Frictional Properties of Automatic Transmission Fluids: Part I—Measurement of Friction–Sliding Speed Behavior

For a wet clutch in an automatic transmission to operate efficiently and smoothly, it is essential that the friction in the clutch contact be high over the whole sliding speed range and also increase with sliding speed. This is achieved by careful design, both of the friction disc material and morphology and of the automatic transmission fluid. This article describes the development and use of a new test rig, based on a mini-traction machine (MTM), to measure the impact on wet clutch friction versus sliding speed properties of a range of additive solutions as well as a fully formulated automatic transmission fluid (ATF). It is found that most organic friction modifiers reduce friction at low sliding speed, whereas succinimide dispersants and some sulfonate detergent additives raise friction over the whole speed range. Overbased and neutral detergents give almost identical friction curves, indicating that, although a calcium carbonate film is deposited on the steel ball when overbased detergent is present, this does not influence friction, which is controlled by the alkyl group on the sulfonate. When long- and medium-chain surfactants are blended in proportions likely to result in a mixed adsorbed film, the resultant blends give higher low-speed friction than either individual surfactant, suggesting that the homogeneity of the adsorbed film plays an important role in determining its low-speed friction behavior. In a companion article, Part II, these results are used in combination with other work and analysis to identify the mechanism by which formulated ATFs are able to provide high friction that increases with sliding speed (Ingram, et al. ( 1 )).