Abstract: Each of two parts has a friction surface engaged under a force against a friction surface on the other to inhibit relative motion between the parts in one direction. One of the parts has at its friction surface strip-like hard zones which extend transversely to that direction and which are laterally spaced apart to have a strip-like soft zones between them. The material of that one part is substantially harder in the hard zones than in the soft zones. The material of the other part, at its friction surface, is of such hardness that under the force urging the friction surfaces together, the differences in hardness as between the hard zones and the soft zones of the one part tend to deform the friction surface of that other part into undulations that increase friction in the mentioned direction without substantially increasing friction transversely to that direction.