spring collar 44 interposed therebetween in addition on the cover 38 of the actuator 31 there is a stop 46 for the spring collar 44 when this spring system 11 is placed under load initially the coil spring 36 is compressed until the spring collar 44 comes into contact with the stop 46 when this happens the stiffer disk spring 37 becomes effective and this results in progressive springing for the most part the embodiments that are shown in fig1 and 14 compare with those shown in fig1 and 12 the difference lies in the fact that instead of an elastomeric disk spring a spring 37 having a progressive spring rate is used in fig1 the elastomeric spring 37 is coaxial with the coil spring 36 with the spring collar 44 interposed therebetween fig1 shows an identical arrangement except that the cover 38 has a stop 46 for the spring collar 44 the manner in which this arrangement works is identical to that shown in the fig1 and 12 embodiments finally fig1 and 16 show two arrangements in which the spring system 11 consists in each instance of three springs in fig 15 three coil springs 36 37 and 50 are arranged coaxially above each.