Heat Generation-dissipation Mechanism And Heat Balance Analysis On The Wet Axle Of Heavy Vehicle

Wet drive axles of heavy vehicle are highly resistant to contamination.They enable vehicles to travel in all types of environments.Wet hub brakes are clean,stable,and reliable.Wheel hub transmissions are lightweight and aid in improving ground clearance.Therefore,wet drive axles that possess all the aforementioned benefits are increasingly used in heavy-duty,cross-country and military vehicles.However,the heat is very unevenly distributed in wet axle owing to its structure characteristics such as three closed parts,compact structure,built-in brake,large span,etc.,and great changes in temperature occur in wet wheel hub on account of the differences of the production efficiency,the size of the workplace or the driving habits of drivers.During the working process of the vehicle,there are many kinds of energy loss in wet axle,such as gear meshing loss,bearing loss,oil churning loss,dragging loss of wet friction pair,brake friction loss and so on.So the large heat quantity is produced.Especially under cycle operation condition,the heat quantity of brake friction is huge.Lower or higher oil temperature will decrease transmission efficiency,higher oil temperature also can accelerate the deterioration of oil,reduce braking performance and lead to the failure of the hub seals.In order to prevent these problems,it is necessary for wet axles to maintain appropriate temperature levels to function properly.Therefore,the thermal analysis on the wet axles is of great significance for improving the economy,braking,reliability and operation efficiency of the vehicle.In this paper,based on the development of the wet axle for wheel loader in Liugong,the braking energy of wheel loader and the energy loss in the wet axles have been investigated in view of the structure characteristics of the wet axles.The temperature rising and distribution in the wet axles under different working conditions were studied.Meanwhile,the factors affecting the heat generation and dissipation of the wet axles were analyzed.A method for balancing and optimizing the heat distribution of wet axle,which can provide a theoretical reference for the design and optimization of wet axle,was put forward.The main contents of the paper are as follows:(1)Theoretical analysis and experimental study on influencing factors of braking energy for wet drive axles.According to the operation modes,braking states and structure characteristics of the wheel loader,the braking dynamic models have been established.Then the braking energy distribution,influencing factors and changing trend were analyzed and compared under two types of operation conditions.After the cycle operation stages were decomposed,the objective operation site,subjective driving habits and enterprise efficiency requirements were considered,and the effects of the vehicle's weight,the initial braking speed and the direction of travel were analyzed.Finally,the changes of braking energy can be tested.(2)System heat sources modeling and energy loss analysis for wet axles based on bond graph theory.The mathematical models,which are suitable for the different types of energy loss of heavy-duty vehicles,have been established.After the energy flow and loss differences of wet axles system were considered,the bond graph theory was used to establish the bond graph model and state equations of the wet axle transmission system.At last the heat sources model of the transmission system for wet axle was determined.In view of the four different stages of the cycle operation,the types,distribution and changes of the energy loss of the wet axle were analyzed by means of the system model of the heat sources.At the same time,the different factors affecting the energy loss were analyzed.It lays the foundation for further analysis and optimization of wet axle.(3)Fluid-solid-heat coupling analysis of wet wheel hub.Aiming at the serious heat accumulation of wet hub,the models for the heat flow field analysis of wet wheel hub,including numerical models and physical models,were established.Then the thermal analysis was carried out for the wet wheel hub,wet brake steel discs and friction disc groove cooling lubricants.In the end,the flow fields of cooling lubricants in wet hub and friction disc groove were analyzed.From the two aspects of heat generation and dissipation,the causes of heat accumulation in the wet axle have been found,which provide a basis for explaining the deterioration of the cooling oil in the wet wheel hub under the high efficiency operation of heavy vehicles,and also provide a reference for the design and optimization of the wet wheel hub.(4)Heat dissipation mechanism and equilibrium analysis on the wet axle.The heat transfer characteristics of the wet axle parts,such as heat distribution,heat conduction,heat convection and heat radiation,have been studied.Accoding to the relationships between parts,the heat transfer components were set as nodes,and the thermal resistances of different nodes were analyzed and determined.The thermal analysis model of the wet axle has been built by the thermal network method based on the heat transfer relations of heat sources and nodes,so the temperature rise process of wet axle was solved.After the instantaneous thermal characteristics and heat balance temperature in the wet axle were studied,the thermal optimization of the wet axle has been completed.(5)Experimental research on the temperature rise of wet axle.According to the working characteristics of whole vehicle and its wet axle,the temperature rise test system of wet axles have been designed and built.The simulation test models were set up in view of d SPACE,and the real-time measurement and control interface was established by using the Control Desk comprehensive test software.Finally,the overall scheme of vehicle test was designed based on the operation principle of wet axle test system on heavy vehicle.The temperature rise tests of wet axle mainly include the temperature rise test of the original and optimized wet axles.The results show that the distribution laws of the test and theoretical values are basically close,which indicate that the thermal analysis models are suitable and the numerical analysis methods are correct.