and 24 b are of sawtooth shape with steep and shallow flanks and are complementary to respective faces 27 a and 27 b of the side members 21 a and 21 b the steep and shallow flanks of the sawteeth of the faces 25 a and 25 b are oriented relative to the directions u 1 and u 2 such that movement of the shoes 24 a and 24 b in the direction u 2 relative to the extension 17 a cams the shoes 24 a and 24 b toward each other and into tighter engagement with the extension 17 a while opposite movement in direction u 1 moves them apart and releases the extension 17 a more particularly as shown in fig1 the brake shoe 24 a has oblique faces 33 a and 35 a that interact with respective opposing faces 34 a and 36 a of the side member 21 a analogously the brake shoe 24 b has oblique faces 33 b and 35 b while side member 21 b has respective opposing faces 34 b and 36 b that interact in the same way the faces 33 a 34 a 35 a and 36 a as well as faces 33 b 34 b 35.