There are many different types of hydraulic motors.
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Most hydraulic motors can be categorised as either a: radial piston motor, hydraulic gear motor or hydraulic vane motor or axial piston/plunger type motor:
Radial piston motors: Generally, there are two basic types of radial piston motor, with either pistons pushing inward (crankshaft type) or pistons pushing outward (multilobe cam ring type).
Commonly found in excavators, cranes, winches and ground drilling equipment, these types of hydraulic motor are able to reach high torques at very low speeds. Radial piston motors can also be referred to as Low Speed High Torque (LSHT) motors.
Gear motors: If your application requires a lower speed, the output speed of the shaft can be reduced by using gears.
The operating pressure of the gears motors is typically very low – ranging between 100 and 150 bar. Some of the more modern gear motors are more capable of working at a continuous pressure of up to 250 bar. However, this motor can end up producing a large amount of noise.
Some of the typical features of this type of hydraulic motor include: lightweight and small unit, relatively high pressures, low cost, variety of speeds, broad temperature range, simple design, large viscosity range.
Hydraulic vane motors: Hydraulic vane motors are found in both industrial and mobile applications, such as screw-drive, injection moulding and agricultural machinery. These motors tend to have less internal leaking than a gear motor, so they are better to use in applications requiring lower speeds.
Some of the main features of hydraulic vane motors are: reduced noise level, low flow pulsation, high torque at low speeds, simple design, easy to service, suitable for vertical installation. For the vane motors to function correctly, the vanes of the rotor must be pressed against the inside of the motor housing. This can be done through spiral or leaf springs, however rods are suitable as well.
Typical technical information for the vane motor include a: displacement volume between 9 to 214 cc, maximum pressure of up to 230 bar, speeds ranging from 100 to 2,500 rpm, maximum torque of up to 650 Nm.
Axial piston motors: Axial piston motors work similarly to piston motors in which they use a bent axis design or a swash plate principle. The fixed displacement type works as a hydraulic motor, whilst the variable displacement type operates like a hydraulic pump. Fixed displacement motors can be used in open and closed circuits.
For the bent axis design, pistons move to and fro within the cylinder block bores. This movement is converted into rotary movement via the piston ball joint at the driving flange. For the swash plate design, pistons move to and fro in the cylinder block so it revolves, which turns the drive shaft via the connected cotter pin.
Gerotor motors: Low speed, high torque hydraulic Gerotor motors (LSHT motors) offer a high starting torque, large range of speeds with a continuous output torque with favourable power-to-weight ratio and a smooth operation, even at low speeds. The Gerotor motors also boast a robust design capable of enduring harsh working conditions.
This is by no means an exhaustive list of hydraulic motor terminology, so if you can’t find an answer to what you’re looking for then get in touch and we’ll do our best to help.
Motor displacement is the capacity of fluid required to turn the motor output shaft through one revolution. Displacements of in.3 or cm3 per revolution are the units usually found in a hydraulic motor. Hydraulic motor displacement may be fixed or variable. A fixed-displacement motor delivers continuous torque. Controlling the amount of input flow into the motor varies the speed. A variable-displacement motor offers variable torque and variable speed. With input flow and pressure constant, varying the displacement can alter the torque speed ratio to meet load requirements.
Torque output is conveyed in inch-pounds or foot-pounds. It exists as a function of system pressure and motor displacement. Motor torque ratings are specified for a precise pressure drop across the motor. Hypothetical figures show the torque available at the motor shaft, presuming there would be no mechanical losses.
Breakaway torque is the torque necessary to get a motionless load turning. Increased torque is needed to start a load moving rather than to keep it moving.
Running torque can refer to a motor’s load or to the motor. When it refers to a load, it specifies the torque needed to ensure the load keeps turning. When it refers to the motor, it identifies the actual torque that a motor can reach to keep a load turning. Running torque takes into account a motor’s inefficiency and is a percentage of its theoretical torque.
Starting torque as far as hydraulic motor terminology is concerned, refers to the capacity of a hydraulic motor to start a load. It indicates the amount of torque that a motor can reach to start a load turning. In some cases, this is significantly less than the motor’s running torque. Starting torque also can be displayed as a percentage of hypothetical torque. Starting torque for common gear, vane, and piston motors varies between 70% and 80% of hypothetical.
Mechanical efficiency is the ratio of actual torque delivered to theoretical torque.
Torque ripple is the difference between minimum and maximum torque distributed at a certain pressure during one revolution of the hydraulic motor.
Motor speed is a function of motor displacement and the volume of fluid transported to the motor.
Maximum motor speed is the speed at a particular inlet pressure that the motor can tolerate for a limited time without damage.
Minimum motor speed is the slowest, non-stop, uninterrupted rotational speed available from the motor output shaft.
Slippage is the leakage through the motor, or the fluid that flows through the motor without accomplishing work.