Friction and discomfort in the design and use of hand tools : exposure to textures at different loads and velocities with reference to contamination
The skin of the palm of the hand has no friction. It is only when in contact with other objects that frictional forces appear. This friction does not, however, conform to the classic laws of friction. The thesis shows that static skin friction is lower than dynamic friction, and that increased velocity increases the coefficient of friction, but increasing load reduces it. Consequently, references to coefficient of friction where palm skin is one of the friction partners require velocity, surface pressure and skin conditions to be specified in addition to contaminants in the friction interface, before reliable conclusions can be drawn. Eleven textured and one non-textured samples all made from the same material were investigated using eighteen male subjects. They were exposed to five contaminants, three skin conditions, three levels ofload and velocities in the range 2-128 mm/so It was concluded that velocity in the friction interface is the most dominant factor contributing to palm friction. Only small, non-significant, differences in friction were found between different types of textures under non-contaminated conditions, but major, and significant differences were observed under contaminated conditions. Coarse textures increased discomfort. For static- and dynamic friction the type of texture, coarse or fine, will affect friction in different ways depending on the skin conditions being "clean" or "contaminated." Experiments show that coarse textures generate less friction than fine under the clean conditions. Under contaminated conditions however coarse textures generate more friction than fine. The highest coefficient offriction 1-1=2.22 (SO=1.12) was recorded under dynamic conditions for a clean hand on a non-textured surface when the surface pressure was low - 6.3 kPa (SO 2.1). The lowest coefficient offriction 1-1=0.05 (SO=0.03) was found under static conditions, with lard present on a non-textured surface when the surface pressure was high - 81.4 kPa (SO=31.0). Two regression models were developed. Regression coefficients are presented for surface topography variables as well as skin condition and contamination, velocity surface pressure and discomfort. Two new surface topography representations explain the generation of friction forces. The uppermost 5% of the volume of texture peaks provided significant information for transfer offriction forces.