Ring Cam working surface compression

A fluid-working machine ring cam comprises plural segments having piston facing surfaces defining a working surface. The segments comprise leading and trailing end cooperating formations each having a working surface potion, which each interlock and rollers are thereby handed over smoothly from one segment to the next irrespective of alignment tolerance or wear variations. The segment piston facing surfaces are placed in compressive stress 112 by providing an unmounted segment 100 with greater curvature 102 than its support 104 and bolting down to remove a formed gap 106 and thus partially or fully compensate for tensile stress arising from the action of rollers. The segments form a wavelike cam surface describing a portion, x, of the wave. The underlying segment has a curvature, y, as a fraction of 360 degrees and x is not an integral multiple of y. Cross apertures 114 provide greater compressibility, assisting in providing compression. Segment attachment is provided through the working surface on whichever of the leading or trailing surfaces thereof is subject to lowest forces from pistons in use.