Hydraulic motors are part of hydrostatic power transmission systems. Their purpose is to convert hydraulic energy into mechanical energy. Depending upon specific applications, these motors prove more efficient, suitable and economical over their electrical or pneumatic counterparts.

The General Construction of Hydraulic Motors Is Quite Simple And Uncomplicated

Hydraulic motors make use of fluid pressure for driving mechanical loads. A hydraulic motor cannot function as an isolated unit like an electric motor does.

It has to be part of a hydraulic circuit that incorporates a hydraulic pump along with other hydraulic gadgetry such as valves, filters, high-pressure hoses, metal tubing, hydraulic fluid reservoir etc.

The pump draws hydraulic fluid from the reservoir and supplies it under pressure to the hydraulic motor linked mechanically to the workload. The pump receives mechanical power for its operation through a prime mover that is either an internal combustion engine or an electric motor.

After going through the hydraulic motor, the hydraulic fluid is returned to the reservoir, filtered, and reused as required. Hydraulic motors are fluid power actuators capable of delivering linear or rotary motion depending on their design.

They are fast becoming popular for a variety of technical applications. Due to their inherent advantages they are being preferred to replace electrical motors or systems incorporating complex mechanical linkages as also for fresh applications.

Where electric motors, which can deliver only rotational power and must be sized to suit the load application, hydraulic motors are much smaller in size even when the application involves heavy loads. In a heavy electromechanical system a big electric motor needs to be directly located on the motion axis which may not be always feasible. It would also require a continuous supply of electricity usually from an external source.

For the same application, a relatively small hydraulic motor can be placed with ease and connected to a pump located remotely within the system through an arrangement of high-pressure flexible hoses that can be conveniently routed even through disadvantageous twists and bends.

Types Of Hydraulic Motors

Hydraulic motors delivering rotary power are mainly of two types and are classified on torque and rotational speed. One is referred to as HSLT or High Speed Low Torque and the other as LSHT or Low Speed High Torque motor.

The LSHT motor can have a speed range from 0.1 to 1000 revolutions per minute whereas HSLT motor speeds can range from 1000 to 5000 revolutions per minute.

The size advantage can be gauged from the fact that the size of a 5hp hydraulic motor will be roughly that of a 350ml beer can. In addition, there would be very low level noise and vibration generation and much higher efficiency. HSLT and LSHT hydraulic motors are available in different types.

Three types that find popular use are piston, gear and vane type hydraulic motors. Piston motors, which use reciprocating pistons to transmit energy to the workload, are basically of two types.

a. Axial piston motors

b. Radial piston motors

The axial piston motor is of the 'swashplate type' and has a bank of cylinders arranged in a circle (360 degrees) parallel to each other. Each cylinder has a piston, which reciprocates with one end of the piston pushing against an eccentric swash-plate located at one end of the bank of cylinders. There is a mechanical arrangement through which the eccentric plate is connected to an output shaft that is axially aligned with the cylinders. During motor operation, the cylinders are filled with high-pressure hydraulic fluid in a particular sequence making the pistons move outwards to push sequentially against the swash-plate causing it to rotate. On the return stroke of the piston the fluid is swept back at low pressure to return to a reservoir. The operation imparts rotational movement to the output shaft, of which one end is connected to the swash-plate and other to the workload. This is a design that caters to a very compact cylindrical hydraulic motor. Most axial hydraulic motors are HSLT.

A radial piston hydraulic motor has a bank of cylinders arranged like a car engine with a series of pistons riding on cams along a camshaft, which is attached to the output shaft. The reciprocating movement of the pistons gives rotary movement to the camshaft/output shaft that is tapped for power. In another variation cylinders are arranged radially like that of an aircraft engine with the pistons moving inwards to push against a cam located in center causing it to rotate. The cam is mechanically linked to the output shaft/workload. Yet another type of radial piston hydraulic motor with cylinders placed radially like an aircraft engine has the pistons moving outwards to push against cams in a housing that surrounds the motor. This makes the housing rotate. The rotating housing is tapped for power. These motors are generally used as wheel motors and for other suitable applications like forklifts etc.

Gear type hydraulic motors can be classified as internal gear or 'gerator' type and external gear motors. Gerator motors are very quiet in operation and designed to transmit rotary power through an output shaft connected to a rotor moving inside an outer stator. Supply of hydraulic fluid under pressure makes the rotor move eccentrically along the inner periphery of the stator. An external gear hydraulic motor has a set of meshing gears enclosed in a sealed housing have passages supply and return of hydraulic fluid. Pressurized hydraulic fluid flowing into the housing has an action on the gear teeth and makes the gears rotate. The rotational movement of the gears is transmitted to the workload through an output shaft connected to the rotating gears and passing through the motor housing.

Vane type hydraulic motors have movable vanes connected to a centrally located output shaft. The whole arrangement is enclosed in a housing/ case that receives hydraulic fluid under pressure from the pump. This fluid exerts force of the vanes to make them move like fan blades. This action results in rotating the output shaft, which is tapped for power.

Hydraulic Motor Applications

Hydraulic motors are mainly used in construction and agricultural equipment. They can be commonly seen in heavy earth moving equipment like excavators, skids, forklifts, heavy dumper trucks, bulldozers etc. where hydraulic cylinders extend and retract as vital operational parts while the machine performs various work. These cylinders are hydraulic motors transmitting linear power.

Due to the high torque at low speeds, loaders and other construction equipment use heavy hydraulic motors to drive the wheels for moving the machines around. There is one motor for each wheel and the diesel engine is used to drive the pump, which deliver hydraulic fluid to the motors. A hydraulic motor with the right specifications needs to be fitted to enable the machine to function properly.

Due to the very heavy loads imposed on car crushers, hydraulic motors are the preferred power delivery system. When the machine stalls' due to something 'uncrushable' creating a snag, the building pressure inside the supply circuit of the hydraulic operates a pressure relief valve preset to a specific pressure to return the supply of the fluid back to the reservoir. It also triggers a mechanism to reverse the direction of rotation to release the snagging materiel and free the machine after which normal rotational direction is resumed. An electric motor would either trip or burn out.

Compact and extremely efficient, small hydraulic motors can be used for various machining operations like boring, reaming, drilling etc. Due to their small size they are tools of choice for applications like: