A New Angle on Torque

8/7/2019 A New Angle on Torque

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A New Angle on Torque

Technologies, Methods and Pitfalls

Whitepaper

by

Frank Skog and Terry Gilbert




Table of Contents The Nuts and Bolts............................................................................................1 Torque Technologies.........................................................................................1 Peak Torque......................................................................................................2 Overshoot..........................................................................................................2 Restart/Breakaway Inflections...........................................................................3 Enter the Solid State Gyro.................................................................................4 Capture Angle ...................................................................................................4 Angle Restart.....................................................................................................6 Conclusion ........................................................................................................7 Glossary............................................................................................................8



A New Angle on Torque 1


Recent months have seen significant new developments in torque technology. Specifically, theuse of the solid state gyro, best-use methodologies associated with it and the incorporation ofthese advances into electronic tools is causing serious rethinking and re-engineering in quality

manufacturing circles and assembly operations. In some respects, the application of the motion-sensing solid state gyro to torque plays out like asolution looking for a problem. We can now sense motion and measure the number of degrees afastener rotates in real time. The question is, how to best harness the innovation? Severaldifferent approaches have emerged. A few look good on the surface, but a disciplined drill downreveals major weaknesses. This study examines the new torque technology, methods andapplications, and provides some useful conclusions.

The Nuts and Bolts

The Torque challenge has been with us for some time. An under-torqued fastener can vibrate or

work loose. Conversely, if tension is too high, the fastener can snap or strip its threads. Precisetorque control can spell the difference between a safe, reliable and economical product andcomplete disaster. Some have argued that with the advent of precise, instrumented DC power tools the need tomeasure or audit residual torque has become less important. Has it? The answer is no, and fortwo reasons.

First, there are dozens of sources of error that could cause an instrumented tool, one thatindicated correct installation torque, to subsequently apply low actual joint torque. For examplesomething as simple as a cracked socket can throw off targeted applied torque. Or consider alonger than normal extension. The longer extension absorbs more rotational energy intended forthe joint than a shorter extension, thus lowering actual applied torque. Power tools in particularcreate variability by their very nature – high speed, constant motion, high volume. As the gears inthe right angle drive of a power wrench collect dirt and wear, increasing friction absorbs torqueand the sensor in the tool picks up less than accurate readings.

Material conditions can leave a fastener with insufficient torque even if the joint was originallyinstalled correctly. An example of this occurred when an over application of lubricant on a jointassembly deposited grease between a washer and the work surface. The grease was tooviscous to be extruded completely from under the washer before the power tool reached cutofftorque. Afterwards it continued to ooze out. A follow-up measurement of residual torqueindicated the joint was nearly one third below installation torque.

The second reason to measure residual torque precisely is the high cost of failure for many joints.Improper torque in safety critical applications such as steering gears or braking assemblies canresult in tens of thousands of dollars in equipment damage, human injury, or even death. As we

continue to observe high levels of recalls attributable to improper torque in the auto and otherindustries, it would seem the need to resolve torque issues carries more weight than ever.

Torque Technologies

There have been a number of competing torque measurement technologies and methodologiesin recent years that have attempted to take on the optimization challenge. Among them are peaktorque, the use of breakaway and restart inflection points, capture angle and angle restart.




Prior to the introduction of the solid state gyro, there were two dominant methodologies used tomeasure residual torque: simple peak measurement, and “restart” or “breakaway” as determinedby inflections in a torque/time curve.

Peak Torque

Assessing residual torque by means of a peak measurement strategy is perhaps the oldest andmost widely employed methodology today. Typically, peak torque wrenches use a simple non-electronic indicating dial to measure peak torque. There are electronic versions as well. In orderfor these devices to effectively and accurately measure residual torque, though, a significantamount of operator training and practice is required. Proper operation requires the operator toslowly and deliberately apply ever-increasing torque until the fastener just begins to move, andthen release pressure. This slow approach is an attempt to reduce the amount of overshoot afterthe fastener starts to turn.

