Some Measurements of Friction in Simple Powder Beds

An apparatus and procedure for measuring the static friction coefficient of powders in simple beds are described. The relative merits of three variations of the shear cell are considered. A sandwich of powder between two surfaces covered with sandpaper is satisfactory under most experimental conditions. Examples of the effect of relative humidity, time under load, mechanical vibration of the powder bed, and variations in the experimental procedure are presented. Reproducible values of the friction coefficient can be obtained readily if the experimental and environmental conditions are standardized. The results of the experiments suggest that a single value of the friction coefficient applies only to a specific powder bed. In practical applications the entire range of values that may be encountered must be within acceptable limits.