A gerotor is a positive displacement pumping unit. The name gerotor is derived from "Generated Rotor". A gerotor unit consists of an inner and outer rotor. The inner rotor has N teeth, and the outer rotor has N+1 teeth. The inner rotor is located off-center and both rotors rotate. During part of the assembly's rotation cycle, the area between the inner and outer rotor increases, creating a vacuum. This vacuum creates suction, and hence, this part of the cycle is where the intake is located. Then, the area between the rotors decreases, causing compression. During this compression period, fluids can be pumped, or compressed (if they are gaseous fluids).

Gerotor pumps are generally designed using a trochoidal inner rotor and an outer rotor formed by a circle with intersecting circular arcs. Although this design works well and is simple to define it does create gaps between the inner and outer rotor when the tooth of the inner rotor rotates into the pocket of the outer rotor. This gap seals during rotation causing inefficiency, noise and wear due to the pump attempting to compress the trapped and incompressible fluid in the gap. In 1996 a Patent was filed (see External Links) by Gavin Whitham, an Engineer with T&N Technology Ltd, defining an outer rotor based upon the envelope of rotation of the inner rotor. This Patent allowed an outer rotor to be designed that was a perfect match for the inner rotor to be created; resulting in improved pumping efficiency, reduced noise and reduced wear. The mathematical formulae contained within this Patent have been included in a software package used by T&N plc that automatically designs the most effective pump for every situation given a few simple parameters such as rotational speed, eccentricity and required pumping volume.

A gerotor can also function as a motor. High pressure gas enters the intake area and pushes against the inner and outer rotors, causing both to rotate as the area between the inner and outer rotor increases. During the compression period, the exhaust is pumped out.

An engine created by the Starrotor Corporation combines both uses of a gerotor. It uses the Brayton cycle, the same thermodynamic cycle employed by jet engines. A first gerotor compresses gas, this gas is then ignited in a combustor. The gaseous products of this combustion have a much higher pressure, which drives a second gerotor. Then, some of the output of the second gerotor is used to drive the 1st. There has been no public demonstration of a running "Starrotor engine" to date.


In common with most mechanical devices, the gerotor has a long history going back 100 years or more. At the most basic level, a gerotor is essentially one that is moved via fluid power. Originally this fluid was water, today the wider use is in hydraulic devices. Mr. Myron F. Hill, who might be called the father of the gerotor, in his booklet "Kinematics of Gerotors," lists efforts by a Mr. Galloway in 1787, by Messrs. Nash and Tilden in 1879, by Mr. Cooley in 1900, by Professor Lilly of Dublin University in 1915, and by Feuerheerd in 1918. These men were all working to perfect an internal gear mechanism with a one-tooth difference to provide displacement.

Mr. Hill made his first efforts in 1906, then, in 1921, gave his entire time to developing the gerotor. He developed a great deal of geometric theory bearing upon these rotors, coined the word GEROTOR (meaning GEnerated ROTOR) and secured basic patents on gerotors.

Gerotors are widely used today throughout industry, and are produced in a variety of shapes and sizes by a number of different methods. The two leading gerotor manufacturers in the United States are Eaton Corporation in its Shawnee, OK facility and [ Parker Hannifin Corporation] its Nichols facility in Portland, Maine.


Eaton Corporation Hydraulic Division developed a derivative of the gerotor called the Geroler. The term is an Eaton Copyright and the design essentially uses bearing rollers instead of lobes on the ring to increase the mechanical efficiency of the gerotor. The gerotor's greater efficiency comes at the price of greater manufacturing complexity and extreme fit tolerances involving single digit micrometres.

