the spring always maintains a pre load set value in the preferred embodiment being illustrated which however is not intended to be limiting said limiting means are constituted by a first and second end cup respectively indicated with the reference numerals 2 and 3 each of which has a radially outward peripheral flange around the lip and are fitted into the corresponding end of the spring which engages against the flange the bottom wall of each spring cup 2 3 is identified with the reference numeral 6 and has a thickness much greater than that of each flange in order to prevent the end turns of the spring from defleting under load said bottom wall is provided with a through hole 7 in which is slidable a pin 8 provided at each end with a circlip 9 which prevents the spring cups 2 3 from coming off the pin itself the depth of each spring cup 2 3 is such that even when the spring 1 is fully compressed the ends of the pin 8 do not project out from the cups 2 3 on the other hand when released the spring 1 as indicated in fig5 is always held by the end cups 2 3 engaged against the circlips 9 to form a spring of length close to but slightly greater than the maximum length which the spring will be held to in use as shown in fig6 each of the end cups 2 and 3 has in an axial direction a length slightly less than half the length of the spring when fully compressed.