Friction coefficients in a longitudinal direction between the finger pad and selected materials for different normal forces and curvatures

Abstract

This study investigated the effect of object curvature, normal force and material on skin friction coefficient. Twelve subjects slid their middle fingertip pad against a test object with small (11 mm), medium (18, 21 mm) or large (flat object) radii of curvature, while maintaining a normal force of 1, 10 or 20 N. Tested materials were aluminium and four rubber hoses. The average friction coefficient was 0.6 for aluminium and 0.9 for the rubber hoses. As normal force increased from 1 to 20 N, the average friction coefficient decreased 46%. Friction coefficient did not vary significantly with object curvature. The citation of friction coefficient data requires careful attention to normal force levels with which they are measured, but not so much to object curvature between 11 mm and infinity. This study provides skin friction coefficient data that are needed for design of objects that are manipulated with the hands. The investigation of the effect of object curvature on skin friction coefficient has important implications to ergonomics practices as many objects handled in everyday activities have curved surfaces.

Figure 1.

Relationship between coefficient of friction (COF) and pressure (P in kPa) for the index finger in log scales using data from Bobjer et al. (1993)

Figure 2.

Expected contact area between the fingertip and an object as a function of object radius of curvature can be calculated using Equation 1 by Timoshenko and Goodier (1970). The radius of the fingertip is assumed to be 8 mm (Buchholz and Armstrong, 1991). The contact area is normalized to that for a flat object.

Figure 3.

Fingertip pad making a contact with a test object on top of a load cell (no friction force applied yet) (left), and fingertip pulled towards the body (right). The picture on the right side was taken right before the fingertip pad slipped on the test object. The longitudinal skin deformation was estimated as a horizontal distance between two black dots, one right below the LED and the other one at the contact between the finger pad the test object (right).

Figure 4.

Longitudinal skin deformation (see Figure 3) as a function of normal force for rubber hose A and B (up, ▲) and for aluminum (down, O)

Figure 5.

Surface character of hose A.

Figure 6.

Friction coefficient between the fingertip and rubber hoses (up) and between the fingertip and aluminum (down) for three different normal force (four rubber hoses and subject pooled)

Figure 7.

Friction coefficient as a function of normal force for rubber (filled symbols) and for aluminum (unfilled symbols) from three studies (“Δ”: Seo et al., accepted, “O”: the present study, “□”: Buchholz et al., 1988) in log scales