Abstract: A shaft (12) rotatively mounted in a support (14) by rolling elements comprising at least one first rolling bearing (18) providing an oblique contact surface axially biased in one direction by an elastic return force and in a direction opposed to the preceding direction by a thrust block (28) carried by the shaft (12). The block (28) is axially movable between a first position and a second position of application of a force opposed to the return force. The magnitude of the opposing force is greater in the second position of the block (28) than in the first position of this block (28). The block (28) is connected to the shaft by a connection (32) which is cooperative by the effect of friction with the shaft (12).