Abstract: Due to the shortcomings of small unit volume displacement, large displacement pulse ratio and poor low-speed stability characteristics for the hydraulic motor with an external mesh spur gear, theoretical studies on performance indexes of hydraulic motor with multi-external mesh spur gears are proposed. On the basis of analysis of the structure and working principle for hydraulic motor with multi-external mesh spur gears, the hydraulic motor’s constitution is presented, and formulae are deduced for calculating its primary performance indexes such as displacement, unit volume displacement, displacement pulse ratio, etc. The influence of the gears’ tooth number on displacement and displacement pulse ratio is analyzed and the selected method of tooth number on the gears mesh is discussed. This method improves unit volume displacement of hydraulic motor with multi-external mesh spur gears and reduces its displacement pulse ratio. And the necessary conditions that tooth number of the center gear should guarantee the reliability of sealing capability are presented. Under the conditions of same displacement and width of gears, the design instance is included to compare specific performance indexes between hydraulic motor with an external mesh spur gear and hydraulic motor with multi- external mesh spur gears. It is verified that the hydraulic motor with multi-external mesh spur gears has the advantages of smaller displacement pulse ratio, larger unit volume displacement, and so on.

Key words: Hydraulic motor, Multi-external mesh spur gear mechanism, Tooth number selection