testing is a deceptively simple method to accurately determine the
“quality” of a rotating part. A source of accurate,
slow, vibration-free rotation is applied to one component of a rotating
part while measuring the force exerted by the opposite component.
The essence of the test is that it is performed at very low speeds,
typically 2 RPM, but in some cases as low as 0.001 RPM. At these
very low speeds, there is no vibration to mask the very small changes
in transmitted force that are caused by microscopic internal inconsistencies
during rotation. When such a rotation system is coupled to a sensitive
force-measuring load cell, the system can detect the smallest variations
caused by bearing brinelling, contamination, raceway misalignment,
lobing and many other issues. Results are graphically displayed
on an easy-to-read trace.
The purpose of this article is to introduce
the reader to the concept of MicroTorque testing and to present
some analytical results from tests on parts with different rotational
problems. All measurements were taken using an M15W MicroTorque™
tester from Measurement Research, Inc.
test – A “normal” test, like beauty, is in the
eye of the beholder. The M15 is a very sensitive device and a
perfectly made part may show many small variations during rotation.
The important thing is to decide what is important to the application.
The test shown here is typical of a disk drive pivot, showing
even rotation over 360°. Small irregularities and the grease
hash do not indicate any problems. In a typical ball bearing the
inner race must rotate 3-4 complete revolutions relative to the
outer race before the balls have moved around the raceways once.
This means that consecutive 360° tests of the same part actually
have different starting positions for the balls, causing apparent
Brinelling Test Trace
Brinelling – Dented raceways (brinelling) are shown as a
series of evenly-spaced spikes with the leading
edge falling (the ball falls into the hole in the raceway before
climbing out). Very severe brinelling may cause a rippling of
the raceway, seen as multiple spikes. At this point, the damage
can usually be felt by hand.
Contamination – Shown as irregular spikes with the leading
edge rising (the ball must climb the contaminating speck before
falling off the back side).
Raceway misalignment – Sinusoidal torque variation over
360° is indicative of slight raceway misalignment.
VCM Test Trace
Voice coil motor (VCM) – This is an automatic test of the
motor efficiency over the entire length of stroke of the motor.
The test automatically reverses when the hard stop is encountered
and the reverse stroke is overlaid as a reversed trace so that
features will align correctly. Built-in current measurement allows
display of Kt (gm-cm per amp).
Lobing Test Trace
Lobing – Two different sinusoidal variations with periods
much shorter and much longer than 360° are typical. This example
is plotted in MS Excel using direct test export feature.
The MicroTorque tester is a versatile
and powerful tool that can be used to pinpoint a number of rotational
faults. The traces shown in this article show simple effects,
but further analysis of test data, including frequency domain,
can expose great detail of the internal structure of any rotating