By Michael Carney, CFPHS, CIHM
A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder. A hydraulic motor is usually designed for the working pressure on both sides of the motor.
Gear motors are used in simple rotating systems. Their benefits include low initial cost, high rpm, higher tolerance to contamination, and durability. Gear motor failures are generally less catastrophic.
A hydraulic gear motor consists of two gears: the driven gear (attached to the output shaft by way of a key) and the idler gear. High pressure oil is ported into one side of the gears, where it flows around the periphery of the gears, between the gear tips and the wall housing, to the outlet port. The gears then mesh, not allowing the oil from the outlet side to flow back to the inlet side. For lubrication, the gear motor uses a small amount of oil from the pressurized side of the gears, bleeds this through the (typically) hydrodynamic bearings, and vents the same oil either to the low pressure side of the gears, or through a dedicated drain port on the motor housing. One very important gear motor feature is that catastrophic breakdown is a lot less common than in most other types of hydraulic motors. This is because the gears gradually wear down the housing and/or main bushings, gradually reducing the volumetric efficiency of the motor. The gear motor can degrade to the point of near uselessness. This often happens long before wear causes the unit to seize or break down.