Abstract: When investigating flow pulsation of an involute helical gear flow-meter with external gear, the instantaneous flow can be regarded as superposition of countless instantaneous flow, staggering a fixed phase angle in turn along the width of the gear tooth, due to the effect of helical angle. Following this guideline, a mathematical expression of maximum variation of flow ripple of the involute external engagement helical gear flow-meter is deduced in this article by obeying the law of energy conservation and the principle of helical gear engagement. On above basis, taking the parameter relationships in the mathematical expression and the helical gear coincidence degree as the theoretical basis, the critical helical angle and the critical helical gear width without oil trapping were deduced in this research. The analysis shows that the level of flow ripple of the involute external engagement helical gear flow-meter is lower than that of the spur gear flow-meter in the case of same tooth capacity, and the flow pulsation will decreases with the increase of helical angle.

Key words: helical gear flow-meter, flow pulsation, critical helix angle, contact ratio, trapped oil