GB2261750A

GB2261750A - Method of monitoring nut runners

Info

Publication number: GB2261750A
Application number: GB9301561A
Authority: GB
Inventors: Akira Takeshima; Akihiko Takahashi; Shigeo Kobayashi; Yasuto Watanabe; Toshikazu Asakura; Hisaya Watanabe
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd
Priority date: 1988-09-21
Filing date: 1993-01-27
Publication date: 1993-05-26
Anticipated expiration: 2013-01-27
Also published as: GB2225130B; GB9301561D0; GB2261750B; CA1332970C; GB8921261D0; GB2225130A

Description

3 METHOD OF MONITORING NUT RUNNERS The present invention relates to a

method of monitoring nut runners, and more particularly to a method of monitoring nut runners such that drive conditions for the nut runners are set up and modified by a master controller, the nut runners are controlled through subcontrollers, res-pectively, and operating conditions of the nut runners are monitored by the master controller through the subCOntrollers.

The assembly process for various industrial products such as automobiles is more and more automatized to meet increased efficiency and labor saving requirements. For example, some threaded parts such as bolts, nuts, or the like are automatically fastened by a s.ystem of nut runners.

Japanese Laid-6pen Patent Publication No. 58(1983)66672 discloses a system comprising a plurality of nut runners for tightening threaded parts. in order to prevent the fastening torques of the nut runners from varying from each other, torque preset means for the respective nut runners preset preliminary torques smaller than desired tightening torcrues, and when the tightening torques have reached the preliminary torques, the nut runners are temporarily stopped, and then the nut runners are operated again to tighten the threaded parts with the desired tightening torques after elapse of a certain period of time.

If the preset preliminary torques are to be changed, however, they have to be modified for. the respective nut runners. It has been quite tedious and timeconsuming to preset and modify the drive conditions for the respective nut runners.

In the nut runner control system, it is desirable to monitor the conditions of how the threaded parts are tightened by the nut runners and how the nut runners are operating, so that any trouble will be prevented from occurring and repairs will quickly be carried out should a problem happen.

For example, Japanese Laid-open Patent Publication No. 55(1980)-18349 shows a system in which the period of time f rom the start of a tightening process of a nut runner to the time at which the tightening torque has reached a prescribed torque is measured to determine whether the threaded part has been successfully tightened or not. Japanese Laid- open Patent Publication No. 59(1984)-118380 discloses a.system in which the rotational angle and tightening torque of a nut runner are detected and compared with preset angle and torque data to determine whether the tightening process is successful or not.

The process of determining whether a threaded part has been tightened as desired in these disclosed systems works well only when the nut runnerand its associated devices operate normally, but cannot determine any tightening failures resulting from malfunctions of the nut runner itself and its associated devices. Therefore, when such a tightening failure hapnens, the cause of the tightening failure cannot be found. As a result, a number of nut runtion line ners cannot efficiently be operated in a product based an the conventional control system.

The present invention provides a method of monitoring conditions of a plurality of nut runners and a workpiece based on torques with which a plurality of threaded parts are tightened on the workpiece by the nut runners, said method comprising the steps of: providing a plurality of nut runners connected, respectively, to subcontrollers for controlling said nut runners; setting predetermined drive conditions for said nut runners in said subcontrollers; measuring the torques with which the threaded parts are tightened; recording a plurality of torques with which the threaded parts are.tightened over a plurality of tightening cycles; and comparing time-dependent changes in said torques with preset values thereby monitoring operating conditions of the nut runners and the workpiece.

1 - 4 0 Preferably the operating conditions include conditions of a drive mechanism of the nut runners and a tightened condition of the threaded parts.

A preferred embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:FIG. 1 is a block diagram of a nut runner control system for use in the method of the present invention; FIG. 2 is a detailed block diagram of the nut runner control system shown in FIG. 1; 1 1 1 - 5 FIG. 3 is a flowchart of a process for initializing the nut runner control system shown in FIG. 1; FIGS. 4(a) through 4(c) are diagrams showing, by way of example, various preset drive conditions, displayed on a control panel shown in FIG. 1, for the nut runner control system; FIG. 5(a) through 5(c) are diagrams showing operating condition data displayed separately on the control panel; FIG. 6 is a diagram showing operating condition data displayed as a whole an the control panel; FIGS. 7(a) and 7(b) are diagrams showing operating conditions and results of nut runners, which are disDIaved on or output from the control panel; FIGS. 8(a) through 8(d) are diagrams of monitored data indicating various abnormal conditions, which are displayed on the control panel; FIG. 9 is a diagram showing torque values for all shafts, displayed an the control panel; FIG. 10 is a diagram showing a torcrue value -L:"Or one shaft. displayed an the control panel; and FIGS. 11(a) through 11(c) are diagrams of torcrue data indicaing various abnormal conditions in the displayed torque value of FIG. 10.

