the motor rotating.

Assuming that the end of the tool is in engagement with a bolt in a threaded hole, the bolt will be turned until a preset datum torque is reached. On reaching this datum torque, the torque sensor 11 passes a signal along the line 12 to a COUNT ENABLE logic circuit 13 which in turn passes a signal along line 14to a Counter/Memory 15.

The COUNT ENABLE logic circuit 13 also passes a signal via the line 16 to FET switch 17. This switch selects, on command from a second one 58 of two test and check modules, either signals from a check count pulse source 45, or from angle encoder 18 via line 19, and passes the signals onto the counter/ memory 15 and logic circuitry via a PULSE DETEC TION logic circuit 20 and a programmable divider 21 and lines 22,23 and 24. The pulses are passed along the data transmission line 25 to a digital comparator 26.A preset control angle is set before operation begins in a control angle preset 27, and the number of input pulses is constantly compared with this control angle and when the number of input pulses corresponds to the control angle, the comparator 26 sends a STOP command signal which passes via a line 28 and an OR gate 29 and line 30 to the STOP/START COMMAND logic circuit 9 to close the solenoid valve 4 to stop the air feed to the motor 1.

However, when the air feed has been stopped, the motor will continue to rotate through a small angle due to the inertia of its rotating parts. Further pulses will therefore be passed along the line 25 to comparators 31 and 32. Comparator 31 is associated with a low angle assessment, and has a low angle preset 32 associated with it. Comparator 32 carries out a similar function for a high angle and has a high angle preset 33. Assuming that the final value of the angle which the tool turns the bolt through after the datum torque has been reached lies between the preset lower and upper angles, a signal will be passed from comparator 31 along line 34, but no signal will be passed from comparator 32 along line 35. Both lines 34 and 35 are connected to a TORQUE and ANGLE ACCEPTANCE logic circuit 36.

Other feeds to this circuit 36 come from the two further torque sensing points of the torque sensor 11. This has lower and upper preset torque limits.

When the lower limit is reached, a signal is passed along line 37 to the logic circuit 36, and if the upper limit is reached then a further signal is passed along the line 38 to the logic circuit 36. If the joint has been tightened correctly, the torque will lie between the lower and upper preset limits, and so there will only be a signal passed along the line 37.

The logic circuit 36 has three status lights 36a, 36b, 36c associated with it. In the initial stages of tightening the bolt, the LO light 36a will be on until a signal is passed along the line 34 from comparator 31 indicating that the lower preset angle limit has been passed. The LO light then goes off. If the rotation of the bolt is such that the higher preset angle limit is passed, then a signal passing along line 35 switches on the HI 36c light. When the signals from comparators 31 and 32 are such that the angle the bolt has turned through is between the present limits, then the third, OK, light 36b will go on, provided that a signal is received from line 37 that the lower preset torque has been passed.If a further signal is received along line 38, indicating that the upper preset torque has been exceeded, then the OK light 36b goes off, and a signal is passed along line 38 to a BLINK ENABLE logic circuit 40 which sends a blink signal along line 41 to a visual display 42.

In addition to the indication of the angle achieved via the three status lights activated by comparators 31,32, the panel 42 gives a digital read out of the angle. There will normally also be digital thumb switches indicating the preset lower angle limit and of the preset upper angle limit so that the actual angle (on display 42) can be visually compared directly with these. When the tool is operating and the upper preset torque limit is exceeded, the blink signal causes the illuminated numerals of the display panel 42 to flash on and offto indicate that the joint is unacceptable because the torque has exceeded the preset upper limit. Assuming that the angle achieved is between the preset angle limits, but that the minimum torque has not been reached, then none of the three status lights will be on.If the angle has not reached the preset lower limit, the LO light 36a will be on whereas if the angle exceeds the upper preset angle limit, then the HI light 36e will be on. It will therefore be possible immediately to determine the nature of fault occurring if the joint is not acceptable, and the OK light is not on.

There are some further subsidiary features.

When using a bank of pneumatic tools, to tighten a number of fastenings on an article, it may be desirable for all of the tools to reach the present datum torque before any of them start to further tighten their fasteners. In this case, the signal produced at the datum torque passing along the line 12 also passes to a SETTLEMENT DELAY logic circuit 43 which will be connected to the controls of the other tools and will pass a signal along line 44 to the STOP/START COMMAND circuit 9 to close the valve 4 until all the tools have reached the same operational stage. This will again be sensed by the logic circuit 43, whereupon a further signal will pass along line 44 to enable the logic circuit 9 to open valve 4 and allow the tightening function to proceed.

Check count pulse 45 can be provided to check the function of the digital system using mains frequency.

The pulse indicating the angle turned through by the fastener can be processed by appropriate BCD circuitry 46, for example to allow the keeping of a permanent record of the operation of each tool. A computer can monitor the performance of each tool and give an indication when a particular tool starts to produce inconsistent results.

