FED-STD-791D
6.4.2.1 Conditions of test.
6.4.2.1.1 Test temperature. The test temperature shall be -40 " 3C (-40 " 5F) or -
55 " 3C (-67 " 5F), as specified. The test shall be started at room temperature (23 " 5C).
The drop in temperature from room temperature to the specified test temperature shall take place
in from 8 to 24 h, following the start of the test.
6.4.2.1.2 Pressure. The initial stroking pressure in the simulated brake system shall be
adjusted at room temperature (23 " 5C) to 3.4 gage MPa (500 psig), and recorded. The
pressure stroke pattern shall be adjusted at 60/40 percent stroke ratio, with the air of the short
cycle pressure recorder so as to produce a relatively smooth curve (see Figure 2). Accurate
record shall be kept of any pressure variations during the test.
6.4.2.2 Break-in or burnishing run. The stroking rate shall be adjusted at 1,000 " 50
strokes/h, using the stroking pressure of 3.4 gage MPa (500 " 10 psig) applied at room
temperature (23 " 5C). While at room temperature the system shall be stroked 16,000 " 500
strokes, at the end of which the system shall be checked for leaks, and proper brake shoe
clearance and the master cylinder checked for fluid level. The cylinder shall be filled to the
given level, if necessary. The stroking rate shall then be adjusted to 720 " 50 strokes/h, using a
40/60 percent stroke ratio. The pressure-stroke pattern shall be adjusted, with the aid of the
short-cycle pressure recorder, to give a relatively smooth curve (see Figure 3). Accurate record
of observed pressure variations shall be compared with the initial pattern at approximately every
20,000 strokes. The stroking cycle, if other than that listed in this paragraph, shall be specified
in the product specification.
6.4.2.3 Duration of test. The test shall be run until 94,000 strokes have been completed.
The number of strokes given in the burnishing run (see 6.4.2.2) and in coming to the specified
test temperature shall be included as part of the total 94,000 strokes. The number of strokes
required, if other than 94,000, shall be specified in the product specification.
6.5 Observations.
6.5.1 Observations during test.
6.5.1.1 Pressure change. Observations at 12-hour intervals shall be made of the stroking
pressure in the simulated system, to determine changes from the initial pressure.
6.5.1.2 Functioning of wheel cylinder pistons. The functioning of the wheel cylinder
pistons shall be observed. Improper functioning of the pistons, such as freezing of the piston in
the cylinder, or failure of the brake shoe to be released from the brake drum, shall be recorded.
However, very slight drag of the brake shoe on the drum, when the drum is rotated by hand, shall
not be considered as improper functioning of the cylinder pistons. Mechanical defects of the
brake mechanism may cause excessive pressure rise or improper functioning of the wheel
cylinder pistons. If inspection of the malfunction shows this to be the case, the test shall be
repeated using new parts.
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