FIG. 1 schematically shows a nut runner control system 10, The nut run- 6 - ner control system 10 basically comprises a sequencer 12, master controller 14, a control panel 16, a plurality of subcontrollers 18, and a plurality of nut runners 20. The subcontrollers 18 are connected in parallel to the master controller 14.

The sequencer 12 sends operation command signals for the nut runners 20, such as start signals, reset signals, tighterring signals, loosening signals, and interlock signals, for example, to the master controller 14, and also sends signals to control the operation of the nut runners 20, such as nonrotation signals, rotation signals, -seating signals, for example, to the subcontrollers 18. The subcontrollers 18 for controlling the nut runners 20 are connected in parallel to the master controller 14. The control panel 16 is connected to the master controller 14 and used to preset and modify a tightening procedure program and tightening torque values, and monitor tightening OKING signals and tightening data.

As shown in FIG. 2, the master controller 14 comprises a CPU 14a, an SIO 14b for delivering serial data between the control panel 16 and the master controller 14, an SIO 14c for delivering serial data between each of the subcontrollers 18 and the master controller 14, a PIO 14d for delivering parallel data between the sequencer 12 and the master controller 14, and a memory 14e for storing a control program that is preset by the control panel 16 and - 7 controls operation of the nut runners 20 through the respective subcontrollers 18, and data on interlock conditions of the nut runners 20. The master controller 14 supplies the sequencer 12 with ready signals, alarm signals, OK signals, NG signals, and interlock signals for the respective nut runners 20 through the P10 14d.

Each of the subcontrollers 18 includes a controller 18a and a nut runngr driver 18b which are electrically connected to one of the nut runners 20. The master controller 14 is supplied with an OK/NG signal indicative of whether a threaded part has successfully been tightened or not by the subcontroller 20, and an interlock signal, from each of the subcontrollers 18. To the subcontroller 18, there are connected a motor power supply 22 for driving the nut runner 20, and a solenoid power supply 24 for changing the rotational speeds of the nut runner 20.

Each nut runner 20 comprises a DC brushless servomotor 20a serving as a drive source, a solenoid unit 20b, a torque detector 20c, and an output shaft 20d. The DC brushless servomotor 20a is associated with a pulse generator PG for detecting the rotational speed of the servomotor 20a. A pulsed signal from the pulse generator PG is applied to the nut runner driver 18b in the subcontroller 18. The solenoid unit 20b has a solenoid 26 which can change the rotational speed of the output shaft 20d to one of a plurality of speed levels by controlling a transmission (not shown). The torque detector 20c has a strain gage 28a for detecting any twist in the output shaft 20d, and supplies the detected strain value to the controller 18a through a rotary transformer 28b.

The controller 18a of the subcontroller 18 comprises a CPU 30 for controlling the nut runner 20, a memory 32 for storing data to be monitored such as a tightening torque for the nut.runner 20, the rotational speed of the servomotor 20a, and the current of the servomotor 20a, an SIO 34 for delivering data between the controller 18a and the SIO 14c in the master controller 14, and a PIO 36 for delivering data between the controller 18a and the sequencer 12. The CPU 30 applies a drive signal to the nut runner driver 18b through a D/A converter 38. The CPU 30 also receives monitor data which have been selected by a multiplexer 40 and converted to a digital signal by an A/D converter 42, and stores the digital signal in the memory 32. The multiplexer 40 is supplied with the monitor data, i.e., the motor speed and the motor current from the nut runner driver 18b, and also with the tightening torque from the torque detector 20c through a detecting circuit. The detecting circuit comprises an oscillator 44 for supplying a carrier voltage to the torque detector 20c through an amplifier al and a synchronous detector 46 for receiving a strain signal through an amplifier a2 to extract a torque signal. The controller 18a also has a switch 48 such as an SSR - 9 (Solid-Stage Relay) for switching over the solenoid unit 20b. The switch 48 is connected to the solenoid power supply 24 and the PIO 36 which outputs a control signal.