Overshoot

The tendency to overshoot is central to many of the problems associated with using a peakreading device to measure residual torque. Contributing to the problem are individual differencesin human reaction time. An operator with quick reaction time tends to take lower readings thanan operator with slower reaction time. Slower reaction time results in greater overshoot. Inaddition, since torque auditors typically take several hundred measurements in a shift,inconsistencies can creep in. Fatigue can cause a weaker pull on the wrench or pressure to meeta schedule can lead to a quicker pull and greater overshoot. On the curve below, an overshoot ofonly about 150 milliseconds resulted in a peak reading more than 10% higher than the torqueapplied at the start of fastener rotation.

While excessive overshoot creates false high readings using a peak reading device, releasing the

wrench before the fastener begins to turn causes false low readings. This all too commonoccurrence is usually triggered when a “bump” or vibration in the work piece is mistaken forfastener rotation. These false apparent indications of fastener rotation are more common whenthe work piece is in motion as on an assembly line. Even if it were possible to stabilize these sources of variance, the peak residual torque method isinherently flawed. It measures torque at the point where the operator stops pulling on thewrench. This may occur before the fastener turns, shortly after the fastener turns, or significantlyafter the fastener turns. Lack of accuracy has a cost. Peak residual torque measurements are

Figure 1. Overshoot.

FastenerMovement

PeakReading

Overshoot

FastenerMovement

PeakReading

Overshoot




Why wait for movement?

Often the question comes up, whenauditing a previously tightened joint,why wait for movement? Why not

capture the torque value at the firstdetected change in angle? Theanswer is that the gyro is in thewrench and not in the fastener. Thewrench, square drive, extension ifused, socket, fastener, and workpiece may all flex to some degreebefore actual fastener rotationbegins. This windup is influenced bymany variables. Capturing a residualtorque measurement before fastenerrotation begins is an error to beavoided.

often so questionable that managers end up takingmultiple measurements attempting to determinewhether a torque problem really exists rather thantake corrective action with the fastening system.

Restart/Breakaway Inflections

Seeking a method whereby residual torquemeasurement is more closely associated withactual fastener motion, a number of vendors havedeveloped algorithms that attempt to detectbreakaway by looking for inflection points in thetorque/time curve. These algorithms leverage thefact that resistance to the wrench changes at thepoint where static friction has been overcome. Thetorque/time curve in figure 2 shows the capture of arestart point. In the hands of a skilled and verycareful operator, breakaway detection algorithmscan produce better quality measurements in lesstime than those taken using peak measurement.

Though faster, restart or breakawayinflection detection presents the user withchallenges. Inflection points in thetorque/time curve can easily be caused byoperator hesitation, resulting in false lowreadings. Conversely, well lubricated softjoints may produce very little or nodetectable inflection at fastener motion.The curve in figure 3 below represents afairly severe case of operator hesitation.The operator was trying to “creep up” on alug nut fastener to minimize overshoot.The series of inflection points that startsnear the 786 millisecond point was causedby involuntary shake in the operator’s armas he tried to slowly apply additional torqueover 170Nm. While torque/time breakawayand restart can produce more accurate

measurements than peak, they are not generally used because they can produce very largeerrors unless used on the right joint type by a very experienced operator.

Figure 3. Operator hesitation.

Figure 2. Restart point.




Another problem area for torque/time inflection point detection is in the case of a well lubricatedsoft joint demonstrating little or no detectable inflection at the start of fastener rotation. The curvein figure 4 is an example of just such a fastener, with very little inflection in the torque/time curveat the start.

Enter the Solid State Gyro

The introduction of the solid state gyro has facilitated development of residual torquemeasurement devices that incorporate the use of sensed angular displacement as a qualifier forthe capture of a torque value. Generally, the purpose of the gyro in these devices is to ensurethat the fastener has actually moved before capturing a torque value, and to eliminate the effects

of overshoot.

Today there are two widely used methods that capitalize on the use of a gyro in a residual torquemeasurement system. One method is referred to as the “Capture Angle” or “Torque at Angle”method, which captures torque at a preset degree of sensed angular rotation. The other methodis called Angle Restart. So we can sense motion. Now what?

