Shear Stability of Gear and Transmission Lubricants

While most mechanisms containing gears will tolerate a considerable variation in viscosity of the gear oil used for lubrication, a radical change in viscosity at a stated temperature, while in use is not desirable. Further, if such a change is due to a partial breakdown of an additive the purpose of the agent may be defeated. Such changes may occur due to shear while in service. The components most often affected are polymers such as V.I. improvers.

At present, products used in vehicles are the lubricants most often influenced by shear. Such changes will become increasingly important as a single fluid is used for several purposes such as a hydraulic fluid, for ATF, and perhaps as an axle lubricant. Further, a multirange gear and transmission oil has advantage in cars and trucks. That is, oil which will cover two or three SAE viscosity ranges.

Under present formulations some of the lubricants offered for the above services contain polymers as V.I. improvers. However, the action of gears or even pumps tends to change the polymers by shear. The shearing action causes either a chemical or mechanical breakdown of the large polymer molecules so that their value is largely lost in the oil. In some cases a viscosity decrease in service is temporary and in such instances there may be an alignment of the polymer molecules at high rates of shear. Of course some polymers or additives are more resistant to break down with shear than others but unfortunately those of high molecular weight, which prove the most effective V.I. improvers, are also most susceptible to loss of viscosity with shear.

Where high V.I. is necessary or desirable in gear oils, tests should be made to determine the viscosity after shear tests. This is most often done by using a test where a pump forces the lubricant through a sharp edge orifice for a stated time at a given temperature. A similar breakdown of polymers occurs with sonic shear, and a method using this procedure has also been used to evaluate the shear resistance of V.I. improvers.

Klaus and Fenske^34 tested fluids containing about 7 per cent of polymer for their permanent decrease in viscosity due to shear. After 5000 cycles in a pump at 100 degree (F) and a pressure drop of 800 psi, decreases of 25.5 to 30.5 per cent occurred. At a pressure drop of 1500 psi, the decreases were 38.5 and 40.5 per cent. The time required to stabilize viscosity will vary both with the mechanism and the fluid used.