The nut runner driver 18b has a servaamplifier 50 which controls the DC brushless servomotor 20a according to a drive signal from the controller 18a. An output signal from the servoamolif-ier 50 is detected by a current sensor 54, and supplied as motor current data to the multiplexer 40 in the controller 18a. The nut runner driver 18b also has an F/V converiter 56 which supplies speed data of the DC brushless servomotor 20a detected by the Dulse generator PG as a voltace sianal to the multiDlexer 40.

Operation and advantages of the nut runner coat-rol system will be described below.

A process for the nuC runner control syste.m will-1 be described, below wit-h- reference.;_--o FIG. 3.

A desired system arrangemer-LL- for the nut runner control syst-em 10 is f ir-st, set up us ing the control panel 16 in a ster) 1. For examD!e, the number of shafts or nut runners 20 to be operated is set up. Then, drive conditions for each of the nut runners 20 to be operated are preset in a s-"s-eD 2. For example, an upper limit torque for the tightening torque of the nut runner 20, an upper. limit time in which to continuously rotate the DC brushless servomotor 20a - 10 of the nut runner 20, a limit value for an overcurrent of the servomotor 20a, an acceleration time in which the servomotor 20a should reach a predetermined motor speed, and other drive conditions are preset (see FIG. 4(a)).

Then, data necessary for tightening a threaded part with each nut runner 20 to be operated are entered in a step 3. For example, there are entered a final tightening torque, a lower linit torque for the final tightening torque, a snug torque which is a torque applied at the end of a mediumspeed operation of the nut runner, a limit time in which to continuously rotate the DC brushless servomotor 20a, maximum and minimum times required to tighten a threaded part, a reverse brake rotational speed upon final tightening, and a reverse brake start timing upon final tightening (see FIG. 4(b)).

After the various data have been entered, as described above, a procedure for actually operating each nut runner 20 is set up in a step 4. For example, as shown in FIG. 4(c), an idling mode before a tightening process, including the threading of a bolt into a workpiece hole, picking up of a nut, etc., and a rotational speed and time for the idling process are preset. Thereafter, an interlock mode for synchronizing the nut runner 20 with the other nut runner 20 and other robots such as assembling robots (not shown) is set up. A slow start mode for rotating the nut runner 20 at a low speed so that a nut or the like can 0 smoothly be threaded over a workpiece bolt at the start of a tightening process is set up. Then, a high-speed mode in which the solenoid unit 20b in the nut runner 20 is to be energized to shift the transmission into a high-speed position to rotate output shaft 20d at a high speed, and a rotational speed for the high-speed mode are set up. A medium-speed mode in which the solenoid unit 20b is deenergized to shift.the transmission into a medium speed position to rotate the output shaft 20d at a medium speed until a snug torque is reached, and a rotational speed for the medium-speed mode are set up. Then, in order to tighten the nuts simultaneously with the nut runners 20, an interlock mode for synchronizing the nut runners 20 is set up. A tightening rotational speed, a mode for applying a reverse brake when increasing the torque, and a time for carrying out such a reverse brake mode are set up. Finally, there is set up a mode in which the tightening data are determined based on the preset values, and displayed on the screen of the control panel 16 and sent to the sequencer 12. That is, a process is set up to determine whether a final tightening torque is to be determined or not, a tightening torque is to be determined or not, and the nut is seated or not. Then the end of the program is entered, thus completing the setup procedure.

The data and program which have been set up for each nut runner 20 through the control panel 16 are deliv- - 12 ered through the SIO 14b and stored in the memory 14e in the master controller 14, and also delivered through the SIOs 14c, 34 and stored in the memories 32 in the subcontrollers 18 corresponding to the respective nut runners 20. Thereafter, a start signal is applied by the sequencer 12 to the master controller 14 and also to each of the subcontrollers 18, which then start executing the program.

In response to the start signal, each subcontroller 18 first executes the idling mode for picking up a nut. More specifically, the CPU 30 in the subcontroller 18 outputs an idling control signal through the PIO 36 to the switch 48 based on the data and program stored in the memory 32. The switch 48 applies a switching signal to the solenoid 26 to enable it to shift the transmission. Then, the CPU 30 applies a drive signal to the servoamplifier 50 in the nut runner driver 18b through the D/A converter 38, and the servoamplifier 50 supplies a drive current to the DC brushless servomotor 20a based on the drive signal. As a result, the output shaft 20d rotates at a prescribed speed for a prescribed period of time to pick up a nut.

