The gerotor is a simple and reliable device commonly used in automotive, industrial and aerospace applications. The lesser known internally generated rotor (IGR) provides excellent wear resistance and performance in specialty pump and hydraulic motor applications.
The gerotor pumping element is a robust and reliable positive-displacement pumping device with excellent cavitation characteristics. It consists of just two components: one known as the inner element and the other as the outer element. Each element of the gerotor is supported by the structure of the pump with their respective centers eccentric from one another. The inner element has four or more teeth (a valid three tooth gerotor can be made but is generally not practical). The outer gerotor element has one more “tooth” or lobe than the inner and therefore rotates at N/(N+1) rpm relative to the inner element. In order to provide for smooth rotation, the profiles of the inner and outer elements of all gerotors, regardless of the number of the teeth, are conjugates of one another and satisfy the fundamental law of gear-tooth action. That is to say that the common normal at all points of contact between the two elements passes through a fixed point thereby maintaining a constant angular velocity. Many of the principles used to define and understand gerotor technology are common to those of other gear mechanisms.
Gerotor history dates to the late 19th century and efforts to understand and perfect the gerotor mechanism were well underway by 1900. Myron Hill was among the early researchers, and after years of study and experimentation, he published the booklet Kinematics of Gerotors in 1927 which is often considered the first comprehensive treatment of useful gerotor geometry and is likely where the term “gerotor” was coined as a contraction of the words GEnerated ROTOR.
Commercialization of the gerotor began in the late 1920s. William H. Nichols, founder of the W.H. Nichols Company and a skilled toolmaker, collaborated with Myron Hill to develop the necessary machinery to manufacture precise gerotor sets. The first commercial application for the gerotor was for a line of oil burner pumps launched in the 1930s. By the early 1940s, the General Electric Aviation division in Lynn Massachusetts had approached the Nichols Company to develop a multi-element gerotor pump for GE’s first jet engine – thus introducing gerotor technology to the aircraft industry and beginning a long history of success in aircraft engine lubrication and scavenge applications. In the late 1940s Arthur Nichols, son of WH, collaborated with “Buck” Charlton to develop gerotors for the low speed high torque motors otherwise known as orbit motors. The subsequent company, known as “Char-Lynn” was ultimately acquired by Eaton in the 1960s. With its start as a small machine shop in Waltham Massachusetts, the W.H. Nichols Company grew over the coming decades and by the 1980’s became the largest manufacturers of gerotors in the world.
Cascon’s founder, Edward H. Gervais II started his career at the W.H. Nichols Company in the 1960s where he and engineer Carle Middlekauff played integral roles in refining the design of gerotors for improved performance and manufacture.
Gerotors can be arranged in a variety of ways to suit the particular performance and packaging requirements of the application. Much like a set of gears, one element of the gerotor set drives the other. It is more common for the inner element to be fixed to the pump drive shaft with a key or spline and to drive the outer element as an idler. However, outer-driven applications are commonly used in gear-train lubrication applications (e.g. transmissions, axles, etc.) for packaging and cost advantages. A few examples of Cascon gerotor designs with inner and outer drive configurations are shown below. Gerotors lend themselves to being arranged in stacks, thus providing for a compact multi-circuit and multi-function pump solution.
In 1971, Ed Gervais and John Shaw were named as inventors on U.S. patent 3,623,829 describing an Internal Gear Set which, when used in pumps and motors, would become known as the Internally Generated Rotor or IGR. The patent was assigned to the W.H. Nichols Company and the device provided the foundation to the company’s entry into the low-speed, high-torque (LSHT) motor market.
The Internally Generated Rotor (IGR) has geometry based on a set of generating equations very similar to those of the gerotor, but with a term or two reversed. Note that the gerotor outer element is comprised of a series of semi-circular forms while the inner element has a continuously changing radius of curvature to form the conjugate. At its most basic, the IGR reverses this arrangement: the outer element has a continuously changing radius of curvature and the inner carries the circular forms. In the IGR, these circular forms are replaced with cylindrical rolls carried by the inner element.
Messrs. Gervais and Middlekauff went on to develop the IGR technology into a line of hydraulic motor products that would be manufactured in a plant that Mr. Gervais managed in Gray, Maine during the late 1970’s and early 1980’s. The motors were successfully sold into the agricultural, mobile and industrial markets where they became known for their durability and efficiency.
Cascon has built on this knowledge and further refined the IGR specifically for challenging chemical metering, fuel metering and hydraulic power applications.
Whether a gerotor or IGR form, the principle of operation is very simple and easily understood through the animations shown below. As the two elements rotate, an expanding “pocket” is formed to draw fluid in through the pump inlet. Fluid is forced out through the pump outlet as the pocket contracts. Porting contained in the adjacent housings provides for the inlet and outlet timing necessary to keep the two sealed from one another.
Gerotor manufacture has, until recent times, predominantly involved grinding and broaching from wrought steel or the use of powdered metallurgy processes using iron-based sintered metal powders. Improvements in wire-type Electrical Discharge Machining (EDM) equipment and milling machinery have paved the way for these more flexible manufacturing methods and the use of a wider range of materials including stainless steels, Stellite, ultra wear-resistant steels, ceramics, and even composites.
Cascon manufactures high precision gerotors in our own factory – using a variety of materials and processes – but also purchases steel and powdered metal units from other manufacturers such as Nichols Portland and Metal Powder Products (formerly NetShape).
To the best of our knowledge, Cascon is the currently the only manufacturer of IGRs. Proprietary methods are used for their manufacture to insure the precision needed to realize the benefits of this technology.
In 1984 the W.H. Nichols Company was purchased by Parker Hannifin to augment their portfolio offering of hydraulic devices and components. While gerotor design and gerotor production remained in Portland, Maine, manufacture of the motor products was absorbed into other Parker operations and the plant in Gray was closed. In 2016 the Portland plant was sold to Altus Capital and today the devices are sold into the automotive and industrial markets under the Nichols Portland brand. The IGR-based motor products were eventually discontinued by Parker and are no longer available.
Ed left the W.H. Nichols Company shortly after its acquisition and went on to start Cascon in 1988 in partnership with Chamberlain Machine. The new company’s focus was providing custom gerotor pump, IGR pump, and gear pump products to OEM manufacturers in mobile, industrial and aerospace markets. The company brought innovative gerotor designs to its customers and developed the IGR for use as a pump element particularly suited for low viscosity and low lubricity applications. As the company matured, its capability expanded. Today Cascon designs products that can include additively manufactured components, specialty ceramic and alloy materials, integrated AC, DC, and brushless DC electric drive, and a variety of electronic control options.
John J. Pippenger’s book, titled Fluid Power: The Hidden Giant, is a good resource for more on the history of the hydraulics industry.