constitutes the bearing point of the spring 20 which is a conventional coil spring in the fig2 example the slide member comprises a longitudinal slot 25 and a locking member in the form of a screw 26 fixed into the wall of said casing projects into the longitudinal slot 25 the movement of the slide member within the casing is therefore limited in a simple way by the cooperation of the screw 26 and the slot 25 as shown in fig3 a the slide member 18a may be a u shape channel member defining a groove inside which is fixed at least one abutment member for example a ball 28 welded to the interior of the section as previously the locking member is a screw 26 in another embodiment fig3 b the slide member 18b incorporates a lateral cavity 30 in the form of a transverse groove defining two opposed abutment members 31a 31b cooperating with the screw 26 which projects into the cavity in all the foregoing examples the casing 22 is preferably covered with a tubular sheath 34 of rubber or a like elastomer material the embodiment of fig4 through 6 has the advantage of being suitable both.