Capture Angle

The capture angle method relies on the operator or engineer to determine a capture angle for

each joint type to be audited. The residual torque for any given joint is the reading taken aftersome degree of sensed angular rotation that includes both windup and actual fastener rotation.That predefined number of degrees has to come from somewhere. Typically an engineer makesa best guess based on the materials used, their properties, the type of joint and anticipatedwindup before fastener rotation begins. Going forward, torque capture angle is often adjustedusing some number of residual measurements and comparing them to in-line installationmeasurements. The method is very much trial and error and fairly subjective.

Figure 4. Undetectable initial torque.

No Obvious Inflection PointNo Obvious Inflection Point




Following are torque/angle curves from anumber of joints that illustrate the issue.In each case a red diamond on thetorque/angle curve indicates the actualstart of fastener rotation.

The chart in figure 5 shows the torquecurve for a fastener on a brake linemanifold shock mounted on an enginecompartment fire wall. On this joint thefastener started to rotate at 6.78 degreespast the starting torque threshold of 10Nm. This large degree of windup is duein large part to flex in the work piece.

A few fasteners later in the study,rotation started at 5.31 degrees asshown in figure 6. Which is the correctcapture angle for this joint?

The charts in figure 7 track torque andangle for a joint consisting of a lug nut onan aluminum wheel. Rotation starts at1.61 degrees. On another lug nut on thesame wheel, the fastener did not beginrotation until windup reached 3 degrees.

How are these different joints to be interpreted? Clearly a capture angle that is correct for one iswrong or very wrong for another. The curve in figure 8 was generated by afastener securing an air bag under the driverside instrument panel. This was a hard joint

where each additional degree of fastenerrotation added approximately 5% of thetorque at the instant of rotation. As in theprevious example, a joint signature of thistype is particularly problematic for thecapture angle method since small changes inthe capture angle or in a joint’s amount ofwindup can significantly alter the reportedtorque value.

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Figure 6. Same joint type, different reading.

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Figure 7. Same joint types, different readings.

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Figure 8. Hard joint – problematic for capture angle.

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Figure 5. Windup due to flex.




Interestingly, this curve in figure 9 wascaptured on the next fastener on the sameair bag. Although the threaded componentswere identical, this joint contained a plasticspacer. The result was a soft joint wherefastener rotation started at only one degree.In this case, capture angles of 1, 2, or 3degrees all produce nearly the same result.On some joints the selection of the captureangle is critical, and on other joints not at all.The problem, once again, is that the user isleft to guess which is which.

Angle Restart

The torque rate differentiation method known as angle restart was developed specifically toeliminate the false high and false low readings associated with peak measurement and toeliminate the error inducing guesswork associated with the capture angle methodology. This isdone by monitoring the changes in angular torque and noting the difference in the curve slope

during windup and actual rotation. Angle restart torque is captured at the start of rotation wherethe curve transitions from the windup slope to the rotation slope (see figure 10 below).

In other words, Angle Restart captures torque at the instant of fastener rotation.

Angle restart can be used to measure torque accurately and consistently independent of the jointtype. The operator does not need any engineering estimates to put him in the ballpark and there

is no need to guess a capture angle. Consequently, the possible chain reaction errors builtupon a bad guess go away. It is no longer possible to have error induced by a capture angle thatdoesn’t match the start of fastener rotation.

There are several other advantages of measuring torque via the angle restart method. Theseinclude:

No second guessing readings. The question of whether an out of spec readingindicates a torque assembly problem or an improperly set capture angle does not arise.

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Figure 9. Soft joint – various capture angles produce the same result.

Figure 10. Angle restart.

WindupRotation

Windup

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WindupRotation

Windup

Rotation




Immunity to gyro drift. All solid state gyros drift over time. While drift error in a captureangle system clearly affects the measured torque value, the angle restart methodeffectively cancels out the drift by comparing torque rates.

Angle restart captures torque at the instant of actual fastener rotation.

Angle restart is equally effective on all joint types.

