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The way a trochoid (also known as gerotor) operates is somewhat difficult to visualize from a 2-dimensional paper drawing. Studying a gear pump with a crescent piece helps to understand the trochoid/gerotor (which does NOT have a crescent piece). Inside of a gear oil pump housing, two gears mesh together. The inner gear is smaller, and it's teeth engage the teeth of the larger outer gear. A stationary crescent piece either cast or machined into the pump housing creates a separation between the two gears where their teeth do not mesh. As rotation continues (the inner gear is powered by an external source - in this case by a gear or chain off the crankshaft) the teeth of the inner gear unmesh (separate) from the outer gear. This creates a void which must be filled by something. An oil hole passage is drilled at this very location. The suction created by the unmeshing gear teeth pulls in oil through this passage. Oil pump housing spur gear pump trochoid oil pump theory of operation page 1 area of decreasing volume oil will be pumped out area of increasing volume oil will be drawn inwards gear pumps with crescent piece as the inner gear rotates counter crescent piece is stationary - separates inner and outer gears crescent piece shaft hole oil hole how motorcycles work.com knowledge is power