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Bearing Runout Measurements
Application Note 243-7
Introduction
Advanced precision machinery,
from centrifuges to computer disk
drives, rely on precision bearing
and spindle assemblies for high
performance. For example, the
spacing of data tracks on a com-
puter disk drive can be limited by
the non-repeatable runout of the
spindle bearing assembly. For
reasons like these, the need to
measure runout and diagnose its
cause has increased in recent
years.
Traditionally, runout has been
measured with the electronic
equivalent of a dial indicator and
oscilloscopes which can deter-
mine the magnitude of runout.
More recently, spectrum analyzers
have been used because they can
help identify the various causes of
runout by providing the frequency
distribution information, as well
as the data available from other
testing methods. Originally con-
fined to design labs, spectrum
analyzers are now finding their This note explores the advantages
way into incoming inspection and of using a dynamic signal analyzer
onto the manufacturing floor, to make runout measurements,
where they are used to measure using both the traditional time
changes in runout caused by domain measurements as well
critical assembly steps. as spectrum measurements.
The measurements shown were
made on a disk memory spindle
assembly.
Test setup to Time domain
measure runout measurements
of repeatable and
nonrepeatable runout
Figure 1:
In the time domain mode, the
Experimental analyzer shows Total Indicated
runout test Runout (TIR) as it changes with
set up
the revolution of the spindle. TIR
has two components. Repeatable
runout, the largest component
(up to 2 mils in this case), is
caused by the center of rotation
being offset from the physical
center of the part, as well as
surface irregularities on the hub.
The runout component of interest
is the nonrepeatable part, which
can be 1000 (60 dB) smaller than
the repeatable runout. In preci-
sion machinery, NRR is caused
largely by imperfections in the
Figure 1 shows a typical test that is proportional to the air gap bearings.
setup to measure spindle runout. between the hub and the probe.
The motor and spindle assembly This signal is fed into a dynamic Repeatable runout in the spindle
is typically loaded with an inertial signal analyzer, where it is digi- assembly is not a great concern
mass to simulate actual running tized into an amplitude vs. time because it is the same for every
loads. A proper load is often record. revolution and can be compen-
required for the spindle servo sated for. For example, a disk
to maintain a constant speed. A once-per-revolution tach pulse drive writes a servo track that is
(INDEX) is needed from the concentric with the center of rota-
The runout in this example is spindle assembly to drive the tion. The non-repeatable runout
measured by placing a proximity external trigger input on the (NRR) can not be compensated
probe close to the hub at the end analyzer. This ensures that data for and therefore is the precision
of the spindle. The probe, with collection starts at the same angle limit for the spindle bearing as-
its electronics, produces a signal of rotation for each average. sembly. In the case of a computer
disk drive, the goal is to maintain
a peak-to-peak NRR less than 5%
of the spacing between the tracks.
In a disk with 1000 tracks-per-
inch, a NRR of <20 microinches
Figure 2: MEASUREMENT PAUSED peak-to-peak is desirable.
Total indicated A Marker X: 3.87573242 ms Y: 97.537 uINCH
runout for two 200
uINCH
Figure 2 shows a single TIR
revolutions
of the hub.
measurement made with a
The fuzziness Real
dynamic signal analyzer. Since
indicates the 50 the spindle speed is 60 Hz
non-repeatable uINCH
runout.
/div (3,600 RPM), the time period
for each revolution is 16.7 milli-
seconds. The time record length
shown in the figure is just over
31 milliseconds, thus showing
almost two complete revolutions
-200
uINCH
of runout.
