FED-STD-791D
and then with acetone, final rinse in clean acetone, air dry and weigh to the nearest 0.1 mg. If
there is to be any delay before weighing, store under n-heptane until ready.
NOTE 1:
As a practical polishing procedure, place a sheet of the abrasive paper on a flat
surface and rub the specimen against the paper with longitudinal strokes,
protecting the specimen from contact with the fingers with clean cloth or ashless
filter paper. An individual abrasive paper should be employed for each metal.
Several specimens of the same metal may be polished with one paper.
6.2 Bearing head assembly.
6.2.1 Main shaft. If the main shaft is not solid, its front end should be plugged with an
aluminum plug.
6.2.2 Screw-thread seal. In installing the seal, the right-hand threads on the O.D. of the
seal must be toward the front (test compartment) end of the rig.
6.2.3 Test-bearing heater mount. The bearing outer race is 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 -0.0005 0.0018 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 may be chrome plated and reground to proper
dimension, or that section of the mount replaced.
6.2.3.1 Test-bearing heater assembly. In reassembling the heater mount, care should be
exercised to insure that the heater coil is centered and that at no point should the heater coils
come within 0.32 cm (1/8 in) of the inside surfaces of the assembly faces. 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 0.32 cm
(1/8 in) of either front or rear inner faces are marked. During final assembly, Transite spacers,
or equivalent, (0.32 0.64 1.27 cm [1/8 1/4 1/2 in]) are placed at these marked points.
The heater mount cavity is repacked with new No. 30-grit aluminum oxide after each test.
Suitably small portions of the oxide are added in increments, followed by brisk tapping of the
mount exterior to eliminate voids. After adding a portion of the oxide, the mount is raised to the
vertical position and tapped with a compositon head hammer over front and rear surfaces
working from bottom to top. The procedure is repeated at frequent intervals until the cavity is
filled.
NOTE 2: Prior to installation of the heater mount and test-bearing assembly, the oil-jet
alignment procedure of 6.2.5 is accomplished.
6.2.3.2 Test-bearing heatermount positioning. Care is exercised to insure that the front
and rear vertical faces are parallel to each other and square with the I.D. of the mount. If these
conditions are not maintained, the concentricity of the bearing's outer race in relation to the shaft
180
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