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how motorcycles work.com knowledge is power. Oil pumps. Trochoid/gerotor type oil outlet hole oil inlet hole inner 4-lobe rotor outer 5-lobe rotor center of inner lobe is offset from center of outer lobe. rotation. a variable number of lobes/teeth can be used. This illustration shows a 4-lobe inner rotor and 5-lobe outer rotor, but higher lobe/teeth combinations are used in some oil pump designs. The trochoid/gerotor type oil pump is an evolution of the gear pump with crescent piece, except the inner gear has grown larger so as to eliminate the crescent piece. Also, the shapes of the gear teeth have changed. Spur gear pump. inlet oil outlet oil oil swept around outside. unmesh area. remesh area. The gears actually mesh together far more closely and precisely than this illustration depicts. Similar to gear pump with crescent piece, the meshing and unmeshing of gear teeth causes spaces that need to be filled and spaces which are being squeezed out. One of the gears is driven by a shaft, while the other gear is driven by the first gear. As the gears unmesh, the increase in available space/volume between the teeth increases, and thus pulls in oil. The oil is then swept around the outside housing, to the point where the teeth re-mesh. Since re-meshing teeth decrease space/volume, the oil is pushed out. Gear pump with crescent piece. inside gear, outer gear area of increasing space between unmeshing teeth oil inlet hole. Area of decreasing space between meshing teeth, oil outlet hole. Crescent piece. Oil swept along teeth. Gear pump with crescent piece effectively works the same as the spur gear pump, except with one gear becoming and inside-toothed ring gear. The meshing and unmeshing of teeth creates the changes in space/volume that pull in and push out oil. In transit inside the pump, oil is swept along between the spaces between teeth, until it meets the outlet. Piston and check valve oil pump. Rotation offset lobe on shaft connecting rod piston inlet oil outlet oil ball check valve pump housing ball check valve outlet oil. Variations of this oil pump design are commonly used in small engines and in older motorcycles due to the manufacturing processes available and economical at the time. An offset (eccentric) lobe rotates on a shaft and reciprocates a connecting rod, which is connected to a piston, which (by the use of 2 ball check valves) effectively pumps oil.