are thus enabled to telescope into and out of the sections b 39 and d 39 it follows then that in the position which the piston 5 assumes for its working stroke i e the position shown in fig1 2 the piston surface area exposed to the pressure medium equals the dimension w max shown in fig5 a conversely when the piston is collapsed i e has its surface area reduced to the minimum for the idle stroke compare fig3 4 the piston surface area exposed and to be moved against the opposition of to the pressure medium equals the much smaller dimension w min since the sections a 39 b 39 d 39 e 39 and element c in fig5 a are of identical dimensions in the plane of fig5 a and there are five of them of which four are telescopable in the illustrated manner the dimension w min will be equal to one fifth of the dimension w max which is to say that for the idle return stroke the piston surface area which needs to be moved against the opposition of the pressure medium is reduced to a mere fifth of the piston surface area which is exposed to the thrust of.