How Gerotors Work

A gerotor pump consists of two main parts: an inner rotor with N teeth and an outer rotor with N+1 teeth. The inner rotor is keyed to the drive shaft, while the outer rotor rotates within a housing. The inner and outer rotors have different centers of rotation, but their meshing profiles create a series of expanding and contracting chambers.

A gerotor, short for “generated rotor,” is a positive displacement pump that uses an inner and outer rotor to transfer fluid. They’re commonly used as oil pumps in engines and transmissions because of their compact size, simple design, and high efficiency. Gerotors are particularly well-suited for low-pressure lubrication systems.

As the rotors turn, these chambers fill with fluid from the inlet port. The fluid is then trapped and carried around to the outlet port as the chamber size decreases. The fluid is then expelled under pressure. The pumping action is continuous and occurs smoothly because of the symmetrical design.

Materials and Manufacturing

In recent years, powdered metallurgy (PM) has become a popular alternative, especially for low-pressure applications like standard lubrication pumps. Powdered metal gerotors offer several advantages:

The main challenge with powdered metallurgy for high-pressure applications is achieving sufficient density and strength. Low-density parts can be porous, leading to fluid leaks and potential failure under high pressure. Advances in material science have led to the development of higher-strength metal powders, making PM a viable option for a broader range of gerotor applications.

Rotor Profiles

The very interesting and critical element of gerotors is the profiles. With the fixed radius of outer lobes, the inner rotor profiles are very often misunderstood as a curve by combination from many radius. But this can only be done by reverse engineering and difficult to be precise. However, through software, but sintering can vary the dimensions even slightly can make difference.

Historically, gerotors for high-pressure applications were machined from steel bar stock. This process is precise and produces high-strength parts, but it’s also expensive and time-consuming.

The most critical aspect of a gerotor is the precise profile of its inner and outer rotors. The shape of the rotor lobes isn’t a simple curve but is mathematically generated to ensure a tight seal and smooth operation.

The outer rotor’s profile is typically based on a circle with a fixed radius. The inner rotor’s profile is then designed as an epitrochoid, which is the curve traced by a point on a circle rolling around the outside of another fixed circle. This precise geometric relationship ensures the teeth of the inner and outer rotors mesh perfectly, creating sealed chambers that efficiently move fluid.

Errors in the rotor profiles, even slight variations from sintering in PM manufacturing, can significantly impact a gerotor’s performance, leading to leaks, noise, and reduced efficiency.

• Specialized Knowledge of Profiles: The text highlights that gerotor profiles are “very often misunderstood as a curve by a combination from many radii” and that precise profiles are critical. It correctly identifies the complexity and the need for specialized software to design these profiles accurately. This demonstrates a clear understanding of the core technical challenge in gerotor design.
• Manufacturing Expertise: The passage directly addresses the manufacturing challenges, contrasting traditional steel bar stock machining with modern powdered metal (PM) techniques. It also mentions a key limitation of PM—low density and strength for high-pressure applications—and then asserts that the company has a solution with “high strength material” and a new “high density PM process.” This shows both knowledge of the industry’s standard practices and an innovative approach to overcoming their limitations.
• Meeting Demanding Quality: The text begins by stating, “We are proud of our capability to provide designs and manufacturing the rotors even quality is demanding.” This directly claims the company’s ability to produce high-quality parts and positions them as an expert capable of handling challenging specifications, such as those for high-pressure applications.

When producing custom designs, one of the main cost factors is the initial investment in tooling. Each project usually requires four tools—two for the outer profile and two for the inner profile (one forming tool and one sizing tool for each).

The sizing tools are fine-tuned versions of the forming tools. They are used in a second operation to ensure the final dimensions are precise—shrinking the outer size or expanding the inner profile as needed.

Because these tools are made from durable tungsten carbide, they are costly to manufacture. This cost is spread (amortized) over the number of parts produced. If the production quantity is too low, the cost per part becomes high, which makes small-volume production less affordable.