FED-STD-791D
torque data obtained from this test method agree with the low temperature performance
of military greases in the field. A correlation was found between the breakaway torque
and the apparent viscosity range of 5,000 - 30,000 poises at a shear rate of 25 reciprocal
seconds.
5. DEFINITIONS
5.1 Breakaway torque. The maximum torque measured at start of rotation.
5.2 Running torque. The torque value after rotation for a specified period of
time (5 minutes).
6. APPARATUS
6.1 Low temperature torque test unit. This test unit consists of an ASTM D
4693 test apparatus which is designed with a cold chamber and two spindle drive
systems, torque measuring system, test specimen assembly, and data acquisition and
control system. The schematic diagram of US Army low temperature torque test unit is
shown in figure 1.
6.1.1 The low temperature test chamber consists of a cascade mechanical
refrigeration system, a blower, and a solid state electronic temperature controller. This
chamber is designed to maintain the test temperature down to -58 C. The chamber
temperature is monitored with a T type thermocouple.
6.1.2 The spindle drive system consists of a 1/3 hp electric motor, a timing belt
and pulley, a gear reducer and a solid stainless steel extension shaft (diameter: 2.2 cm,
length: 25.6 cm). The gear reducer provides 84 N.m of maximum output torque and
approximately 1 rpm of fixed speed. Two spindle drive systems are mounted externally
and connected to the inside of the cold chamber using a solid coupling and extension
shaft, respectively. These shafts are supported by two sealed roller bearings which are
lubricated with MIL-G-23827 grease. Only the outside supporting bearings are rotated
with the extension drive shafts during the test time.
6.1.3 The torque measuring system consists of a strain-gage load cell (capacity:
136 kg), a cylindrical rod (diameter: 1.9 cm length: 22.5 cm), a load button and a DC
power supply (capacity 20 V). Two torque measuring systems are mounted on a plate
which is situated below and external to the cold chamber. To measure the grease torque,
rods are connected to the inside of the cold chamber with the load cell. A spherical load
button is attached to the top of the rod to allow for the adjustment of the level of the
torque arm. A DC power supply is connected to both load cells and applies 10 volts for
the input bridge excitation.
6.1.4 The test specimen assembly is as specified in the ASTM D 4693 test
method.
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