White Paper Outline by katiebelonga

VIEWS: 50 PAGES: 11

									A New Angle on Torque




 Technologies, Methods and Pitfalls




                Whitepaper

                    by

        Frank Skog and Terry Gilbert
A New Angle on Torque



 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

Recent months have seen significant new developments in torque technology. Specifically, the
use of the solid state gyro, best-use methodologies associated with it and the incorporation of
these 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 a
solution looking for a problem. We can now sense motion and measure the number of degrees a
fastener rotates in real time. The question is, how to best harness the innovation? Several
different approaches have emerged. A few look good on the surface, but a disciplined drill down
reveals major weaknesses. This study examines the new torque technology, methods and
applications, 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. Precise
torque control can spell the difference between a safe, reliable and economical product and
complete disaster.

Some have argued that with the advent of precise, instrumented DC power tools the need to
measure or audit residual torque has become less important. Has it? The answer is no, and for
two reasons.

First, there are dozens of sources of error that could cause an instrumented tool, one that
indicated correct installation torque, to subsequently apply low actual joint torque. For example
something as simple as a cracked socket can throw off targeted applied torque. Or consider a
longer than normal extension. The longer extension absorbs more rotational energy intended for
the joint than a shorter extension, thus lowering actual applied torque. Power tools in particular
create variability by their very nature – high speed, constant motion, high volume. As the gears in
the right angle drive of a power wrench collect dirt and wear, increasing friction absorbs torque
and 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 originally
installed correctly. An example of this occurred when an over application of lubricant on a joint
assembly deposited grease between a washer and the work surface. The grease was too
viscous to be extruded completely from under the washer before the power tool reached cutoff
torque. Afterwards it continued to ooze out. A follow-up measurement of residual torque
indicated 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 can
result 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 other
industries, 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 methodologies
in recent years that have attempted to take on the optimization challenge. Among them are peak
torque, the use of breakaway and restart inflection points, capture angle and angle restart.



A New Angle on Torque                                                                            1
Prior to the introduction of the solid state gyro, there were two dominant methodologies used to
measure residual torque: simple peak measurement, and “restart” or “breakaway” as determined
by inflections in a torque/time curve.


Peak Torque

Assessing residual torque by means of a peak measurement strategy is perhaps the oldest and
most 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 order
for these devices to effectively and accurately measure residual torque, though, a significant
amount of operator training and practice is required. Proper operation requires the operator to
slowly and deliberately apply ever-increasing torque until the fastener just begins to move, and
then release pressure. This slow approach is an attempt to reduce the amount of overshoot after
the fastener starts to turn.


Overshoot

The tendency to overshoot is central to many of the problems associated with using a peak
reading device to measure residual torque. Contributing to the problem are individual differences
in human reaction time. An operator with quick reaction time tends to take lower readings than
an operator with slower reaction time. Slower reaction time results in greater overshoot. In
addition, 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 meet
a schedule can lead to a quicker pull and greater overshoot. On the curve below, an overshoot of
only about 150 milliseconds resulted in a peak reading more than 10% higher than the torque
applied at the start of fastener rotation.


                                         Figure 1. Overshoot.

                            Fastener                                       Peak
                            Movement                                      Reading
                                                    Overshoot




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 common
occurrence is usually triggered when a “bump” or vibration in the work piece is mistaken for
fastener rotation. These false apparent indications of fastener rotation are more common when
the 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 is
inherently flawed. It measures torque at the point where the operator stops pulling on the
wrench. This may occur before the fastener turns, shortly after the fastener turns, or significantly
after the fastener turns. Lack of accuracy has a cost. Peak residual torque measurements are



A New Angle on Torque                                                                             2
often so questionable that managers end up taking
multiple measurements attempting to determine
whether a torque problem really exists rather than                            Why wait for movement?
take corrective action with the fastening system.
                                                                         Often the question comes up, when
                                                                         auditing a previously tightened joint,
                                                                         why wait for movement? Why not
Restart/Breakaway Inflections                                            capture the torque value at the first
                                                                         detected change in angle?        The
Seeking a method whereby residual torque                                 answer is that the gyro is in the
                                                                         wrench and not in the fastener. The
measurement is more closely associated with
                                                                         wrench, square drive, extension if
actual fastener motion, a number of vendors have                         used, socket, fastener, and work
developed algorithms that attempt to detect                              piece may all flex to some degree
breakaway by looking for inflection points in the                        before actual fastener rotation
torque/time curve. These algorithms leverage the                         begins. This windup is influenced by
fact that resistance to the wrench changes at the                        many variables. Capturing a residual
point where static friction has been overcome. The                       torque measurement before fastener
torque/time curve in figure 2 shows the capture of a                     rotation begins is an error to be
restart point. In the hands of a skilled and very                        avoided.
careful operator, breakaway detection algorithms
can produce better quality measurements in less
time than those taken using peak measurement.

