Contact surface pressure P increases rather drastically in proportion to skew angle .theta. within the range of 0<.theta..ltoreq..theta..sub.T, but because the critical skew .theta..sub.T is restricted to be small, the contact surface pressure P changes in a low range below a certain level P.sub.0 at which wear occurs in contact portions. Thus restricting the critical skew angle .theta..sub.T to a small angle reduces the contact surface pressure P and suppresses heat generation and wear in the contact portions.