Ring cam ensuring smooth follower handover between segments

A ring cam for a fluid-working machine comprises plural segments 5, 7 having piston facing surfaces 15, 16 together defining a working surface. The segments comprise leading and trailing end cooperating formations each having a potion of the working surface, which each inter-engage with respective adjacent formations and which are recessed from the working surface towards their respective ends or cross at less than 180 degrees. The cooperating formations interlock and rollers are thereby handed over smoothly from one segment to the next irrespective of slight variations in alignment due to manufacturing tolerance or wear. The segment piston facing surfaces are placed in compressive stress to partially or fully compensate for tensile stress arising from the action of rollers in use. The segments form a wavelike cam surface and attachment means 3 are provided, through the working surface on whichever of the leading 70 or trailing 72 surfaces thereof is subject to lowest forces from pistons in use.