The Variation of the Coefficient of Static Friction with the Rate of Application of the Tangential Force

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Abstract
The coefficient of static friction has been measured for a number of different materials under impact conditions. An analysis of the conditions of impact, derived from collision between solid bodies, and from a suitable hydraulic system, showed the latter to be the more convenient and equally effective method of applying tangential force. The method adopted consisted of feeding oil at a constant rate into an oil cylinder, the piston of which constituted the striking head. The friction members were so arranged that the struck member was in the form of a bar that was clamped between two massive plates. The normal force was applied hydraulically and the tangential force was measured by wire resistance strain-gauges. The static friction was measured for rates of application of the axial force up to 103 tons per sec. Under dry conditions the friction increased with the rate of applied force, an effect more pronounced between a non-metal and a metal than between metals. Lubrication with oil diminished the variation of friction with rate of application of force for all combinations of materials tested. The magnitude of this effect was sufficient to explain a type of failure commonly encountered with safety clutches, and similar mechanical devices.

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