When a number of tools are connected together into one unit, a master board 47 is provided to monitor the performance of all the tools. The circuitry associated with each tool is provided with a NOR gate 48 which receives signals from the upper torque limit sensor, via lines 38 and 39, and from the signal activating the OK status light 36b, and passes a resulting signal along a line 49 to a logic circuit 50 with master inspection lights which reflect the status of all the fasteners of a unit. If just one fastener is not accepted, then the master inspection light will show REJECT, and it will be necessary to scan the displays associated with each tool to discover which tool is at fault.

When the COUNT ENABLE logic circuit 13 is actuated, a signal is also passed via the lines 14 and 51 to an OR gate 52 and then to a DELAY logic circuit 53. This passes a reset inhibit signal along a line 54 to the logic circuit 7 to prevent any tool from being recycled after it has completed its preset control angle.

A blink oscillator 55 passes a blink signal along a line 56 to the BLINK ENABLE logic circuit 40 of each tool.

A front panel 57 provides two test facilities 58 and 59 for testing the operation of the encoder 18 and of the counting system. There is also a locai reset control 60. The operation of this reset control 60 will not be subject to the delay produced by the logic circuit 53.

The torque sensor 11 may be of a conventional caliper type where the torque exerted by the motor tends to close two calipers (one of which is stationary and the other of which is fixed to the body of the motor) against the force of a spring. The three preset torque limits can be provided by signal logic switches which are operated at different distances apart of the two calipers. These logic switches can be operated by abutments of varying length on one of the caliper arms. A caliper operated control of this type is shown in our British Patent Specification No.

1,095,495, although in this Patent Specification, closing of the caliper operates an air valve rather than a logic switch.

Figures 2 and 3 indicate schematically the condition of a bolt during tightening as described. In these Figures, torque is plotted against bolt rotation in degrees, and this bolt rotatation relates to the extension of the bolt shaft, due to the micrometer effect of the screw threads.

The first step is to decide how far the particular joint in question is to be tightened. This can be calculated empirically from a consideration of the optimum tension in the bolt, which produces the clamping force, and then by considering through what angle the bolt has to be turned, in view of the pitch of the thread, to achieve the extension corresponding to that tension. The graphical illustration shows that the tension and extension increase linearly. The final tension will normally be close to the top of the linear portion of the graph relating the tension and extension, or slightly above this linear portion beyond the permanent set limit. A control angle must then be set to switch off the tool so that when the tool stops running, the bolt has been turned through the desired angle.

The first stage in running down the bolt is to get to the beginning of the linear relationship between tension and extension (or bolt rotation). Once the beginning of this relationship has been reached, torque starts to rise as the bolt is turned, and a datum torque is preset. The angle corresponding to this datum torque is taken as datum for the counting circuitry (0 ), and a counter is set in operation by a signal from the torque sensor 11. The control angle is preset in the device 27, and when this control angle has been reached, the air feed to the motor is shut off. Because of the inertia of the rotating parts of the tool, the bolt will continue to be rotated through a small further angle, and the counter will continue to indicate this further rotation.It will normally be necessary to set the control angle, which shuts off the air feed, below the lower preset angle limit 61. The overshoot will then carry the bolt into the correct angular range between the lower limit 61 and the upper limit 62. At the same time as the apparatus is indicating whether the desired angle has been reached, the torque sensing mechanism 11 is indicating whether the torque reached lies between two preset torque limits 63 and 64. Only if both angle and torque are between the respective preset limits will the joint be accepted. This is shown graphically in both Figures 2 and Figure 3 by a region 65 which is the part of the curve which corresponds to the torque/rotation position for an acceptable joint. A LO light will appear if the torque/rotation relationship lies in the region 66 and a HI light will come on if the relationship lies in the region 67.In the region 68 there will be no lights, and if the upper torque limit is exceeded, there will again be no status lights showing but the digits on the display panel 42 will be flashing. In addition, the valve 4 is closed immediately the upper torque limit is exceeded.

In certain circumstances, it may be desirable to control the apparatus in a different way, by providing a shut off signal at a predetermined torque level below the desired torque level, so that the overshoot carries the bolt to the desired level and then observing the angle turned through to check whether this lies between desired limits before indicating whether or not the joint will be acceptable.

The visual angle display 42 has a decimal point avaiiable. This point can be in either one of two possible positions. One of these positions is used to indicate when the datum torque has been achieved, and the other of the positions is used to indicate that the lower of the preset torque limits has been passed.

When setting the present torque limits, the logic circuitry change on passing the upper of the present limits is signalled by blanking the visual display 42.

Claims (7)

1. A method of tightening a threaded fastener using a pneumatic tool, wherein the tool is automatically stopped when the fastener has been tightened, and signals are produced when the fastener has been tightened to indicate the angle through which the fastener has been turned beyond a datum point, and the torque which has been reached, the signals indicating whether the tightened fastener is accepted and has acceptable torque and angle values, or is rejected for having an unsuitable value of torque, or angle, or both.

2. A method as claimed in claim 1, wherein the tool is switched off when a predetermined angle value is reached, and the fastener is accepted or rejected depending on whether the torque reached lies within or outside preset torque limits.