Start: 0 s Stop: 31.219 ms
TIR
2
Determining non-repeatable
runout with Dynamic
Signal Analyzers
NRR can be computed by sub- Figure 3: MEASUREMENT PAUSED
tracting repeatable runout from Repeatable A Marker X: 3.87573242 ms Y: 58.409 e-6 INCH
runout is 200
a single TIR measurement. The e-6
obtained by using INCH
repeatable runout is measured Vector/Time
by time averaging many TIR Averaging to Real
measurements together. The average out the 50
non-repeatable e-6
nonrepeatable parts of the TIR contribution to
INCH
/div
are averaged out. This method is runout.
more precise than drawing limit
lines on an oscilloscope or eyeing
peak-to-peak fuzziness of many
TIR measurements superimposed
-200
on each other. e-6
Start: 0 s Stop: 31.219 ms
REPEATABLE RUNOUT VECTOR: 64
To determine the repeatable
runout, time records used in the
Figure 4: MEASUREMENT PAUSED
average must be synchronized;
Non-repeatable A Marker X: 3.87573242 ms Y: 29.128 e-6 INCH
data collection for each time runout is the 200
e-6
record must start at the same difference INCH
angular location on the spindle. between a TIR
measurement Real
To do this, an external trigger is and the 50
used to start data collection for repeatable e-6
INCH
each time record at the same runout. /div
angle of rotation on the disk
spindle. The average smooths out
and converges on the repeatable
runout (figure 3). For vector aver-
aging to work properly, the speed -200
of the spindle should be regulated e-6
Start: 0 s Stop: 31.219 ms
within 1% or better1. NON REPEATABLE RUNOUT
Next, capture a new time record
of TIR. Subtract the repeatable Unfortunately, this time domain
runout from the total runout. The approach can not be used to diag-
difference is the NRR versus time. nose the causes of NRR because
This subtraction removes all re- the result is simply a display
peatable eccentricities and sur- of amplitude versus time. A
face runout effects (figure 4). different measurement method
is needed to break NRR into
The results of such testing are components that can be related
evaluated by looking for the to bearing defects.
peak-to-peak NRR. The one sigma
variance or repeatability of the
NRR versus time measurement
is typically 20-50%.
1
Computed order tracking can handle
conditions or speed regulation.
3
Frequency domain
measurements of
synchronous and
asynchronous runout
The next step in runout testing is Figure 5: MEASUREMENT PAUSED
to use a spectrum analyzer to look Synchronous B Marker X: 131 Hz Y: 5.6465 u*
at the different components of runout is 1
m*
measured
runout. When runout is displayed by using
as amplitude versus frequency, the Vector/Time LogMag
runout contributions from varia- Averaging. 10
Note the dB
tions on the inner race, outer race, markers set
/div
and rolling elements stand out as at 60 Hz and
separate spectral components its harmonics.
(figures 5 & 6). Variation analysis
becomes more quantitative and
less guesswork. This approach of
measuring runout in the frequency 100
THD: 44.173 %
e-9
domain is referred to as synchro- Start: 0 Hz Stop: 400 Hz
S: Spectrum Chan 1 VECTOR: 64
nous and asynchronous runout,
differentiating it from the time
domain measurements of repeat- Figure 6: MEASUREMENT PAUSED
able runout and NRR. Total runout is B Marker X: 131 Hz Y: 19.966 u*
measured using 1
m* * = INCH rms
rms averaging.
Synchronous runout is the spec- Notice the noise
trum of the repeatable runout, floor is higher LogMag
and the 10
which is the result of vector aver- amplitude of dB
/div
aging many TIR measurements. the 131 Hz
Figure 5, the synchronous runout component is
greater, than
spectrum, contains runout compo- adjacent
nents that are due to the hub ec- harmonics
centricity, hub finish and any of 60 Hz.
other repeatable bearing runout.
100
The off-synchronous runout com- e-9 THD: 48.444 %
Start: 0 Hz
ponents reduce in amplitude at a Stop: 400 Hz
S: TOTAL RUNOUT RMS: 64
rate of 1 k where k is the number
of averages. So 64 averages will
attenuate the asynchronous con- Table 1:
tributions by a factor of eight. Bearing
Characteristic
Frequencies
Figure 6 shows a spectrum of to-
tal or average runout, measured Defect on outer race (n) (RPM) (1 - Bd cos