| Gear transmission passes motion and dynamic through gear tooth meshing and yet gear tooth failure is a common failure mode. Failure mode of tooth surface is varied such as teeth surface pitting, teeth wear, teeth bonding and teeth plastic deformations and so on. Both theory and practice show that gear tooth failure is connected with lubrication effect, what is more, the gear rotational speed and gear lubrication are inseparable. Therefore, it is necessary and feasible to explore the effect of gear rotational speed on the gear contact fatigue life.In order to explore the effect of gear rotational speed on subsurface stress in the contact area, through changing gear rotational speed, more than 130 sets of numerical calculations are completed, when other parameters of gearing and lubricant are constant. Under the light, medium and heavy load condition, the relations between the subsurface maximum principal shear stressτmax in the contact area and gear rotational velocity na are respectively established. The numerical results show that the gear contact fatigue life is connected with gear rotational velocity. Under light, medium and heavy load conditions, there are some critical values n acr and they are varied under different loads. When the gear rotational velocity na reaches these critical values, the subsurface maximum principal shear stressτmaxin the contact area reaches minimum; subsurface maximum principal shear stressτmaxis decreased as the gear rotational velocity na rises when na≤nacr; When na > nacr, however, the subsurface maximum principal shear stressτmaxis increased as the speed. This is clearly contrary to the people's traditional knowledge. In order to demonstrate the correctness of the results of this study, this paper analyzes the relation between subsurface maximum principal shear stressτmax and the amplitude of pressure spike Pmaxin teeth surface pressure distribution. The results show that subsurface maximum principal shear stressτmaxand the amplitude of pressure spike Pmax assume the parabola relations. The amplitude of pressure spike also have critical values Pc r and they are basically correspond to n acr. Subsurface maximum principal shear stressτmaxis decreased as the amplitude of pressure spike Pmax rises, when Pmax≤Pcr; however, the subsurface maximum principal shear stressτmaxis increased as Pmax rises, while na > nacr. This means that higher gear rotational speed will cause the amplitude of pressure spike Pmax increase, and higher Pmaxnot only increasesτmaxbut also increases the phenomenon of subsurface stress concentration in contact area, thereby causing gear contact fatigue life reduced.It is necessary to point out that this paper's findings are not consistent with the lubrication theory recommended by current design standard of gearing contact fatigue strength (ISO/TC60 423E). In order to consider the effect of gear rotational speed on the lubrication effect, this standard recommended speed factor Zv, But its value increases with the raise of gearing rotational velocity, which means that the faster speed is, the better lubricating effect is, the longer gear fatigue life is. Therefore, based on the above research results, this paper considers that the speed factor only fits in certain working conditions, and has certain limitations.The numerical calculations of current study are based on the full film EHL state, but fail to take into account of time-dependent nature caused by roughness effect. In addition, the results only question the applicable range of speed factor Zv recommended by the current design standard of gearing contact fatigue strength, but fail to explore how to amend this factor.This numerical calculation is carried out under full film lubrication, but not considered tooth surface roughness effects caused by time variation. In addition, this paper just brought force questions to the applicable scope of speed factor Zv recommended by the current design standard of gearing contact fatigue strength, but did not explore how to fix this coefficient. |
PDF Full Text Request