3. A method as claimed in claim 1, wherein the tool is switched off when a predetermined torque has been reached, and the fastener is accepted or rejected depending on whetherthe angle turned through lies within preset limits.

4. A method as claimed in any preceding claim, for tightening a plurality of threaded fasteners with respective pneumatic tools, wherein each tool stops when it reaches a preset datum torque, and once all the tools have reached said datum torque, they are all restarted to tighten their respective fasteners.

5. A method as claimed in any preceding claim, wherein the air feed to the tool or tools is shut off at a certain point before the desired angle and/ortorque has been reached, so that the inertia of the tool will take the fastening to the correct angle and/or torque.

6. Apparatus for tightening a threaded fastener to produce a clamped joint, the apparatus comprising a rotary pneumatic tool for turning the fastener, means for sensing the torque applied by said tool to the fastener, counting means for counting the number of degrees of angle through which the tool turns the fastener, and electronic circuitry connecting the tool and the said means and adapted to switch on the counting means when a first preset torque has been reached, to switch off the tool when the tool has turned the fastener through a preset number of degrees, to indicate acceptance of the joint only if the counting means indicate that the angle turned through lies between two preset limits and the torque sensing means indicate the maximum torque reached also lies between two present limits, and to indicate rejection of the joint if these conditions are not satisfied.

7. Apparatus as claimed in any one of claims 4 to 6, for tightening a plurality of threaded fasteners with a respective pneumatic tool for each fastener, wherein all the tools are connected into a single unit, and the electronic circuitry controlling the tools is also connected into one unit with a master display panel carrying displays relating to each tool and a further display relating to the operation of all the tools of the unit.

7. Apparatus as claimed in claim 6, wherein means are provided to switch off the tool if the torque sensor senses an upper torque limit before the tool has turned the fastener through said preset number of degrees.

8. Apparatus as claimed in claim 6 or claim 7, wherein the electronic circuitry is connected to a display panel which includes a digital angle display which starts to run when counting is started, and three status lights indicating that the angle turned through once the tool has stopped is either below the lower limit, between the limits or above the upper limit.

9. Apparatus as claimed in any of claims 6 to 8, wherein the circuitry includes a delay system for preventing restarting of the tools immediately they have stopped.

10. Apparatusfortightening a plurality of threaded fasteners with a respective pneumatic tool for each fastener, wherein all the tools are connected into a single unit, and the electronic circuit controlling the tools is also connected into one unit with a master display panel carrying displays relating to each tool and a further display relating to the operation of all the tools of the unit.

11. A method of tightening a threaded fastener substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus for tightening a threaded fastener substantially as herein described with reference to Figure 1 of the accompanying drawings.

New claims or amendments to claims filed on 18 July 1979 Superseded claims 1 to 10 New or amended claims:

1. A method of tightening a threaded fastener using a pneumatic tool, wherein a datum torque value, a preset angle value and acceptable minimum and maximum torque and angular rotation limits for the fastener are set, the fastener is rotated by the tool until the datum torque is sensed, whereupon a digital count of angular rotation of the fastener is started, and rotation of the fastener continues until the tool has turned the fastener through the preset angle, whereupon the tool is shut down and digital circuitry indicates that the tightened fastener is accepted and has acceptable torque and angle values, or is rejected for having a torque or angle value, or both, outside the limits set, the torque and angle status reached once the tool has come to a stop being visually displayed for inspection by an operator.

2. A method as claimed in claim 1, for tightening a plurality of threaded fasteners with respective pneumatic tools, wherein each tool stops when it reaches a preset datum torque, and once all the tools have reached said datum torque, they are all restarted to tighten their respective fasteners.

3. A method as claimed in claim 1 or claim 2, wherein the air feed to the tool or tools is shut off at a certain point before the desired angle and/or torque has been reached, so that the inertia of the tool will take the fastening to the correct angle and/or torque.

4. Apparatus for tightening a threaded fastener to produce a clamped joint, the apparatus comprising a rotary pneumatic tool for turning the fastener, means for sensing the torque applied by said tool to the fastener, digital counting means for counting the number of degrees of angle through which the tool turns the fastener, digital electronic circuitry connecting the tool and the counting means and adapted to switch on the counting means when a first preset torque has been reached, to switch off the tool when the tool has turned the fastener through a preset number of degrees, to indicate acceptance of the joint only if the counting means indicate that the angle turned through lies between two preset limits and the torque sensing means indicates the maximum torque reached also lies between two preset limits, and to indicate rejection of the joint if this condition is not safisfied, the electronic circuitry being connected to display panel which includes a digital angle display which starts to run when counting is started, and three status lights indicating that the angle turned through once the tool has stopped is either below the lower limit, between the limits or above the upper limit.

5. Apparatus as claimed in claim 4, wherein means are provided to switch off the tool and to indicate rejection if the torque sensor senses an upper torque limit before the tool has turned the fastener through said preset number of degrees.

6. Apparatus as claimed in claim 4 or claim 5, wherein the circuitry includes a delay system for preventing restarting of the tools immediately they have stopped.