After the nut has been picked up, each subcontroller 18 supplies an interlock signal to the master controller 14 to put the nut runner 20 in a standby condition. The interlock signal is stored in the memory 14e in the master controller 14. when the master controller 14 has confirmed that all the subcontrollers 18 are interlocked based on the received interlock signals, the master controller 14 applies an interlock signal to the sequencer 12.

In response to the interlock signal, the sequencer 12 outputs an answer signal to the master controller 14 in the event that each nut runner 20 can start a next mode of operation. When the answer signal is received by the master controller 14, the master controller 14 applies an interlock disable command to-.qll the subcontrollers 18 which have been interlocked.

In response to the interlock disable command from the master controller 14, the subcontrollers 18 then execute a sl-ow start command. More specifically, the CPU 30 in each subcontroller 18 applies a control signal through the PIO 36 to the switch 48 to enable the solenoid 26 in the nut runner 20 to shift the transmission into a slow start position. Then, the CPU 30 applies a drive signal through the D/A converter 38 to the servoamplifier So in the nut runner driver 18b. The servoamplifier 50 then energizes the DC brushless servomotor 20a based on the drive signal applied thereto. As a result, the output shaft 20d rotates at a predetermined low speed for a prescribed period of time, thus starting the slow start mode. The nut is then threaded over the workpiece bolt.

The"output signal from the servoamplifier 50 is detected by the current sensor 54, and is fed as a motor current, i.e., data to be displayed, through the multiplexer - 14 40, the A/D converter 42, and the CPU 30 and stcred in the memory 32 in the controller 18a. The rotational speed of the DC brushless servomotor 20a is detected by the pulse generator PG, converted to a corresponding frequency by the F/V converter 56, and then stored as motor speed data in the memory 32.

These data to be monitored are then transferred through the SI0s -34, 14c to the master controller 14, and can then be displayed as time-dependent changes on a monitor such as a CRT or the like of the control panel 16. Therefore, the operator can know the operating conditions of the nut runner 20 based on the displayed data. FIGS. 5(a) and 5(b) show by way of example time-dependent changes in the motor current and speed which are displayed on the moni tor of the control panel 16.

After the slow start mode of the nut runner 20 is finished, the CPU 30 effects a high-speed nut tightening operation in the nut runner 20. The switch 48 energizes the solenoid 26 to shift the transmission to L highspeed position. The output shaft 20d is then rotated at a high speed based on a drive signal from the CPU 30. The current sensor 54 detects the motor current and transfers its data to the controller 18a. The pulse generator PG also detects the motor speed and transfers its data to the controller 18. The operator can therefore also confirm the operating conditions of the nut runner 20 on the control panel 16 based on the delivered data.

- is - When the DC brushless servomotor 20a is rotated at a high speed, the nut is tightened with respect to the workpiece, and hence a tightening torque is produced on the output shaft 20d. This tightening torque is detected by the strain gage 28a of the torque detector 20c and sent to the detecting circuit in the subcontroller 18. More specifically, the detecting circuit supplies the carrier voltage from the oscillator 44 through the amplifier al to the rotary transformer 28b, and applies a certain voltage to the strain gage 28a. The strain gage 28a detects the twist in the output shaft 20d which is caused by the tightening torque, and supplies the detected twist as a strain signal 1-o the anlDli'L-ie-a2 through the rotary transformer 28b. The strain signal is then applied to the synchronous detector 46 which extracts a torque voltage. The torque voltage is introduced as data to be monitored through the multiplexer 40 and A/D converter 42 into the memory 32 in the subcontroller 18, and applied to the master controller 14 and the control panel 16 for monitoring. FIG. 5(a) shows by way of example time-dependent changes in the torque voltage displayed on the monitor of the control panel 16. The moni tor data shown in FIGS. 5(a) through 5(c) may be displayed as a whole on a multimonitor as shown in FIG. 6 so that the relationship between the displayed data can be recognized more clearly.

