Hydraulic motors are mechanical actuators that convert hydraulic pressure and fluid flow into rotational motion and torque. They are widely used in industrial, mobile and marine applications where high power density, variable speed control and reliable performance are required. There are several types of hydraulic motors, including gear motors, vane motors and piston motors (radial and axial designs such as swash plate and bent axis). Each type operates on the principle of positive displacement but offers different characteristics in terms of efficiency, pressure capability, speed range and cost. This page provides an overview of the main hydraulic motor types, explains how they work, and compares their performance characteristics to assist with selection and design.
Hydraulic motors operate by converting the energy of pressurised fluid into mechanical rotation. Fluid enters the motor under pressure and acts on internal components such as gears, vanes or pistons. This force produces torque and rotational motion at the output shaft. The speed of the motor depends on the flow rate of the hydraulic fluid, while the torque is determined by the pressure. Because of this relationship, hydraulic motors are ideal for applications requiring high torque at low speeds.
These notes include basic descriptions the following types of hydraulic motors
Below are identified various types of positive displacement motors with notes identifying
operating information. These notes and sketches are rudimentary in nature and are provided to show
the principles involved in the designs. For detailed notes on the design and performance it is advised
the the manufacturers literature is referenced.
Basic Calculations relating motor fluid flow, power and, shaft torque are found on my webpage. Hydraulic Calcs
The motor includes two gears one gear is includes the drive shaft. The motor
bearings are generally provided with internal bearings and packed glands or mechanical
seals. The most popular gear types are straight spur. These can be
noisy and subject to vibration if they are not manufactured to high standards.
These motors are reliable low cost units which can be run for long periods if operated correctly. They have good high pressure
operating characteristics. Close tolerances are required between the internal components for the motor
to operate effectively
The gear motor has moderate volumetric and overall efficiency
These motors can operate with fluid pressures of up to 200 bar. For pressures
above 50 bar the motors have to be specially designed with hydraulic balancing.
Approx Efficiencies of a typical gear pump /motor
The pump is based on an external gear located within, and meshing with, a larger internal gear.
A crescent vane is included to seperate the inlet volume from the discharge
volume between the two gears. The hyddraulic fluid enters enters the inlet volume and the pressure
tend to expand the volume which results in the gears rotating. At a certain point in the rotation the
volume is connected to the outlet port and as the gears continue to rotate the fluid is force out to the discharge route.
the.
The internal gear motor has similar characteristics to the external gear motor. The motor is smoother in
operation.< The internal gear motor is however more complicated and expensive to manufacture and maintain.
The internal gear pump can operate at pressures of 200 barg.
Note: The Gerotor hydraulic motor is basically the internal gear motor without the crescent vane. Gerotor
motors are one of the most
common types of muli-lobe motors whose operation is quite similar to that of an internal gear motor.
The inner gear has one tooth less than the outer gear, and the volumetric displacement is determined by the space formed
by the extra tooth in the outer gear. The tips of the inner and the outer lobes make contact to seal the inlet and discharge chambers from
each other.
The vane motor includes a ring mounted inside a cylindrical case The ring
includes a number of radial slots in which are located sliding vanes. The ring
is mounted eccentric to the case and the vanes are designed to press against the inside
wall of the case. The vanes are forced against the wall by hydraulic pressure or
spring force or due the centrifugal force resulting as the ring is rotated.
The fluid flow at pressure is use to rotate the ring and liquid flow into compartments between the
vanes and the case inner circumference. The pressure of the liquid causes the ring to rotate to increase
the space trapped between the vanes and the case. As the space is reduced as the ring rotates the fluid
is discharged.
The older designs of vane motors are based on an eccentric ring as described above . These are not hydraulically
balanced and are thus limited in the hydraulic pressure which can be developed. More modern designs include
for an elliptical inner ring which results in two pressure cycle per revolution. These pumps can develop much
higher pressures at high rotational speeds.
The vanes outer edges are subject to continuous wear and the vanes need to be replaced
after periods of continuous use. Modern motors are designed for convenient
maintenance by having the internal components design as cartridges.
These motors are smooth operating and have higher efficiency compared to gear motors.
Certain designs can tolerate significant vane wear. (carbon vanes)
This type of motor can operate
at pressures of up to 200 barg.
The principles of the adjustable displacement vane type hydraulic motor are illustrated in the sketch below
Approx Efficiencies of a typical vane pump /motor
The motors are extensively used for power transfer applications in the off shore ,
power transmission , agricultural, aerospace and construction industries,.. to list just a few.
All of these motors work on a similar principle.
The motor includes a block with a number of symmetrically arranged cylindrical
pistons around a common centre line. The pistons are caused reciprocate in and
out under the action of a Separate fixed or rotating plate (axial Pistons) or and eccentric bearing ring (radial pump)
or some other mechanical feature. Each piston is interfaced with the inlet and
outlet port via a special valve arrangement such that as it moves out of its cylinder under fluid pressure
and as it moves back it pushes the fluid out. The motors are
engineered to allow rotational speeds from less the 0,5 RPM to over 25,000RPM.
