Motors 101: Hydraulics at Work

motors

This article, Motors 101, continues in HPS’ series of articles focused on “the basics” of major categories of hydraulic parts. If you are in fluid power sales or service, and need a reputable repair resource for hydraulic motors, pumps, or valves, HPS is ready to help.

Of many available types of fluid power products, motors are defined as “the actuators that release the power to perform the physical work of hydraulic systems.”

While hydraulic pumps get energy moving, and hydraulic valves control fluid energy, hydraulic motors do the work - in tandem with pumps - to transfer fluid energy into power.

Motors work by converting fluid pressure into motion. The pressure from the pump turns the motor’s output shaft by pushing on the gears, pistons, or vanes of the hydraulic motor.

Selecting a Hydraulic Motor

The design of an entire hydraulic system will change depending on the type of motor selected, so it’s important that motor selection comes first in the process, with that decision largely depending on the system’s load requirement, performance goals, and application.

For example, an important consideration in the design and decision-making process is how motors are rated by displacement (volume of fluid it takes to rotate the output shaft once), measured in cubic inches per revolution (CIR) or cubic centimeters per revolution (CCR).

Besides displacement, additional operating considerations include:

Operating torque, or the torque that the motor is capable of delivering
Maximum system pressure
Fluid temperature range, including minimum and maximum operating temperatures
Maximum operating speed
Power limits based on maximum volumetric flow through the motor
Maximum fluid viscosity

Attention to meeting these specific limits are vital in assuring the motor provides the most power.

Speed Classification and Motor Types

Hydraulic motors can be sorted in two ways:

Revolving speed - Defined as either a high speed, low torque (HSLT); or low speed, high torque (LSHT)
Design configuration - Three main types are gear, vane, and piston motors

In terms of revolving speed, high-speed hydraulic motors operate at 500 rpm and above, and have a low output torque (these are known as “HSLT” motors).

A low-speed motor operates at fewer than 500 rpm, and has a large displacement, size, and low revolving speed (equating again to high output torque – abbreviated as “LSHT”).

In terms of design, each type has a specific profile to meet the requirements, performance goals, and desired applications within a hydraulic system.

Hydraulic Motor Design Configurations in Detail

Gear Motors

Gear motors consist of rotating gears that move when hydraulic fluid enters the motor. They are categorized as either internal or external.

External gear motors consist of matched gears enclosed in one housing. Internal gears can either consist of a direct-drive gerotor motor or orbiting gerotor motor, and generally have less gear teeth than external gear motors.

Key features of gear motors include:

Small size and light weight
Low cost
Wide range of speed
Wide temperature range
Wide viscosity range
Low inertia

Some drawbacks include:

Large pressure and torque ripple
Low volumetric efficiency and input pressure
Low starting torque
High noise level
Poor stability at low speed

Therefore, gear motors are only applicable under the high-speed and low-torque (HSLT) classification and are most suitable for mechanical equipment that does not demand much in torque uniformity.

Vane Motors

A vane motor uses a rectangular vane to generate energy. When pressurized fluid enters the motor, the blades in the vane cause it to rotate and produce output torque.

Key features include:

Low noise level
High torque at low speeds
Low flow pulsation
Simple design
Versatility
Vertical installation friendly

Some drawbacks include:

Large leakage
Low input pressure
Unstable at low speed
Anti-pollution capability not as good as gear motors

Vane motors are typically classified as HSLT units, although larger displacements will fall into the LSHT category. They satisfy demands of sensitive motion so are used often in mobile applications.

Piston Motors

Piston motors work by using mounted pistons to generate energy. They come in a variety of designs, such as radial-piston and axial piston, with both LSHT and HSLT classifications.

A radial-piston hydraulic motor uses pistons mounted radially around a central axis, sometimes using multiple interconnected pistons in a star pattern, to generate energy. It is classified as LSHT, and generally operates efficiently for a long life, despite a higher cost and complex design.

An Axial-piston hydraulic motor produces an axial motion, rather than radial. It is similar to radial piston motors, and also offers a high efficiency and long life at a high cost. Unlike the radial design, however, they are not as efficient at low speeds and are classified as HSLT.

As Always with Hydraulics, It’s About Parts Working Together

Pumps and motors have overlapping characteristics and types (vane, gear, piston), and for good reason. They are both working together to provide the energy needed to put hydraulic equipment in motion.

The pump converts mechanical energy into fluid energy in the form of oil flow. The oil flow created by the hydraulic pump is then directed to the motor through the system’s valve.

But it is the motor that creates the output for this cohesive unit, converting the fluid energy into mechanical energy to perform the physical work.

Hydraulic Parts Source is here to help – if you have questions about motors or any kind of hydraulic system component, we’re here to help you learn. Call (888) 477-7278 to get connected to expert help for hydraulic parts.