There are three main types of hydraulic motors that include gear, vane, and piston types.
Hydraulic motors utilize fluid pressure in the operation of the mechanical loads. (See Reference 1) Hydraulic motors are unable to function separately from a circuit, pump, valves, filters, hoses, metal tubing and other hydraulic components. There are three major types of hydraulic motors that each operate within this same capacity but are unique in their own respects.
Gear motors
Gear motors come in two types, internal and external. Internal gear motors generally are constructed of an output shaft and an inner-outer gear set. (See Reference 2) The outer gear has an additional tooth compared to the inner gear, and the shape of these teeth makes it so the two gears are constantly in contact with one another. Motor housings for the motors have kidney-shaped inlet and outlet ports. In addition, the center rotations of both gears are separated by a predetermined amount called the eccentricity. In this design, the center of the output shaft and the inner gear are located in the same place. (see Reference 2) In contrast to the internal design, external gear motors are constructed of matched gears contained within a single housing. A similar tooth formation is seen in both gears and propelled by fluid pressure, with one gear connected to an output shaft and the other an idler. The force of the pressure fluid creates the rotation of the gears along the outside of the housing, and the fluid flows at a low pressure from the other side of the motor. (See Reference 2)
Vane motors
Vane motors are constructed of a slotted rotor affixed to the drive shaft, which is then operated through the rotor. (See Reference 2) The vanes move in a radial direction to seal the cam ring. This ring, with dual major and minor sections, is connected by transitional ramps or sections. Grooves and holes in the vanes are used to provide balance to the radial forces throughout the operation of the motor. (See Reference 2)
Piston
There are two types of piston motors, axial and radial piston motors. With the axial piston motor, the cylinders are in a 360-degree circle and are parallel to one another. (See Reference 1) Each individual cylinder contains a piston that "reciprocates with one end of the piston pushing against an eccentric swash-plate located at one end of the bank of cylinders." (See Reference 1) Through this arrangement of cylinders, the plate is connected to an output shaft that is "axially aligned with the cylinders." (See Reference 1) Generally seen in cylindrical hydraulic motors, axial piston motors are largely meant for compact designs. Radial piston motors are defined by the arrangement of the pistons on cams alongside the camshaft that is then attached to the output shaft. Using a reciprocal movement, the pistons create a rotary motion to the variant shafts to create power. Radial piston hydraulic motors are often seen in vehicle and airplane engines but are additionally useful in large equipment such as forklifts.
