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A few relevant points on the 4 Ball method under ASTM D4172:
1. This method is for "fluids" (unspecified), not grease, and therefore is valid for motor oils:
1. Scope
1.1 This test method covers a procedure for making a
preliminary evaluation of the anti-wear properties of fluid
lubricants in sliding contact by means of the Four-Ball Wear
Test Machine. Evaluation of lubricating grease using the same
machine is detailed in Test Method D 2266.
2. The ASTM method calls for 1200 rpm, not the 1800 rpm used in the Amsoil paper:
4. Summary of Test Method
4.1 Three 12.7-mm [1⁄2-in.] diameter steel balls are clamped
together and covered with the lubricant to be evaluated. A
fourth 12.7-mm diameter steel ball, referred to as the top ball,
is pressed with a force of 147 or 392 N [15 or 40 kgf] into the
cavity formed by the three clamped balls for three-point
contact. The temperature of the test lubricant is regulated at
75°C [167°F] and then the top ball is rotated at 1200 rpm for
60 min. Lubricants are compared by using the average size of
the scar diameters worn on the three lower clamped balls.
3. The Repeatability of the ASTM 4172 method is 0.12 mm:
11.1.1 Repeatability
The difference between successive results obtained by the same
operator with the same apparatus under constant operating
conditions on identical test material would, in the long run,
in the normal and correct operation of the test method, exceed
the following value only in one case in twenty:
Repeatability = 0.12 mm scar diameter difference
This means that all results between 0.35 mm and 47 mm for a single run are considered the same.
Source:
http://www.dieseltechsolutions.com/files/FE26PremierTestBooklet.pdf
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Significance and Use
This test method can be used to determine the relative wear preventive properties of lubricating fluids in sliding contact under the prescribed test conditions. No attempt has been made to correlate this test with balls in rolling contact.
The user of this test method should determine to his own satisfaction whether results of this test procedure correlate with field performance or other bench test machines.
1. Scope
1.1 This test method covers a procedure for making a preliminary evaluation of the anti-wear properties of fluid lubricants in sliding contact by means of the Four-Ball Wear Test Machine. Evaluation of lubricating grease using the same machine is detailed in Test Method D 2266.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
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Four Ball Wear Test ASTM D4172
The Four Ball Wear Test ASTM D4172 evaluates an industrial gear lubricant’s wear-preventing properties in steel-on-steel sliding conditions. This test is not intended to predict wear characteristics with other metal combinations other than steel-on-steel and does not measure the extreme pressure characteristics of the industrial gear lubricant. The Four Ball Wear Test ASTM D4172 utilizes the same test apparatus and set-up as the Four Ball Extreme Pressure Test ASTM D2783. The test is run at 1,200 rpm under a 40 kgf load at 167ºF (75ºC) for one hour. After the one-hour test period, the three stationary balls are washed and the wear scars are measured to the nearest 0.1 mm. Another set of measurements across each scar, 90 degrees from the first, is then made. The average of the six measured scars in millimeters is reported as the average scar diameter.
The lower the scar diameter, the higher the level of wear protection the industrial gear lubricant will provide under sliding conditions. When comparing the capabilities of industrial gear lubricants, the results obtained in both the ASTM D2783 Four Ball EP and the ASTM D4172 Four Ball Wear Test should be considered. Some industrial gear lubricants that have good extreme pressure properties may not be effective in reducing wear rates at lower loads.1
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"The 4-ball test (ASTM D4172) is often used as a screening test for many different lubricant types that contain antiwear additives or similar base oil properties. Other tribo-mechanical bench tests are often used as well, including the Timken Test (ASTM D2782) and the Pin and V-Block (ASTM D2670). Because engines have different contact geometry, loads, metallurgy and speeds, numerous bench tests and test protocols are needed.
It is not uncommon for several oils to be tested using two such methods and to find that the performance rankings between the oils to reverse (no correlation). This is why, among other reasons, Passenger Car Motor Oils and Heavy Duty Oils (diesel crankcase) are tested in actual engines using controlled methods such as ASTM D5533 Sequence IIIE and D5302 Sequence VE.
Jim Fitch, Noria Corporation"
