442 The degree of mirror tilt is somewhat arbitrary and does not affect the accuracy of the readings. This  degree of mirror tilt is measured by the autocollimator and provides the scale factor for the bearing measurements. Also it was determined that if mirror tilt was too small, bearing wobble would cause crossover patterns of the reflected beam, such as a “figure eight” pattern. Therefore, the mirror to bearing axis alignment was intentionally perturbed for all testing cases to trace a nominal 500-microradian diameter path, as a seen by the autocollimator detector. Note that the effect of this approach was to introduce a calibrated, constant tilt of the mirror normal axis relative to the bearing axis.    Each test consisted of ten clockwise revolutions, followed by ten counter-clockwise revolutions. All tests were performed using a constant, low-speed rotation rate of 4.28 RPM (14 seconds per revolution), taking approximately five minutes to complete the full 20 revolutions. Note that the particular motor control speed was selected in order to determine slow-speed, or kinematical, bearing behavior, neglecting the dynamic effects.[4] The autocollimator azimuth and elevation signals were sampled by the data acquisition system at 250 Hz.   Bearing Summary  A total of six unique bearings were tested from three separate vendors. Although the same diameter, cross section and duplex arrangement were used throughout, each configuration had a unique retainer. ABEC ratings include 3, 5 & 7. Also, note that SN6 had a single outer race for the duplex set. The complete bearing summary table is shown in Table 1.   Table 1. Test Bearing Summary 

Serial  Number ABEC OD Cross Section Vendor Ball Dia Ball Retainer Type  (in) (mm) (in) (mm) (in) (mm) Qty  1 3 3.0 76.2 0.25 6.35 A 0.125 3.175 52 Nylon DB  2 3 3.0 76.2 0.25 6.35 A 0.125 3.175 52 Nylon DB  3 3 3.0 76.2 0.25 6.35 A 0.125 3.175 52 Nylon DB  4 7 3.0 76.2 0.25 6.35 B 0.125 3.175 52 Brass DB  5 7 3.0 76.2 0.25 6.35 B 0.125 3.175 52 Brass DB  6 5 3.0 76.2 0.25 6.35 C 0.125 3.175 60 Teflon  Toroid DB (1-Piece  Outer Race) 

Test Results  The type of results presented can be described as kinematic or quasi-static since they are done at a relatively low rotation rate. This is to avoid structural resonances in the system so that the measurements can be directly related to bearing behavior.  Measured bearing wobble was processed using several  methods. The primary method for evaluating synchronous and asynchronous runout was obtained from the elevation versus azimuth plots. A cursory, qualitative analysis compares the acquired test data trace to the perfect circle at 500-microradian diameter, which we will refer to as the “reference diameter”. Several performance aspects should be noted. First, the deviation from the average test data and the reference diameter is defined as “Synchronous Error”. It is synchronous in that it repeats itself every revolution. Second is the portion of the error that does not repeat itself, forming a radial hash band about the average test data. This is defined as the “Asynchronous Error”. These two definitions are illustrated in Figure 4. Finally, hysteresis is observed during cycle reversal, which is most evident in SN5 and SN6 traces. All six traces are shown in Figures 5 to 10, labeled sequentially from SN1 through SN6.