Abstract
The hydro-viscous drive (HVD) has been widely used in fan transmission in vehicles, fans, and scraper conveyors for step-less speed regulation or soft starting. In the mixed friction stage, the contact, friction, and torque characteristics of friction pairs are very complex and change at any time. The characteristics of the frictional and hydrodynamic lubrication states were studied in order to calculate and predict the friction and torque characteristics of the friction pairs in the mixed friction stage. The fluid torque was calculated by applying the average shear stress model and the load-carrying capacity of asperity was determined on the basis of the fractal contact theory. In addition, the contact friction coefficient of the friction pairs was taken into consideration and measured by using the MM1000-III friction and wear testing machine. The asperity friction torque and total torque in the mixed friction stage were obtained and finally, the test rig for the torque characteristics was set up. The results show that the contribution to the total torque is shared by the oil film and the asperity friction. The friction coefficient decreases sharply at first and then increases with a change in the relative rotational speed, following the Stribeck curve closely, and the contact frictional coefficient slowly decreases with increase in the pressure between the friction pairs. The torque between the friction pairs is provided by the asperity friction, and the torque due to the oil film reduces to zero. When the thickness of the oil film is small, a major contribution to the total torque is due to the asperity friction. The total torque also increases with the decrease in the film thickness ratio. Therefore, by theoretical analysis and experimental verification, the torque of the friction pairs in the mixed friction stage can be accurately calculated using the average shear stress model and asperity friction torque model.
Highlights
The hydro-viscous drive (HVD) which is widely used in fans, pumps, belt conveyors, and scraper conveyors is a kind of torque and power transmission device based on oil film shearing and friction for step-less speed regulation, soft starting, and overload protection [1]
7 Conclusions The calculation of the torque characteristics of friction pairs in the mixed friction stage was carried out using the theoretical model as well as experimental research
The conclusions are as follows: 1. The friction coefficient of the friction pairs in the mixed friction stage was determined by using the MM1000-III friction and wear testing machine
Summary
The hydro-viscous drive (HVD) which is widely used in fans, pumps, belt conveyors, and scraper conveyors is a kind of torque and power transmission device based on oil film shearing and friction for step-less speed regulation, soft starting, and overload protection [1]. The load-carrying capacity and torque are shared by the oil film and asperity contact. The contact model of a rough surface was set up in the mixed friction stage of the HVD by outlining the asperity contact properties [16]. The frictional torque of the wet clutch was calculated considering the effects of surface roughness, permeability, elastic modulus of the frictional material, lubricant viscosity, and temperature by using the analytical model and experimental measurement [20]. Assuming that the asperity peaks on the surface satisfy the Gaussian distribution with zero average value, Greenwood and Tripp calculated the actual contact area ratio based on the G-W model as [5]. The actual contact area of the friction pairs changing with the film thickness ratio is given as. In Eq (5), the shear stress will be infinite when the thickness of the oil film approaches zero, and when it is where R1 and R2 are the inner and outer radii of the friction pairs respectively
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