Hydraulic Motors (Orbital): 36–395 cm³/rev Low-Speed High-Torque Motors

A hydraulic motor does the opposite of a hydraulic pump: where the pump turns mechanical rotation into pressure, the motor turns pressure into mechanical rotation. A cylinder gives linear motion; a hydraulic motor gives continuous rotation — conveyor drums, mixer impellers, crane winches, sweeper brushes and agricultural drive shafts all turn on hydraulic motors.

Most of the motors in stock are of the orbital (gerotor) type. In this design an inner gear tracks eccentrically inside an outer gear ring; because a large number of chambers fill and empty on each revolution, shaft speed is low and torque is high. That characteristic — direct drive — removes the need for a gearbox in most applications and saves considerable space.

Displacement sits at the centre of the selection: the volume of oil the motor swallows per revolution, in cm³/rev. The number in the product code is exactly that figure (HMR.100 = 100 cm³/rev). Speed follows from it: speed (rpm) ≈ flow (l/min) × 1000 ÷ displacement (cm³). So 40 l/min turns a 100 cm³/rev motor at roughly 400 rpm. If slower, stronger rotation is wanted at the same flow, choose a larger displacement; if speed is wanted, choose a smaller one.

The second determinant is torque, which comes from working pressure: torque ≈ pressure (bar) × displacement (cm³) ÷ 63. The third is the shaft and mounting geometry — plain, splined and keyed shafts, and flange hole patterns, decide whether the motor bolts onto the machine; this has to be specified alongside displacement when ordering.

When building a hydraulic motor circuit, leaving the return line free to tank matters, because it lets the motor draw oil during the moments it keeps turning under its own inertia; otherwise the motor cavitates internally. For the pump that drives the motor see «Hydraulic Pumps», and for direction and speed control see «Hydraulic Valves».