ASTM D5183 Determination of the Coefficient of Friction of Lubricants
ASTM D5183 Standard Test Method for Determination of the Coefficient of Friction of Lubricants Using the Four-Ball Wear Test Machine
10. Procedure
10.1 Thoroughly clean four test balls, clamping parts for upper and lower balls and the ball cup by first soaking in heptane for 1 min and then with sonic agitation for 10 s. Drain and rinse with fresh heptane.
10.1.1 Repeat 10.1 using acetone. Blow dry under a stream of nitrogen gas. After cleaning, handle all parts using only a fresh wipe. No trace of solvent should remain when wear-in lubricant is introduced and the machine assembled.

10.2 Tighten one of the clean balls into the spindle of the test machine.

10.3 Assemble three of the clean balls in the test-oil cup.

10.4 Pour the wear-in lubricant indicated in 7.2 into the test-oil cup to a level at least 3 mm (1/8 in.) above the top of the balls. Observe that this oil level still exists after the lubricant fills all of the voids in the test oil cup assembly.

10.5 Install the test-oil cup/three balls in the machine and avoid shock loading by slowly applying the test load 392 N (40 kgf).

10.6 Turn on the heaters and set controls to obtain 75 +/- 2°C (167 +/- 4°F). Heater voltage or offset on proportional controllers should be capable of bringing stabilized temperature within the prescribed limits.

10.7 When the test temperature is reached, start the drive motor that was previously set to drive the top ball at 600 +/- 30 rpm. Machines with automatic start using a proportional controller will start below the set temperature. Set the proportional band so that test machines start at temperatures 2°C (4°F) below the set point temperature.

10.8 After the drive motor has been on for 60 +/- 1 min, turn off the heaters and drive motor and remove the test-oil cup and three-ball assembly.

10.9 Measure the wear scars on the three lower balls to an accuracy of +/-0.01 mm by the following method described in 10.10.

10.10 Drain the test oil from the three-ball assembly and wipe the scar area with a tissue. Leave the three balls clamped and set the assembly on a special base of a microscope that has been designed for the purpose. Make two measurements on each of the wear scars. Take one measurement of the scar along a radial line from the center of the holder. Take the second measurement along a line 90° from the first measurement. If a scar is elliptical take one measurement with the striations and the other across the striations. Take care to ensure that the line of sight is perpendicular to the surface being measured. Average the six readings and report as wear-in scar diameter in millimetres. If the wear-in scar measures 0.67 +/- 0.03 mm, the following test sequence is performed; otherwise, the wear-in sequence is repeated using new specimen balls. (The wear-in scar can measure outside this range in different machines, but for a given machine, repeated result should be +/- 0.03 mm. The user should first determine the wear-in average for the machine.)

10.11 Add heptane to test-cup and let soak for 1 min with occasional agitation. Drain off heptane. Rinse inside of the cup with heptane using a squeeze bottle to remove entrained oil. Repeat soak and rinse stages twice. Rinse inside of the cup with two successive rinses of acetone using a squeeze bottle. Blow dry with nitrogen.

10.12 Wipe surface of top ball and entire surface of ball chuck assembly using a clean tissue dampened with heptane. Repeat with a tissue dampened with acetone. Blow dry with nitrogen.

10.13 Wipe wear scars on the bottom three balls and wear track on the top ball with a tissue dampened with methyl ethyl ketone.

10.14 Pour the sample to be tested into the clean test oil cup assembly, containing the worn-in test balls, to a level at least 3 mm (1/8 in.) above the top of the balls. Observe that this oil level exists after the lubricant fills all of the voids in the test-oil cup assembly.

10.15 Install the test-oil cup containing three balls in the machine and avoid shock loading by slowly applying a test load of 98.1 N (10 kgf).

10.16 Turn on the heater and set controls to obtain 75 +/- 2°C (167 +/- 4°F).

10.17 When the test temperature is reached, start the drive motor that was previously set to drive the top ball at 600 +/- 30 rpm. Run for 10 min. Do not allow test machine to stop. At the end of the 10 min interval, record the torque value. Increase the load 98.1 N (10 kgf) every 10 min until the friction trace indicates incipient seizure is occurring. Record the torque value at the end of each 10 min interval.

10.18 Measure the final wear scars on the three lower balls to an accuracy of 0.01 mm as described in 10.10 and note the appearance of the wear scar.

11. Calculation
11.1 Determine the coefficient of friction using one of the two following equations:
µ = 0.00223 fL/P
where:
µ = coefficient of friction,
f = friction force, grams force,
L = length of friction lever arm, cm, and
P = test load, kg.

NOTE 3 - Current test machines are supplied with a friction lever arm having a length of 7.62 cm (3 in.), and friction force is displayed in units of grams force. Therefore, for these instruments, µ = 0.0170 f/P can be used.

or
µ = 0.00227 fL/P
where:
µ = coefficient of friction,
f = friction force, N,
L = length of friction lever arm, cm, and
P = test load, kg.

NOTE 4 - For test instruments with a lever arm of 7.62 cm (3 in.) and friction force displayed in Newtons, µ = 1.73 f/P.

NOTE 5 - Friction force is read directly from the test machine and is the force measured at a distance equal to the length of the friction lever arm from center of rotation. Test machines may have friction force displayed as grams force or Newtons. Users should consult their operating manuals for their equipment to verify the calculation equation.