FED-STD-791D
6.1.8 Test bearing. For qualification tests a new 10.0-cm test bearing should be used for
each 100-hour endurance run. The new bearing should be cleaned in solvent and dipped in test
oil prior to installing.
6.2 Bearing head assembly.
6.2.1 Screw thread seal. In installing the seal (P/N IRBB 6204) the right hand threads on
the O.D. of the seal must be toward the front (test compartment) end of the rig. Reversing the
seal position will cause the test and support oil to be whirled toward the center of the seal by the
rotating action of the shaft and enhance the possibility of mixing the two oils.
6.2.2 Test bearing heater mount. The bearing outer race should be lightly pressed into
the bearing heater mount with the serial number facing the front of the rig. The recommended fit
on the bearing outer race is -0.000508 to +0.00178 cm (-0.0002 to +0.0007 in). On repeated
usage, the I.D. of the bearing mount may become worn and result in a loose fit which might tend
to cock the outer race. It may also result in erroneous temperature readings of the bearing outer
race. When this happens, the I.D. of the mount should be chrome plated and brought back to
proper dimension or that section of the mount replaced.
NOTE: The heater mount must be disassembled and repacked with No. 30 grit aluminum
oxide after each test.
6.2.2.1 Reassembly of heater mount. Prior to assembly of the heater mount, the heater
coils should be centered and checked to insure that no point of the coils comes within 0.318 cm
(1/8 in) of the inside surfaces of the front and rear mount plates. A method of accomplishing this
is to assemble the unit without retainer ring; this allows visual inspection of the location of the
heater- coils within the assembly. Areas where the coils come within 0.318 cm (1/8 in) of either
front or rear inner faces are marked. During final assembly, transite spacers 0.32 H 0.64 H 1.3
cm (1/8 H 1/4 x 1/2 in) are placed at these marked points. In reassembling the heater mount, care
should be exercised to insure that the front and rear vertical faces are parallel to each ther and
square with the I.D. of the mount. If these conditions are not maintained, the concentricity of the
bearing outer race in relation to the axis of the shaft may be affected. Before installing the heater
mount in the rig, the bearing outer race thermocouples should be checked for continuity.
6.2.3 Test bearing. Before installing the test bearing on the hub, it should be examined
visually for any obvious mechanical defects. If the bearing appears satisfactory, the inner race
and cage assembly should be pressed onto the hub with the serial number facing the front of the
rig. The recommended fit of the bearing inner race on the hub is 0.0020 to 0.0036 cm (0.0008 to
0.0014 in) tight. After repeated usage the O.D. of the bearing hub may become worn and result
in a loose bearing fit. When this happens, the hub should be chrome plated back to proper
dimension or replaced. After the bearing has been installed in the bearing head, turn the shaft by
hand to make sure it turns freely. Bearing roughness can usually be detected in this manner.
When properly installed the rollers should be evenly centered in the outer race. If the rollers are
more than 0.32 cm (1/8 in) off center on the outer race, check for missing shaft assembly parts
or an improperly installed bearing outer race. It is important that the lock nut on the test bearing
retainer be tight since it imparts the axial pinch on all of the shaft assembly parts back to the load
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