FED-STD-791D
bearing and in addition helps to prevent the bearing inner race from turning on the hub during
operation.
6.2.4 End cover. Before installing the end cover on the rig, check the test oil jet tube
alignment to make sure it is aimed midway between the test bearing outer race and the cage. A
gasket, made from material to withstand high temperatures, should be installed under the end
cover.
6.2.5 Bench check of seal operation. After assembly of the bearing head, the screw
thread seal operation should be checked under a 2240-N (500-lb) radial load to the test bearing
by applying 352 gage kPa (51 psig) shop air pressure to the loading cylinder and hand rotating
the shaft to detect rubbing. If any rubbing occurs, the bearing head should be disassembled and a
slight amount of material removed from either the screw threads or the seal plate, depending on
the location and severity of the rubbing. The material is removed from the seal plate if the
rubbing is localized, and the screw thread is ground if the rubbing is severe and occurs over a
considerable area. On a new build, the seal plate may be coated with Prussian blue to determine
the area of contact. On later rebuilds, the rubbing can usually be detected without the use of the
dye. The seal should be checked for rubbing after every build.
6.2.6 Miscellaneous joint compounds. No pipe dopes, thread compounds, or gasket
sealants should be used in the assembly of the rig or test oil system. Teflon tape (no adhesive) is
an effective sealant when applied to pipe threads.
6.3 Rig installation and stand checks. After assembly, the bearing head is installed on
the stand. The test oil tank is filled with 7.6 L (2 gal) of the oil to be tested. This oil is heated to
121C (250F) in the tank and then circulated for 20 min through the test oil system, including
the bearing head. The oil is then drained from the tank and the bottom sections of the two oil
filter housings. Fresh test oil is then added to the tank until the fluid level is 1.3 cm (1/2 in)
above the baffle plate with the test oil pressure and scavenge lines filled in preparation for the
test.
6.3.1 A qualified MIL-L-23699 or MIL-PRF-7808 lubricating oil is used in the support
system and is not normally changed after each run. Drain periods on the support oil are
governed by the amount of deterioration as determined by laboratory tests and the degree of the
filter plugging in the support oil system.
6.4 Critical test items. Before starting a test, care should be taken that critical items such
as dimensional tolerances and heater electrical connections conform to those listed in Table II.
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