Radial Piston motors include a rotating cylinder containing equally spaced radial pistons
arranged radial around the cylinder centre line. A springs pushes the pistons
against the inner surface of an encircling stationary ring mounted eccentric to the cylinder.
The fluid flow at pressure causes the pistons to move out,resulting in rotation, for half of a revolution
and the pistons move in ,driving the fluid out
during the other half. The greater the ring eccentricity the
longer the pistons stroke and the less the rotation speed per unit fluid flow rate.
A variation to this principle is shown with an internal shaped cam replacing the eccentric ring this results in the pistons
making a number of strokes per revolution of the shaft resulting in smoother more efficient motion.
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A variation of the radial piston motor is the Staffa motor as shown below. For this type of motor the radial pistons are arranged in the outer case and move against an eccentric section of the rotating shaft. This motor is produced in a number of sizes enabling high torques at low speeds.
Swashplate motors have a rotating cylinder containing parallel pistons arranged radially around the cylinder centre line.
A spring pushes the pistons against a stationary swash plate located at one end of the cylinder ,
which sits at an angle to the cylinder. The fluid pressure causes the pistons to move out during half a revolution and
pistons drive the e fluid out
during the other half. The greater the swashplate angle relative to the cylinder centre line the
longer the pistons stroke and the less the rotation speed per unit fluid flowrate.
Bent axis piston motors have a rotating cylinder containing parallel pistons arranged radially
around the cylinder centre line. The pressure in the fluid causes the pistons to reciprocate over a stroke
based on the relative angle of the shaft and cylinder. The motion of the pistons results in the rotation of the shaft.
The cylinder is driven by an shaft which is arranged at an
angle to the cylinder axis. The shaft includes a flange with a mechanical connection to each
piston. The greater the angle of the cylinders to the shaft axes the
longer the pistons stroke and the less the rotation speed per unit fluid flowrate.
Approx Efficiencies of a typical piston pump /motor
| Type | Max Speed | Min Speed | Pressure range | Running Efficiency | Variable option | Duty | Cost | Flow Range |
| RPM | RPM | bar | % | - | - | - | litres.min | |
| Gear | 2500 | 500 | 0-200 | 80-90 | No | Light | Low | 5-500 |
| Int Gear | 2500 | 500 | 0-200 | 70-85 | No | light | Low | 5-750 |
| Gerotor or Geroller | 350 | 25 | 0-200 | Fair | No | Light | Low | Low |
| Vane | 2000 | 200 | 0-200 | 80-95 | Yes | Light | Medium | 5-300 |
| Axial | 12000 | 100 | 0-400 | 90-98 | yes | Heavy | Medium/High | 5-750 |
| Radial | 12000 | 0 | 0-400 | 85-95 | Yes | Heavy | Medium | 5-750 |
Hydraulic motors are used in a wide range of industries including construction, agriculture, marine and manufacturing. Common applications include excavators, winches, conveyors, drilling equipment, injection moulding machines and aerospace systems. They are particularly suited to applications requiring high torque, compact size and precise speed control under varying load conditions.
A hydraulic motor is a mechanical device that converts hydraulic pressure and fluid flow into rotational motion and torque. It is the opposite of a hydraulic pump and is commonly used in industrial and mobile machinery.
What are the main types of hydraulic motors?The main types of hydraulic motors are gear motors, vane motors and piston motors. Piston motors include radial piston and axial piston designs such as swash plate and bent axis motors.
How does a hydraulic motor work?A hydraulic motor works by allowing pressurised fluid to act on internal components such as gears, vanes or pistons. This force generates torque and causes the motor shaft to rotate.
Which hydraulic motor is most efficient?Piston motors are generally the most efficient, often achieving efficiencies above 90%. Vane motors offer moderate efficiency, while gear motors are typically the least efficient but are simpler and lower cost.
What is the difference between a hydraulic motor and a hydraulic pump?A hydraulic pump converts mechanical energy into hydraulic energy by moving fluid, while a hydraulic motor converts hydraulic energy back into mechanical rotation.
What factors affect hydraulic motor performance?Key factors include fluid pressure, flow rate, motor design, internal leakage, operating speed and temperature. Proper selection and maintenance are essential for efficient performance.
Where are hydraulic motors used?Hydraulic motors are used in construction equipment, agricultural machinery, marine systems, industrial drives, conveyors and aerospace applications where high torque and controlled speed are required.
What are the advantages of hydraulic motors?Hydraulic motors provide high torque, compact size, smooth speed control and the ability to operate under heavy loads. They are also well suited to harsh environments.
What are the limitations of hydraulic motors?Hydraulic motors can suffer from efficiency losses due to leakage and friction, require clean fluid to avoid wear, and need proper sealing and maintenance to ensure reliable operation.
