Abstract
A thorough survey has been made of the literature on static-friction measurement. It is shown that there are large discrepancies among the results of different investigators and it is concluded that the differences are largely due to lack of control of experimental technique. A complete bibliography is presented.
An experimental study of several of the more important variables influencing static friction is described. The variables studied are the effect of load, vibration, and cleanliness of the surfaces. An inclined-plane and a horizontal-plate apparatus are used. It is concluded that the method of cleaning the surfaces is the factor which has the greatest influence upon the results and it is shown that invisible films which may be deposited upon the surfaces during the cleaning process can exert large effects upon static-friction measurements.
A method of cleaning is described which involves polishing the surfaces with moist abrasive. This method yields high and reproducible values for static friction between unlubricated surfaces. It is shown that sulphide and oxide films on steel, copper, and brass produce a lowering in static friction and that the amount of lowering depends upon the thickness of the film.
A comparison is made between the static and the kinetic values obtained on several bearing metals lubricated with typical oils. It is shown that the static values are in general considerably higher than the kinetic values, and it is concluded that the differences which are observed are due in large part to viscosity effects in the kinetic measurements.
An analysis of static-friction measurements on typical lubricating oils leads to the conclusion that moisture and corrosion effects are responsible for many of the phenomena observed. It is concluded that static-friction measurements do not lend themselves to routine testing of oils, but that they can be used to considerable advantage for studying single variables influencing boundary lubrication.