uggestions for using gerotor elements

# Gerotor pump elements should be used in sets as received from the factory.
# Gerotor elements are manufactured with close dimensional control and require similar controls in their installation.
# The following tolerances can be used as a guide to insure good performance and life.
#*Eccentricity: i.e.: distance between center around which the outer member rotates and the center of rotation of the inner member to be as shown, +/-. 0005 inch.
#*Side clearance: i.e.: difference between gerotor thickness (inner and outer are the same) and the depth of recess gerotor runs in is 0.0015 to 0.003 inches for moderate pressures, speeds and oil viscosity.
#*Running clearance: i.e.: difference in diameter between outer gerotor member and recess .002 to .004 inch.
#*Bearing alignment: The bearings should be in line with each other and square with the gerotor cavity within .0005' per inch.
#*Housing clearance: zero to 0.002 inches maximum over eccentric ring O.D.
#Eccentric rings: An eccentric ring with correct eccentricity, side clearance and running clearance is available as a third piece of each gerotor set. Its use eliminates difficult machining operations, all boring and turning then are concentric and to normal machining tolerances. In addition, accurate bearing alignment can be assured by piloting the housings on the O.D. of the eccentric ring. Proper side clearance is assured by bolting up tight against the eccentric ring which is the proper amount thicker than the gerotors.
#Surface Finishes: The sides of the recess should be flat and at least 64 RMS in smoothness. The outside diameter of the recess should be at least 64 RMS and the corners must be perfectly sharp or undercut slightly to insure noninterference with the corners of the outer gerotor.
#Bearings: may be anti friction or sleeve type and sized according to good engineering practice. When using sleeve bearings, pressed in place, we urge final sizing be accomplished by boring rather than reaming, to insure maintaining eccentricity tolerance.
#Shafting: As the inner and outer gerotor members are held in, correct running position by the O.D. of the recess and the shaft, the latter should be reasonably sturdy and free from deflection. Preferred construction is a hardened shaft ground .0005 to .0015 free in the inner gerotor bore.
#Driving: between shaft and gerotor can be accomplished by key, spline or press fit. However, the last two are second choice. If a press fit must be used, we must know how much the press is and have the gerotor sets especially manufactured to maintain proper fit. Splines ~ are expensive and will not locate the inner gerotor properly unless made with minimum radial freedom. We recommend a key drive, or, in very small sizes, a cross pin works well. Multiple keys can be used, although usually not necessary. The shaft may be driven by a coupling spline, or gear, etc. It may be positioned axially by use of snap rings on the shaft either side of the inner gerotor, which can take a reasonable amount of end thrust.
#Porting: Porting should be in accordance, with that shown in "Suggested Port Configuration" (p. 4). These ports are suitable for rotation in either direction.
#Line Size: We recommend particular attention is paid to providing adequate line size. Page 4 lists acceptable sizes for various capacities. Inlet oil velocities of about 4'/see. Will permit satisfactory performance. The long inlet cycle of the gerotor makes for high pumping efficiency, but line sizes must be sufficient to permit complete filling; otherwise, cavitation, loss of efficiency, and possible damage to the elements may occur.
#Pump Housing: We recommend the two halves of the pump housing be positioned together with a pilot. Doweling can be used if great care is taken to insure concentricity of bearings and proper eccentricity of gerotor recess. The parts of the housing in contact with the gerotor can be made of cast iron, anodized aluminum, bronze or other material having good bearing characteristics.

REVERSIBILITY FEATURE Changing the direction of rotation of a gerotor pump changes the direction of flow. Changing the eccentricity of a gerotor pump 180° also reverses flow. If eccentricity is changed 180° with each reversal of rotation, direction of flow will remain unchanged. This is accomplished by using a free turning eccentric ring with stop pin in housing to limit rotation of 180° either way. A simple Friction drive between outer rotor and eccentric ring assures proper positioning of the ring to maintain direction of flow unchanged regardless of drive direction reversals.


* Oil pumps
* High speed gas compressors
* Engines
* Hydraulic motors
* Power steering units

ee also

*Gear pump
*Wankel engine

External links

* [ Nichols Portland Division of Parker Hannifin]
* [ Cascon, Inc. Custom Engineered Gerotor Pumps]
* [ Step by step drawing]
* [ Gerotor Patent for Improved Outer Rotor]
* [ Starrotor]

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