Angle restart more fully captures the effect of material failure as shown in figure 11below. This joint was installed to 5.3Nm, but due to material failure of a plasticcomponent the restart point was at only 3.3Nm.

Conclusion

Most manufacturers continue to face challenges relating to proper torque, and demand isincreasing. As the number and types of mechanized consumer and commercial products beingdeveloped and offered in the world expands, so does the need to assess and manage torqueprecisely and accurately. Current electronic, software-controlled DC power tools do not have a

means by which to measure and manage the joints that they create, and so by themselves offerno solution.

Of the various torque measurement and management solutions discussed, it is clear that peaktorque introduces too much variability to be considered viable. The use of restart and breakawayinflection points provides some improvement, but only in the hands of very experiencedoperators, and only if there are ways to overcome inconsistencies. Employing a solid state gyro to sense angular motion is a critical step in the right direction. Thistechnology was initially put to use in the capture angle method. Unfortunately this approachdoesn’t really help us pinpoint optimal torque. In fact the “capture angle” is something that has tobe estimated and inserted into the process before any torque auditing can actually begin. Thereis a great deal of guesswork involved, considerable variability across joints and joint types, andinconsistency between operators performing follow-up measurements. Thinking of this as asound and accurate torque method is dangerously deceiving. The angle restart approach does away with the guesswork and variability because it employs atechnology that can be applied consistently to joints of all types. It measures residual torque andprovides a basis for setting optimum torque on each fastener independently. This is achieved bymeans of a motion-detecting gyro used in conjunction with a sophisticated software algorithm thatallows the device to differentiate between the windup and rotation angular torque curves. Thisadvancement delivers significant precision and optimization advantages to the torquemanagement challenge and seriously raises the bar over other approaches.

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Figure 11. Angle restart curve captures effects of material failure.




Glossary

Joint. A mechanical connection of two or more typically metalcomponents held together by a threaded fastener.

Clamp force. Clamp force is the compressive force exerted by athreaded fastener in a joint. This force results from stretching thefastener during installation.

Residual torque. The amount of tension thatremains in a joint after fastening a threadedfastener. Installation torque. Installation torque is theamount of torque applied to a threadedfastener joint to effect installation. Peak. Peak torque is simply the highest torqueduring a torque event. Restart. Restart torque is a residual torquemeasurement strategy that measures the

torque required to turn a previously tightenedfastener after static friction has beenovercome. In a well lubricated low staticfriction joint the restart point and the breakawaypoint can be one and the same. The joint thatgenerated the curve shown at right had a veryhigh component of static friction. This strategyprovides the best measure of power toolperformance and the best indication of clampforce. Breakaway. Breakaway torque is a residual

torque measurement strategy that measuresthe torque necessary to overcome static frictionin a previously tightened fastener. Torque at angle. Torque at angle is a residualtorque measurement strategy where torque iscaptured after a given amount of sensedangular rotation past a torque threshold. On thecurve at right, the chosen capture angle was

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1 144 287 430 573 716 859 1002 1145 1288 1431 1574 1717

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End

three degrees. The capture angle needs to be large enough to avoid the flex portion of thecurve at the extreme left. Flex or windup. Flex or windup in residual torquemeasurement is sensed angular motion due to theinherent metallic elasticity in the wrench, drive,extension, socket, fastener, and work piece. Thecurve below shows about one degree of windupbefore the start of actual fastener rotation. Hard joint. A hard joint is a joint where thecomponents are firmly joined and non-compressible so that additional fastener rotationwill almost exclusively result in fastener elongation. Soft joint. A soft joint is a joint where additional fastener rotation results in little or no fastenerelongation, but the joint components are either pulled closer together, or are compressed

further.

Authors

Frank Skog is a Sr. Product Manger and Applications Engineer for ASI DataMyte, Inc. Mr.Skog is recognized as an industry leader in the field of Torque Technology. He holds anumber of measurement-related patents, and is published regularly in top qualitymanufacturing and assembly periodicals across the globe. Terry Gilbert is a Marketing Manger for ASI DataMyte.

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A New Angle on Torque

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