ASTM D4693 for low-temperature torque of grease-lubricated wheel bearings
ASTM D4693 standard test method for low-temperature torque of grease-lubricated wheel bearings
8. Calibration
8.1 Torque Calibration:
8.1.1 Place the load cell in the cold chamber at -40°C and allow it to come to temperature equilibrium. Electrically null the load cell and recording potentiometer using the adjusting potentiometer of the matching network. Adjust or standardize the range potentiometer as needed to achieve the desired zero-point and full-scale calibration. Select a millivolt range on the recording potentiometer, and record pen displacement as a known force is applied to the load cell. (It may be necessary to fabricate a platform to attach to the load cell to support the calibrating weights.) Apply several forces (up to about 200 N or more) in stepwise fashion while recording pen displacement as a function of applied force. Calibrate the other millivolt ranges in similar fashion until limited by either maximum displacement or maximum load. Remove forces stepwise to determine possible hysteresis.
8.1.2 Construct a plot of pen displacement as a function of force for each millivolt range. Use these plots for the subsequent determination of torque for the test greases. Ordinarily, the plots will be linear, and simple conversion factors can be calculated.
NOTE 5 - Data logging software may be uesd to collect this data.
8.1.3 This calibration need be done only at the time of initial setup and when occasional checks indicate that it is required. However, the torque measuring system is to be standardized before each test.
8.2 Temperature Calibration:
8.2.1 In order to obtain precise torque measurements, an accurate temperature calibration is essential. Meticulous attention to the details described in the several ASTM methods of temperature calibration is of paramount importance.
8.2.2 Calibrate the temperature-measuring system, including both the potentiometer and the spindle thermocouple, at 0°C and -40°C by comparing the observed temperature with that of the certified thermometer as described in Test Method E 220. Alternatively, both thermometer and thermocouple can be calibrated using the freezing point of mercury as described in Test Method E 77. The reference ice bath is to be made in accordance with Practice E 563.
NOTE 6 - A suitable low-temperature liquid bath can be made with a 65 volume per volume percent commercial, automotive-type, ethylene glycol antifreeze (7.2) in water.
8.2.3 The temperature correction determined by the thermocouple calibration shall be applied when measuring the temperature of the spindle.
8.3 Spring Calibration:
8.3.1 Remove the spring (Part 8, Fig. 3), and outer and inner compression plates (Parts 9 and 11, Fig. 3) from the test unit.
8.3.2 Reassemble spring and compression plates in correct order. Apply a force of 400 N and measure the distance, +/- 0.03 mm, between the compression plates. Since the distance between the plates varies slightly around the circumference of the plates, make the measurement at the point of shortest distance. Mark the spring and both plates to ensure repeatable alignment when assembling the test unit.
8.3.3 Using the dimension determined in 8.3.2, construct a custom spring gage of equal length. When assembling the test unit, this gage is to be used to apply a 400 N load.
NOTE 7 - Some low-temperature torque-test apparatus have been supplied with a metal cylinder to be used to calibrate the spring. A small number of these have been found to be incorrect. If an apparatus includes such a cylinder, its calibration should be certified by the manufacturer or verified by the test operator before use in this test. The fabrication and use of a custom spring gage is preferred.
