about hydraulic motor

Ryszard Daniel, Tim Paulus, in Lock Gates and Other Closures in Hydraulic Projects, 2019

11.3.7 Hydraulic Motors

There are three types of hydraulic motors: gear, piston, and vane. Gear Motors are compact and provide continuous service at rated power levels with moderate efficiency. They have a high tolerance for contamination of the hydraulic oil which is a consideration for applications in dirty environments. External gear motors consist of a pair of matched gears enclosed in one housing. Both gears have the same tooth form and are driven by the pressurized fluid. One gear is connected to an output shaft and the other to an idler. Pressurized fluid enters the housing at a point where the gears mesh. It forces the gears to rotate, and follows the path of least resistance around the periphery of the housing. The fluid exits at low pressure at the opposite side of the motor. Close tolerances between the gears and the housing help to control fluid leakage and increase volumetric efficiency. There are several variations of the Gear Motor, including the gerotor, differential gear motor, and roller-gerotor. All of these variations produce higher torque with less friction loss.

All hydraulic piston motors are available in fixed and variable volume versions. The most common type of hydraulic motor available is the axial piston type. Axial piston hydraulic motors have high volumetric efficiency. This allows steady speed under variable torque or fluid viscosity conditions. Axial piston hydraulic motors are also among the most adaptable to variable loading conditions. They are available in two basic design types including swash plate and bent axis. The swash plate design is the most commonly available but the bent axis design is the most reliable and generally more expensive.

Radial piston hydraulic motors have a cylinder barrel attached to a driven shaft and can usually produce more torque than axial piston hydraulic motors. They do have a limited speed range, however, and are more sensitive to hydraulic fluid contamination. The barrel contains a number of pistons that reciprocate in a radial bore. The outer piston ends bear against a thrust ring and pressurized fluid flows through a pintle in the center of the cylinder barrel to drive the pistons outward. The pistons push against the thrust ring and the reaction forces rotate the barrel. Motor displacement is varied by shifting the slide block laterally to change the piston stroke. When the centerlines of the cylinder barrel and housing coincide, there is no fluid flow and therefore the cylinder barrel stops. Moving the slide past center reverses direction of motor rotation. Radial piston motors are extremely efficient and rated for relatively high torque. In many USACE sector gate drives radial piston hydraulic motors are used. The hydraulic motor shown in Fig. 11.23 is a radial piston type and provides a torque of 260 Nm/bar. The rated speed is 50 rev/min. The hydraulic motor drives a pinion gear which in turn drives a rack gear on the sector gate.

Vane motors are compact, simple in design, reliable, and have good overall efficiency at rated conditions. They have limited low-speed capability, however. Vane motors use springs or fluid pressure to extend the vanes. Vane motors have a slotted rotor mounted on a drive shaft that is driven by the rotor. Vanes, closely fitted into the rotor slots, move radially to seal against the cam ring. The ring has two major and two minor radial sections joined by transitional sections or ramps. Vane motors generally use a two or four-port configuration. Four-port motors generate twice the torque at approximately half the speed of two-port motors. The high starting torque efficiency of vane-type motors makes them adaptable to hoist winch drives allowing the motor to start under high load. Vane motors provide good operating efficiencies, but not as high as those of piston motors. The service life of a vane motor typically is shorter than that of a piston motor.