The torque voltage is compared with a clutch operating torque (see FIG. 4(a)) which is a preset system - 16 parameter, by the CPU 30. When the tightening torque represented by the torque voltage has reached the clutch operating torque, the CPU 30 issues a control signal to the switch 48 through the PIO 36. The solenoid 26 is energized to shift the transmission, and the DC brushless motor 20b is slowed down by the nut runner driver 18b. As a result, the output shaft 20d of the nut runner 20 enters the mediumspeed mode of operation. The medium-speed mode of operation is continued until the tightening torque detected by the torque detect'-or 20c reaches a preset snug torque.

when the tightening torque has reached the preset sd-ug torque, the CPU 30 applies a shutdown command to the nut runner 20, and also applies an interlock signal to the master controller 14. The masiter controller 14 receives interlock signals from the respective subcontrollers 18, and stores them in the memory 14e. when the interlock signals have been received from all the subcontrollers 18, the master controller 14 applies an interlock mode disable command to the subcontrollers 18, thus restarting operation of the nut runners 20. The nut runners 20 then simultaneously starts finally tightening the respective nuts on the workpiece.

Based on the interlock mode disabling command applied, the CPU 30 in each of the subcontrollers 18 enables the nut runner driver 18b to drive the nut runner 20, the output shaft 20d of which finally tightens the nut on the workpiece. At this time, the CPU 30 determines whether the nut is successfully tightened or not based on the tightening torque detected by the torque detector 20c. More specifically, the CPU 30 determines whether the tightening torque is of a value between the preset upper and lower limits for final torque (see FIG. 4(b)), whether the final tightening time is of a value between the maximum and minimum tightening times (see FIG. 4(b)), and whether there is an input signal from a limit switch that detects when the nut is seated on the workpiece.

while the nut runners 20 are in operation, the control panel 16 displays OK/NG monitor data on the control panel 16. The displayed data are updated on a real-time basis each time a tightening completion signal or an error alarm signal is issued from the subcontrollers 18. Therefore, the operator can immediately confirm operating conditions of the nut runners 20 and which nut runner 20 is malfunctioning by looking at the OK/NG monitor data. The operator can be provided with a printed list of operating conditions of the nut runners 20, check results, and errors based on torque data, speed data, and current data of the nut runners-20 which are transferred to the control panel 16, as shown in FIG. 7(b).

If a check result indicates NG, then the operator may locate the position where the malfunction has occurred by comparing the time-dependent changes (see FIGS. 5(a) - is - through 5(c) and 6) in the torque data, the motor current data, and the motor speed data displayed on the control panel 16 with normal time- dependeng changes in these data.

Each of FIGS. 8(a) through 8(d) shows a pattern of varying speeds of the DC brushless servomotor 20a, a pattern of varying currents flowing through the DC brushless servom otor 20a, and a pattern of varying torques of the output shaft 20d, when arf-alarm signal or NG signal indicative of an abrormal condition of the nut runner 20 or the tightening process.

FIG. 8(a) indicates that whereas the rotational speed of the motor 20a does not increase, the motor current flows abruptly. The patterns of FIG. 8(a) suggest that the mechanisms (such as gears, the transmission clutch, e.'--c) of the nut runner 20 may be having a problem.

FIG. 8 (b) shows that the current remains unchanged but the rotational speed of the motor 20a increases, indi cating that a thread of the nut may have been broken, the nut may have been dislodged from the output shaft 20d, or a nut tightening failure may have occurred.

FIG. 8(c) shows that the current and rotational speed change, but no torque variation is seen unlike a nor mal pattern with which the nut is normally tightened. These patterns suggest that the torque detector may have failed.

FIG. 8(d) shows that the torque and current abruptly change, indicating a tightening failure. If the 19 - rotational speed of the motor has reached a predetermined 0 speed, then the nut may have been jammed or some foreign matter may have entered. If the rotational speed has not reached the predetermined speed, then the rotational speed or the tightening speed may have been preset in error.

The tightening torque values stored in the memory 14e in the master controller 14 may be displayed as L torque value chart 60 for-aill shafts as shown in FIG. 9. The torque value chart 60 displays a combination of a torque value charts 64 each for a single shaft, each chart 64 show- ing a tightening torque value 62 of the nut runner 20 for ea-ch nut. The torque value chart 64 for a single shaft can be displayed singly at an enlarged scale as shown in FIG. 10. The torque value chart 64 shows time- dependent changes in the tightening torque 62 plotted with respect to a final torque 66 which has been preset in the step 3 (FIG. 3) through the control panel 16. The torque value chart 64 also includes upper and lower limits 70, 72 for the final torque.

