Basic Principle of Hydraulic Motor
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.