fairly linear force acting on the system attention is next directed to fig2 of the drawings there the coil spring 24 is shown about the mandrel 20 the mandrel 20 extends the full length of fig2 except that the central portions thereof are omitted for sake of clarity to show details of the arm construction further the lower portion of fig2 shows the coil spring 26 which is bearing upwardly against the crosshead assembly to be described the topmost crosshead assembly is identified by the numeral 30 it is preferably identical to the bottom crosshead assembly identified by the same numeral they differ only in location and the two are spaced from one another both being telescoped on the exterior of the mandrel 20 that is they are received on the mandrel for sliding movement the sliding movement occurs upon deflection of the arms as will be described such movement may require the two crossheads to move closer together or further apart in the latter event the springs 24 and 26 are somewhat compressed if this occurs the crosshead assembly 30 is better shown in fig3 there the upper crosshead assembly has been disclosed to the degree that internal details of construction are revealed this internal construction is duplicated with the lower crosshead assembly the mandrel 20 again is shown in the upper view a spring collar 32 is located at the upper end of the crosshead assembly and has a protruding shoulder 34 which supports the coil spring 24 the collar 32 slides on the exterior of the mandrel it is forced downwardly and received inside a crosshead shell or housing 36 the shell or housing is closed at the upper end by means of an.