Using the torque value chart 60 for all shafts or the torque value chart 64 for a single shaft, the operator can monitor time-dependent changes in the operating conditions of the respective nut runners 20 or workpiece positioning conditions.

More specifically, if the tightening torque value 62 varies as shown in FIG. 11(a), for example, the operator - 20 can recognize in advance that the functions of circuit camponents of the nut runner 20 have been gradually degraded. if the tightening torque value 62 is abruptly changed as shown in FIG. 11(b),- the operator can know that the workpiece may have been changed or the amplifier a2 may have malfunctioned. A different warkpiece may be handled by varying the final torque 66, the upper limit 70 for the final torque, and the lower limit 72 for the final torque. if the tightening torque value 62 varies between the final torcrue 66 and the upper limit 70 for the final torcrue as shown in FIG. 11(c), the preset rotational speed for tightening the nut with the nut runner 20 may have been inamoroDriate, or t-he drive power transmitting mechanism of the nut runner 20 may have been malfunctioning. To solve t hese -problems, the preset rotational speed for the nut runner 20 may be varied, or the nut runner 20 may be replaced with another nut runner.

With the present invention, as described above, the operating conditions of the subcontrollers are monitored through the master controller at all times. The master controller can control the nut runners as a whole. The drive conditions for the nut runners can quickly and efficiently be varied through the master controller, 1 j Since the operating conditions of the nut runners are always monitored through the master controller, the operator can immediately confirm whether the nut runners are being operated normally or not, or whether the nuts or other threaded parts are being tightened properly or not. Accordingly, when any malfunction takes place, such an error can quickly be removed. Defective components are thus prevented -from being produced, and the efficiency of production is increased.

Furtihlenrziore, at least one of the rotational sDeed of the drive source for the nut runner, the current flowing Lhrough the drive source, and the torque Of the output shaft of the nut runner is detected and its data are accumulated, and the accumulated data are output as time-dependent c. nances. Bv compariLng the above data with a reference level -car the rotational speed of the drive source, the current Llowing through the drive source, or the torque of the output shairt of ithe nut runner, 'the reference level being of a value reached when the threaded part is normally tightened, it %_ is possible to determine a failure of the nut runner itself and a tightening failure resulting from a wrong -ion given to the nut runner, and also to locate the instruct. cause of such a failure.

Moreover, a final tightening torque applied by the nut runner is successively measured and measured values are 22 - 1 accumulated for eachthreaded part, and the accumulated Jfinal tightening torque values are displayed as timedependent data for evaluating the conditions of the nut runner or the workDiece. The driven conditions of the nut runner, entry of foreign matter into the workpiece, jamming of the threaded part, or a positioning error thereof can appropriately be determined based on the tendency of the time-dependent changes in the final tightening torque. Troubles can thus be c-uicklv removed or avoided in advance by repairing 'he nut runner or the workpiece based on the determined result.

Although a cer.:-ain preferred embodiment has been shown arid described, it should be undersCood that many -ications may be made - in without changes and modi' 1-here deoarj-ina from 4k--he SCO0e OJE appen-ded. claims.

Preferred embodiments of the present invention can provide inc nut runners, wh4 Ch a method of.moni4t-or- prevent the nut runners from having troubles, and permit quick repairs when any of the nut runners suffers from a trouble.

23 -

Claims

CLAIMS:

A method of monitoring conditions of a plurality of nut runners and a workpiece based on torques with which a plurality of threaded parts are tightened on the workpiece by the nut runners, said method comprising the steps of: of nut runners connected, for controlling said nut providing a plurality respectively, to subcontrollers runners; setting predetermined drive conditions for said nut runners in said subcontrollers; measuring the torques with which the threaded parts are tightened;- - recording a plurality of torques with which the threaded parts are tightened over a plurality of tightening cycles; and comparing time-dependent changes in said torques with preset values thereby monitoring operating conditions of the nut runners and the workpiece.
2. A method according to claim 1, wherein said operating conditions include conditions of a drive mechanism of the nut runners and a tightened condition of the threaded parts.
3. A method of monitoring conditions of a plurality of nut runners and a workpiece substantially as hereinbefore described with reference to the accompanying drawings.

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