Various tests can be used to determine the characteristics of lubricating fluids for manual transmissions and differentials such as ASTM D 5570 (evaluation of the thermal stability of MT lubricants in a cycle durability test), the ASTM D 5704 (evaluation of the thermal and oxidative stability of lubricants for MT and final drive axles), and the CEC L-37-T-85 using the FZG rig.
Since most lubricants require a Viscosity Index Modifier or VII to meet a targeted VI, one needs to know how much viscosity loss will occur after VII shearing.
Most shearing tests involve rotating a cylinder, a cone-shaped pintle, or a bearing assembly with the test oil added under specific rpms, temperatures, and loads. These rotating assemblies shear the VII molecules at a rate of so many shears per second. Some VII’s result in a permanent loss, some only result in a temporary loss.
Neither the base oil nor the additive package will shear to any extent.
The KRL taper roller bearing test method is concerned with the determination of the mechanical shear stability of lubricating oils with polymer additives, such as MT or gear oils. For this test a tapered roller bearing assembly is used to shear the fluid. The purpose of the test is to determine the drop in viscosity caused by mechanical shearing stresses under practical conditions.
The DIN 51350/6 Testing of Shear Stability of Lubricating Oils Containing Polymers conforms to the CEC L-45-T-93 Viscosity Shear Stability of Transmission Lubricants test using the Taper Roller Bearing Rig, or equivalent rig.
For a typical test, about 40mL of test lubricant is placed in the testing machine and the temperature is brought up to 60C and rotated at approx. 1475 rpm with a load of 5,000N or 510 kgf for 4, 8, or 20 hours.
So when viewing specifications, one should determine how long the test was conducted.
After shearing, the Shear Stability Index (SSI), in percentages, of the formulated lubricant is determined by:
(Viscosity Loss of sheared fluid)/(Polymeric Viscosity before shearing) X 100.
For example:
I start out with a base oil viscosity of 6 cSt. I add a certain amount of VII to get to the final “adjusted” viscosity of 15 cSt. The VII’s contribution to the overall viscosity is thus a plus 9. After the shearing test the viscosity falls to 13.0 cSt, a drop of 2 cSt. The SSI is thus 2/9X100 = 22.2%.
For HD transmission lubricants and differential lubricants the tests would be run according to CEC L-45-A-99 Viscosity Shear Stability of Transmission Lubricants again using the Taper Roller Bearing Rig.
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