The Versatile Gerotor: An Evolving Positive Displacement Pump

For over a century, engineers and inventors dreamed of efficient rotary machines capable of attaining high pressures at variable speeds. In 1906, the principles of mutually generative rotors, or gerotors, fulfilled that dream.

Having only two moving parts, gerotors are simple and versatile in design. Yet, they can produce a multitude of flows and pressures, pumping liquids of almost any viscosity. They can be used virtually anywhere a gear, crescent, or vane pump is operating in fluid power and transmission applications.  Additionally, in comparison to other positive displacement pumps, they have the added benefits of requiring less input horsepower, of having fewer parts, lower noise levels, delivering high torque at low speeds while performing extremely well at high speeds, and being small in size and low in cost. Gerotors are also extremely efficient, operating from fractional flows to over 100 gallons per minute, and at pressures from vacuum to over 3000 pounds per square inch.

Gerotor applications include: engine lubrication, fuel transfer and hydraulic pumps, hydrostatic transmissions, hydraulic motors, cooling systems, air motors, and vacuum pumps. They are also used in off-highway and lawn care equipment hydraulic systems, and are particularly suited for applications where a small profile package is required. Still, as gerotor design variations evolve, so do the applications. Indeed, by partnering with a supplier, prospective buyers can obtain a custom-designed gerotor that meets their design standards for performance, reliability, and economy. To help effect such a partnership, this article provides an overview of gerotor design, as well as the fundamentals of choosing and working with a supplier.

Figure 1. – The unique geometry of a gerotor creates pockets between the elements that work together much like a piston and cylinder in a combustion engine.

Basic Gerotor Design

To design and generate the correct gerotor form for a particular application, a thorough understanding of geometric and mathematical principles is needed. Accuracy of gerotor design can be further enhanced by the use of computer technology, as well as by modern machine tools capable of producing parts that meet exacting design specifications.

Several features must be contained in an ideal rotary machine. Rotary machines require a minimum of two relatively active members. In the gerotor, the active components are called rotors. To reduce wear to the lowest possible degree while a rotor is under pressure, the travel of one rotor must be as near the travel of the other as possible. They should travel together in the same direction and at near the same speed as design permits. Additionally, the chambers created by the two rotors must open and close to perform the function of a piston and cylinder (see Figure 1). This dictates that the rotor contours must maintain continuous contact (see Figure 2).

Conditions of steady flow of fluid and steady torque are best achieved by having one rotor inside the other, with one rotor having one less curved division than the other. As can be seen in Figure 3, the cusps of the rotor curves are rounded, and the curved divisions are essentially a series of convex-concave curves. When there are suitable continuous contacts and driving relations between the two rotors, a mutually generative relation is created.

Figure 2. – Continuous constant contact is maintained between the inner and outer elements.

Choosing and Working with a Supplier

Only a handful of suppliers worldwide are recognized as quality designers and manufacturers of gerotors. Most are experts in the field of powdered metal parts design and manufacturing. To evaluate a potential supplier’s capabilities, a visit to the supplier’s plant is strongly recommended. This will also provide an opportunity to meet the key players in the organization and evaluate the supplier’s commitment to quality and on-time delivery. Suppliers should have the following minimum capabilities:

  • Experience in powdered metal parts design and technology
  • Engineering staff with experience in gerotor design
  • In-house tool design
  • Quality control
  • Statistical process control
  • Hydraulics industry experience
  • Secondary machining capability for providing finished parts
  • On going research and ongoing development in powdered metal technology
  • Prototyping experimental and sample parts
  • Batch production to large lot production capability
Figure 3. – The inner tooth form is designed to provide constant contact with all lobes on the outer element.

An informed purchaser and potential customer should be prepared to supply application data required by gerotor designers and manufacturers. When designing a gerotor set, the supplier will need to know:

  • Theoretical displacement in in3/rev or cm3/rev
  • Flow requirement in gallons per minute or liters per minute
  • Operating pressure in lbs/in2 or kg/cm2
  • Shaft speed in rpm
  • Fluid type
  • Viscosity
  • Maximum and minimum temperature range in degrees Fahrenheit or Celsius

Look for a supplier that will evaluate your needs and design a gerotor set for a specific application. Failure to do so will limit one’s possibilities of purchasing the smallest overall gerotor package at the lowest cost. For a long-term and mutually beneficial business relationship, designers and buyers from the purchasing firm should work with a supplier to insure that final gerotor design meets with the desired specifications.

The gerotor is a unique machine. When specifying a hydraulic pump, or motor in fluid power or fluid transfer, consider the gerotor for high reliability, economy, and performance.