                  Figure 2. Restart point.
                                                        Though faster, restart or breakaway
                                                        inflection detection presents the user with
                                                        challenges.       Inflection points in the
                                                        torque/time curve can easily be caused by
                                                        operator hesitation, resulting in false low
                                                        readings. Conversely, well lubricated soft
                                                        joints may produce very little or no
                                                        detectable inflection at fastener motion.
                                                        The curve in figure 3 below represents a
                                                        fairly severe case of operator hesitation.
                                                        The operator was trying to “creep up” on a
                                                        lug nut fastener to minimize overshoot.
                                                        The series of inflection points that starts
                                                        near the 786 millisecond point was caused
                                                        by involuntary shake in the operator’s arm
                                                        as he tried to slowly apply additional torque
                                                        over 170Nm. While torque/time breakaway
                                                        and restart can produce more accurate
measurements than peak, they are not generally used because they can produce very large
errors unless used on the right joint type by a very experienced operator.


                                             Figure 3. Operator hesitation.




A New Angle on Torque                                                                                             3
Another problem area for torque/time inflection point detection is in the case of a well lubricated
soft joint demonstrating little or no detectable inflection at the start of fastener rotation. The curve
in figure 4 is an example of just such a fastener, with very little inflection in the torque/time curve
at the start.


                                    Figure 4. Undetectable initial torque.




                  No Obvious Inflection Point




Enter the Solid State Gyro

The introduction of the solid state gyro has facilitated development of residual torque
measurement devices that incorporate the use of sensed angular displacement as a qualifier for
the capture of a torque value. Generally, the purpose of the gyro in these devices is to ensure
that 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 torque
measurement 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 method
is 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 after
some 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 makes
a best guess based on the materials used, their properties, the type of joint and anticipated
windup before fastener rotation begins. Going forward, torque capture angle is often adjusted
using some number of residual measurements and comparing them to in-line installation
measurements. The method is very much trial and error and fairly subjective.




A New Angle on Torque                                                                                 4
Following are torque/angle curves from a                                                             Figure 5. Windup due to flex.
number of joints that illustrate the issue.
                                                                   35
In each case a red diamond on the
                                                                   30
torque/angle curve indicates the actual                            25
start of fastener rotation.




                                                          Torque
                                                                   20
                                                                   15
The chart in figure 5 shows the torque                             10
curve for a fastener on a brake line                               5
manifold shock mounted on an engine                                0
                                                                          0                      2         4       6             8         10            12            14
compartment fire wall. On this joint the
                                                                                                                       Angle
fastener started to rotate at 6.78 degrees
past the starting torque threshold of 10
Nm. This large degree of windup is due
in large part to flex in the work piece.                                                         Figure 6. Same joint type, different reading.
                                                                   30
A few fasteners later in the study,                                25
rotation started at 5.31 degrees as                                20




                                                          Torque
shown in figure 6. Which is the correct                            15
capture angle for this joint?                                      10
                                                                   5
The charts in figure 7 track torque and                            0
angle for a joint consisting of a lug nut on                              0                      2         4       6             8         10            12            14
an aluminum wheel. Rotation starts at                                                                                  Angle
1.61 degrees. On another lug nut on the
same wheel, the fastener did not begin
rotation until windup reached 3 degrees.

                                   Figure 7. Same joint types, different readings.

          160                                                                    250
          140
          120                                                                    200
          100
 Torque




                                                                        Torque




                                                                                 150
           80
           60                                                                    100
           40                                                                        50
           20
            0                                                                            0
                0   1   2   3           4     5       6                          7           0         1       2       3          4        5         6             7        8

                                Angle                                                                                          Angle




How are these different joints to be interpreted? Clearly a capture angle that is correct for one is
wrong or very wrong for another.

The curve in figure 8 was generated by a
fastener securing an air bag under the driver
                                                                                             Figure 8. Hard joint – problematic for capture angle.
side instrument panel. This was a hard joint                                     20
where each additional degree of fastener                                         18
                                                                                 16
rotation added approximately 5% of the                                           14
torque at the instant of rotation. As in the                                     12
                                                                        Torque




previous example, a joint signature of this                                      10
                                                                                     8
type is particularly problematic for the                                             6

capture angle method since small changes in                                          4
                                                                                     2
the capture angle or in a joint’s amount of                                          0
                                                                                         0            2        4           6           8        10            12            14
windup can significantly alter the reported
                                                                                                                               Angle
torque value.



A New Angle on Torque                                                                                                                                                   5
Interestingly, this curve in figure 9 was                                Figure 9. Soft joint – various capture
captured on the next fastener on the same                                  angles produce the same result.
air bag. Although the threaded components                        9
                                                                 8
were identical, this joint contained a plastic                   7
spacer. The result was a soft joint where                        6
fastener rotation started at only one degree.




                                                        Torque
                                                                 5

In this case, capture angles of 1, 2, or 3                       4

degrees all produce nearly the same result.                      3
                                                                 2
On some joints the selection of the capture
                                                                 1
angle is critical, and on other joints not at all.               0
The problem, once again, is that the user is                         0     2          4           6          8    10

left to guess which is which.                                                             Angle




Angle Restart

The torque rate differentiation method known as angle restart was developed specifically to
eliminate the false high and false low readings associated with peak measurement and to
eliminate the error inducing guesswork associated with the capture angle methodology. This is
done 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 where
the curve transitions from the windup slope to the rotation slope (see figure 10 below).

