Introduction to Hydraulic Motor Terminology

Motor displacement

This equals the fluid capacity required to turn the motor output shaft through one revolution. A hydraulic motor usually has a in.3 or cm3 per revolution displacement. Moreover, the hydraulic motor displacement is either fixed or variable. A fixed-displacement motor delivers continuous torque. Controlling the input flow amount varies the speed. A variable-displacement motor, on the other hand, offers variable torque and variable speed. Displacement variation alters the torque speed ratio to meet load requirements.

Torque output

Torque output is conveyed in inch-pounds or foot-pounds. It measures the precise pressure drop across the motor.

Breakaway torque

The torque required to get a stationary load turning. More torque is required to start a load moving 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 indicates the torque required to keep the load turning.

Running torque

This can refer to a motor’s load or to the motor itself. 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

This refers to a hydraulic motor’s capacity 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. In addition, starting torque 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

This is the ratio of actual torque delivered to theoretical torque.

Torque ripple

This is the difference between minimum and maximum torque distributed at a certain pressure and during the hydraulic motor’s one revolution.

Motor speed

Is a function of motor displacement and the fluid volume transported to the motor.

Maximum motor speed

This is the motor’s maximum tolerated speed at a particular inlet pressure for a limited time without damage.

Minimum motor speed

This is the motor’s output shaft slowest, non-stop, uninterrupted rotational speed available.

Slippage

Slippage is the motor leakage, or the fluid, that flows through the motor without accomplishing work.