The Hydraulic motor described here
(pdf) is a radial-piston type motor. The motor has a rotating cylinder
shaft and stationary housing. The cylinder block is mounted in fixed roller
bearings in the housing. Even numbers of pistons are located in bores
inside the cylinder block. The valve plate directs the incoming and outgoing
oil to and from the working piston. Each piston is working against a cam
roller. Because of the hydraulic pressure the pistons pushes the cam rollers
when going down on the cam ring and transmits no power when going up.
This principle produces torque and therefore the shaft starts rotating.