                                            Figure 10. Angle restart.




                                                     Rotation
                                                            Rotation
                                     Windup
                                Windup




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 joint
type. 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 built
upon a bad guess go away. It is no longer possible to have error induced by a capture angle that
doesn’t match the start of fastener rotation.

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

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




A New Angle on Torque                                                                                             6
         Immunity to gyro drift. All solid state gyros drift over time. While drift error in a capture
          angle system clearly affects the measured torque value, the angle restart method
          effectively 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 11
          below. This joint was installed to 5.3Nm, but due to material failure of a plastic
          component the restart point was at only 3.3Nm.


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

                         6
                         5
                         4
                Torque




                         3
                         2
                         1
                         0
                             0            1            2            3            4            5         6
                                                                 Angle



Conclusion
Most manufacturers continue to face challenges relating to proper torque, and demand is
increasing. As the number and types of mechanized consumer and commercial products being
developed and offered in the world expands, so does the need to assess and manage torque
precisely 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 offer
no solution.

Of the various torque measurement and management solutions discussed, it is clear that peak
torque introduces too much variability to be considered viable. The use of restart and breakaway
inflection points provides some improvement, but only in the hands of very experienced
operators, 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. This
technology was initially put to use in the capture angle method. Unfortunately this approach
doesn’t really help us pinpoint optimal torque. In fact the “capture angle” is something that has to
be estimated and inserted into the process before any torque auditing can actually begin. There
is a great deal of guesswork involved, considerable variability across joints and joint types, and
inconsistency between operators performing follow-up measurements. Thinking of this as a
sound and accurate torque method is dangerously deceiving.

The angle restart approach does away with the guesswork and variability because it employs a
technology that can be applied consistently to joints of all types. It measures residual torque and
provides a basis for setting optimum torque on each fastener independently. This is achieved by
means of a motion-detecting gyro used in conjunction with a sophisticated software algorithm that
allows the device to differentiate between the windup and rotation angular torque curves. This
advancement delivers significant precision and optimization advantages to the torque
management challenge and seriously raises the bar over other approaches.



A New Angle on Torque                                                                                       7
Glossary


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


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



   Residual torque. The amount of tension that                                                 Peak Torque
   remains in a joint after fastening a threaded                  60
   fastener.                                                      50

                                                                  40




                                                         Torque
                                                                  30

   Installation torque. Installation torque is the                20

   amount of torque applied to a threaded                         10

   fastener joint to effect installation.                          0
                                                                       1    144 287 430 573 716 859 1002 1145 1288 1431 1574 1717
                                                                                                        Time



   Peak. Peak torque is simply the highest torque
                                                                                                    Restart
   during a torque event.
                                                                  250

                                                                  200

   Restart. Restart torque is a residual torque
                                                         Torque




                                                                  150

   measurement strategy that measures the                         100

   torque required to turn a previously tightened                 50

   fastener after static friction has been                         0
                                                                        0       1      2        3         4         5    6   7    8
   overcome. In a well lubricated low static                                                            Angle
   friction joint the restart point and the breakaway
   point can be one and the same. The joint that
                                                                                           Breakaway
   generated the curve shown at right had a very
   high component of static friction. This strategy               250
   provides the best measure of power tool                        200
   performance and the best indication of clamp
                                                         Torque




                                                                  150
   force.                                                         100

                                                                  50

                                                                   0
   Breakaway. Breakaway torque is a residual                            0       1      2        3         4         5    6   7    8
                                                                                                        Angle
   torque measurement strategy that measures
   the torque necessary to overcome static friction
   in a previously tightened fastener.
                                                                                       Torque at Angle

                                                                  20
   Torque at angle. Torque at angle is a residual                 15
   torque measurement strategy where torque is
                                                         Torque




                                                                  10
   captured after a given amount of sensed
   angular rotation past a torque threshold. On the                5

   curve at right, the chosen capture angle was                    0
                                                                        0        2         4        6           8       10   12       14
                                                                                                        Angle


A New Angle on Torque                                                                                                             8
three degrees. The capture angle needs to be large enough to avoid the flex portion of the
curve at the extreme left.


Flex or windup. Flex or windup in residual torque                                    Flex or Windup
measurement is sensed angular motion due to the
                                                                       250
inherent metallic elasticity in the wrench, drive,
extension, socket, fastener, and work piece. The                       200


curve below shows about one degree of windup                           150




                                                              Torque
before the start of actual fastener rotation.                          100


                                                                       50



Hard joint. A hard joint is a joint where the                           0
                                                                             0   1         2           3   4   5

components are firmly joined and non-                                                          Angle


compressible so that additional fastener rotation
will almost exclusively result in fastener elongation.


Soft joint. A soft joint is a joint where additional fastener rotation results in little or no fastener
elongation, 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 a
number of measurement-related patents, and is published regularly in top quality
manufacturing and assembly periodicals across the globe.

Terry Gilbert is a Marketing Manger for ASI DataMyte.




                                             End

								
To top