The aim of this article is to study the theoretical flowrate characteristics of the conjugated involute internal gear pump. The conjugated involute internal gear pump has a different internal gear, the profile of which is completely conjugated to that of pinion, from the conventional involute internal gear pump. To describe briefly, the former pump is called 'conjugated pump' while the latter pump 'conventional pump'. This structure makes the conjugated pump have smaller volumes of trapped fluid than the conventional pump, which indicates the conjugated pump has a better flowrate performance. Although there have been numerous investigations into gear pumps, none of them have dealt with the conjugated pumps yet. This article presents a systematic study on the theoretical flowrate characteristics of the conjugated pumps. It is expected to obtain some generalized conclusions to guide the design of internal gear pumps. Firstly, a double envelope concept is applied to derive the mathematical models of the conjugated involute internal gear couple. Next, mesh characteristics of the gear couple, which mainly decide the fluid delivery performance of the pumps, are analyzed. Finally, using a control volume approach, the theoretical flowrate characteristics of the pump under different design parameters of gears are investigated. Results show the conjugated pump has a better performance than the conventional one. To design a conjugated pump with good flowrate characteristics, it's feasible to choose a proper shifting coefficient, a larger tooth number of the pinion, a smaller tooth number of the internal gear, a larger pressure angle and a larger fillet radius, under the condition that no tooth interference occurs.
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