Understanding the Working Principle of Hydraulic Orbital Motors - Impro Precision


Understanding the Working Principle of Hydraulic Orbital Motors

August 1st, 2023

For high torque in mobile and industrial applications, it’s hard to beat a hydraulic orbital motor. These are compact enough to use as wheel motors in mobile equipment like skid steers, and for driving machinery like that found in agriculture, construction, forestry and manufacturing. They deliver copious quantities of torque, smoothly and at low speed. Here’s how they do it.

Hydraulic Vs. Electric Motors

While an electric motor uses AC or DC power, a hydraulic motor generates rotation and torque from a flowing fluid. They need a pump, filter and piping, but don’t require long cable connections, high voltage supplies, or complex drive controllers.

In mobile applications a hydraulic motor is driven by a generator or engine, such as that used in a tractor or skid steer. They need little maintenance and are available for use in potentially explosive environments.

Hydraulic motors take many forms, using vanes, pistons or gears to turn fluid pressure into rotary motion. In principle they are little different to hydraulic pumps running in reverse.

Gerotor Basics

The hydraulic orbital motor is a form of gear motor that uses a gerotor design. This has two main elements: a star-shaped inner gear or rotor and a stator with a matching tooth form on the inside. The rotor has one tooth less than the stator and is placed inside the stator where it can roll around in a circular path.

The difference in number of teeth results in one rotor tooth meshing with a recess in the stator while the tip of the diametrically opposite rotor tooth brushes the tip of a stator tooth. This divides the volume in the gerotor between rotor and stator into two chambers.

Pressurized fluid is introduced into one of these chambers. This pushes the rotor towards the lower pressure side, and the mesh gear teeth ensure the rotor turns as it moves.

In a DC electric motor, where current always flows in the same direction, commutation is used to switch current direction in the motor windings. A hydraulic orbital motor also needs commutation, in this case to ensure delivery of high pressure fluid processes around the gerotor as the rotor turns.

This commutation is provided by a valve plate mounted at one end of the gerotor cavity. As it rotates it allows low pressure fluid to flow back to the storage tank and pump while simultaneously letting fluid flow into the high pressure chamber.

Coupling the circular motion of the rotor to an output shaft results in the rotation the motor needs to deliver. Torque generated by the motor is a function of rotor surface area and fluid pressure. Gerotor designs have a higher power density (torque produced for motor volume), than competing alternatives.

Start-Up Loads

As with an electric motor, getting a hydraulic motor turning needs more force than is needed to maintain it at a steady speed. Impro Fluidtek hydraulic motors address this with a proprietary design of pressure-compensated balance plate. This is engineered to improve volumetric efficiency at startup and in steady-state operation.

Specifying a Hydraulic Orbital Motor

The primary metric defining these motors is displacement. Specified in cubic inches (in3), this is the volume of fluid needed to produce one revolution and correlates with torque produced. At a given fluid pressure larger displacement motors make more torque. When selecting a hydraulic orbital motor, start with the speed and torque required, then consider the pressure available. This will lead to the motor needed.

High Power Density for Mobile and Industrial Applications

Impro Fluidtek produces three families of hydraulic orbital motors, all engineered for smooth torque delivery and high output at low speed. Applications include wheel motors, mobile rotary power, and heavy duty industrial equipment. Contact us to learn more.


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