How to pick the perfect hydraulic motor

We all wish selecting components for con-ag equipment was as easy as playing eenie-meenie-minie-mo. However, for each piece of compact equipment, application constraints like performance, efficiency, reliability, serviceability, productivity, control and cost have to be considered. This is especially true when designing hydraulic systems and selecting components such as hydraulic motors to operate the equipment's drivetrain and attachments.

With two types of motors, three basic motor constructions, numerous variations on these designs and more than 20 hydraulic motor manufacturers and hundreds (if not thousands) of models, selecting the right motor for an application can lead to a confusing array of options and configurations.

Luckily, with some basic hydraulic motor knowledge, a buyer can certainly take much of the confusion out of the selection process.

First, motors are broken into two segments: high-speed, low-torque (HSLT) motors and low-speed, high-torque (LSHT) motors.

Generally, a LSHT motor's speed can range between 0.1 and 1000rev/min depending on the motor and control system, whereas HSLT motors range between 1000 and 5000rev/min.

In this article, the focus will be on LSHT motors because these motors transmit a large amount of torque in a relatively small envelope - for example, a 5hp motor can be the size of a 350ml beer can.

Additionally, LSHT motors generate low noise levels and vibration, have a high-energy efficiency - in the 80 to 90% plus range - and operate smoothly at low speeds.

All of these attributes are ideal for con-ag and mobile equipment applications.

The combination of these LSHT motor characteristics gives it the ability to virtually eliminate the need for gearboxes in many applications.

Because of this, LSHT motors are great for con-ag equipment such as skid steers, mini excavators and their attachments.

LSHT motors generally come in three basic constructions: gear, piston or vane.

Gear motors are available in either external or internal gear configurations.

External motors will not be discussed as these motors are HSLT motors.

Internal gear motors - also known as gerotor, geroller or orbiting motors - have an inner (rotor) and outer (stator) gear.

The rotor has fewer teeth than the stator, giving it a large mechanical advantage.

Both gears move as the fluid passes through the motor, causing the rotor to orbit around the stator.

The drive coupling transmits the motion of the rotor to the output shaft.

Piston motors come in two basic designs: radial-piston and axial-piston.

These motors use reciprocating pistons to transmit the energy from the fluid to the drive trains or attachments.

Although there are numerous radial piston designs, the two most common are the telescoping piston and the multi-cam-lobe piston.

The telescoping piston design looks like the old airplane prop engines.

In this design, telescoping cylinders capture the pressurised fluid and push against an eccentric cam attached to the output shaft.

This design is not typically used in con-ag or mobile applications, except for large, heavy-duty applications like winches.

The multi-cam-lobe design is similar to an internal combustion engine whereby the pressurised fluid pushes against a series of pistons riding on a cam, which is attached to the output shaft.

A variation of this design is the rotating housing.