Table of Contents

What is Hydraulic Motor OR Rotary Actuator | Types , Application

Introduction

Hydraulic motors are rotary actuators. However, the name rotary actuator is reserved for a particular type of unit that is limited in rotation to less than 360 Degree . A hydraulic motor is a device which converts fluid power into rotary power or converts fluid pressure into torque. Torque is a function of pressure or, in other words, the motor input pressure level is determined by the resisting torque at the output shaft. A hydraulic pump is a device which converts mechanical force and motion into fluid power. A hydraulic motor is not a hydraulic pump when run backward. A design that is completely acceptable as a motor may operate very poorly as a pump in a certain applications.

About Rotary actuator or Hydraulic Motor :

The function of a rotary actuator is to convert hydraulic energy into rotary mechanical energy.
Rotary actuators are the hydraulic equivalents of electric motors. Hence rotary actuators are also called as hydraulic motors.
The hydraulic motors are very much identical in construction and size to rotary type pumps. They work on exactly the reverse principle, to that of rotary pumps.
Instead of pushing the fluid as pumps do, in a hydraulic motor the rotating elements (z.e., vanes, gears, pistons, etc.) are pushed by the pressurized fluid. This enables the hydraulic motor to develop the necessary output torque and rotating motion.
The hydraulic motors are usually rated/specified in terms of the torque developing capacity or differential pressure.

Applications of Hydraulic Motor :

Hydraulic motors have become popular in industries. Hydraulic motors can be applied directly to the work.They provide excellent control for acceleration, operating speed, deceleration, smooth reversals and positioning. They also provide flexibility in design and eliminate much of bulk and weight of mechanical and electrical power transmission. The applications of hydraulic motors in their various combinations
with pumping units are termed hydrostatic transmission.

A hydrostatic transmission converts mechanical power into fluid power and then reconverts fluid power into shaft power. The advantages of hydrostatic transmissions include power transmission to remote areas, infinitely variable speed control, self-overload protection, reverse rotation capability, dynamic braking and a high power-to-weight ratio. Applications include material-handling equipment, farm tractors, railway locomotives, buses, lawn mowers and machine tools.

New fields of applications are being discovered constantly for hydrostatic transmissions. Farm implements, road machinery, material-handling equipment, Numerical Control(NC) machines high performance aircrafts, military uses and special machinery are only a few of new fields expanding through the use of fluid power transmission. Many automobiles, railway locomotives and buses use a hydrostatic transmission.

Comparison Between a Hydraulic Motor and an Electric Motor

Electric Motor	Hydraulic Motor
Electric motors cannot be stopped instantly. Their direction of rotation cannot be reversed instantly. This is because of air gap between the rotor and stator and the weak magnetic field.	Hydraulic motors can be stalled for any length of time. Their direction of rotation can be instantly reversed and their rotational speed can be infinitely varied without affecting their torque. They can be braked instantly and have immense torque capacities.
Electric motors are heavy and bulky	Hydraulic motors are very compact compared to electric motors. For the same power, they occupy about 25% of the space required by electric motors and weigh about 10% of electric motors.
Moment of inertia-to-torque ratio is nearly 100.	Moment of inertia-to-torque ratio is nearly 1.

Classification of Hydraulic Motors

There are two types of hydraulic motors:

(a) High-speed low-torque motors and

(b) low–speed high-torque motors.

In high-speed low-torque motors, the shaft is driven directly from either the barrel or the cam plate, whereas in low-speed high-torque motors, the shaft is driven through a differential gear arrangement that reduces the speed and increases the torque.

Depending upon the mechanism employed to provide shaft rotation, hydraulic motors can be classified as follows:
1. Gear motors.
2. Vane motors.
3. Piston motors:

Axial piston-type motors.
Radial piston-type motors.

Gear motors are the least efficient, most dirt-tolerant and have the lowest pressure rating of 3.

Piston motors are the most efficient, least dirt-tolerant and have high pressure ratings. Vane and piston motors can be fixed or variable displacement, but gear motors are available with only fixed displacement.