rotate relative to the working shaft 2 but the second mass 9 is prevented from rotating relative to the housing 4 by means of a pair of parallel bars 10 the first mass 7 has a generally frustoconical outer surface 11 which mates with a corresponding frustoconical surface 12 on integral gear 3 such that when the frustoconical surfaces 11 12 are fully in contact with each other the cone angle which is around 15 u00b0 causes a relatively large frictional torque for a relatively small amount of axial force pushing the first mass 7 into contact with the integral gear 3 the cone angle is not less than the friction angle tan u2212 1 u03bc where u03bc is the coefficient of friction between first mass 7 and integral gear 3 as a result of which the first mass 7 does not become stuck in engagement with frustoconical surface 12 when the spring 8 exerts any traction force tending to pull the first mass 7 away from the integral gear 3 at the limiting value of this condition i e when the cone angle is exactly equal to tan u2212 1 u03bc the net frictional torque between the integral gear 3 and the first mass 7 has a maximum value of rf s where r is the mean radius of frustoconical surface 11 and f s is the compression force in the spring 8 the characteristics of the helical spring 8 are such that it causes a coupling between twist and axial compression extension deformation for some limited range of deformation.