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Choosing a Gerotor Motor Cheat Sheet (DRAFT) by [deleted]

Considerations choosing Gerotor motors

This is a draft cheat sheet. It is a work in progress and is not finished yet.


There are two motor types using what is termed as the orbit principle, which is what gives the motors their tremendous power density and compact size. A gerotor motor star has six teeth and seven lobes. The spaces among them are pressure chambers. Pumped fluid flows into these pockets, creating high pressure in one chamber and low pressure in another. This creates an imbalance of forces, which causes the gerotor motor star to rotate, or orbit. A gerotor motor star orbits multiple times, Figure 1, typically six to eight times depending on the specific star and ring geomet­ry—for each complete single revolution within the outer ring. Geroler motors use the gerotor principle, but use rollers instead of lobes. This reduces friction and wear—and improves low-speed perfor­mance, extending the motor’s life.

Gerotor and Geroler motors deliver from 10 to 50,000 in.-lb of torque and can operate at speeds up to 2,000 rpm. Because of their simple design and compact size, they can be used in both mobile and industrial applic­ations. However, they are well-s­uited for mobile applic­ati­ons­—es­pec­ially agricu­lture, material handling and constr­uct­ion­—be­cause of their incredible power density.
Credit: By Todd Degler, Product Manager, Eaton Hydraulic Motors

Gerotor Motor

The speed and torque requir­ements of the applic­ation will determine the size of motor. This will point users into what displa­cement (how much fluid is needed to turn one revolu­tion) is needed. Speed and torque can be determined with the following basic motor equations. These are the starting points for finding the properly sized motor.
Theore­tical torque (in.-lbs) = in.3/rev x pressure differ­ent­ial­/(2Pi)
Theore­tical speed (rpm) = gpm x 231 / in.3/rev

1. Perfor­mance

Ask for mechanical and volumetric efficiency data, and make sure the test data is over an extended period of time. Some manufa­cturers may tune perfor­mance to peak in the first few hours of operation, but then degrade quickly. Perfor­mance that degrades quickly will not do your machine justice.
Compare apples to apples; hydraulic motor perfor­mance data is not standa­rdized. Be wary of ratings and test data that do not include all of the perfor­mance test parame­ters.

Gerotor & Geroler Motors

2. Quality

Does the manufa­cturer have a history of consistent quality? Ask for the quality data.
Motor grinding is not all the same. Grinding star profiles outside the capabi­lities of the form grinding machine will cause incons­istent results.
Look for high-q­uality materials and ensure that inferior materials (with processes like heat treating and form grinding) are not being used.
Compare the warranty options.

3. Reliab­ility

Reliab­ility is quality over time. Look for a product with a track record­—hi­story is the best predictor of the future.

4. Support

Invest­igate the options for additional support. Would you benefit from CAD modeling support or the creation of a custom solution? Remember that this can differ­entiate your machine from the compet­itor.
Do you just want a part or do you want a system of solutions? Some manufa­cturers can provide a custom system of solutions for your machine, while others specialize in providing a singular part.

5. Cost

Obviously, cost is very important, but it is really a function of all the previous items. Sacrif­icing the above items can provide a lower cost, but consider what will happen to your machine (and your customer) when issues arise.
Some companies try to keep costs low by having a “one size fits all” approach to their motor portfolio. This may work for some, but make sure the solution you choose really works the way it should on your machine.