大功率牵引传动CVT的设计原理

Design principle of High power Traction Drive CVT

人们不断要求汽车传动系具有最佳的操纵性和油耗 ,可满足这个要求的方法之一是 CVT(无级变速器 )。已经有报告表明 V型钢带推块 CVT改善燃油消耗大于 1 5% [1 ],已经生产了 70 0 ,0 0 0台以上的 CVT在实际应用中取得了很好的成果 ,近 1 0年来 CVT已经缓慢地走向世界。但到目前为止 ,市场上所有的这种汽车都是小型的 ,排量低于 1 30 0 cc。最近有报告表明大功率的 V型钢带已在 3.3L带篷货车和赛车上进行试验 ,但对于大功率发动机的 CVT仍未投放市场。所几年来 ,汽车已经对大功率 CVT提出需要 ,满足这种要求的牵引传动 CVT已有可能。在牵引传动中 ,动力是由两滚动体间存在的弹性流体润滑油膜 (EHL)的剪切力来传递的。牵引传动的性能在很大程度上取决于弹流接触液体的流变特性。 1 960年 [3]已经形成了该理论。大家知道 ,Hewko[4]在 1 960年制成的复曲面变速器是用于汽车的牵引传动 CVT的一个实例 ,Kraus[5]进行了车辆试验。 Lowenthal[6]和 Coy[7]完成了基础研究。由 Monsanto开发了牵引油 ,但 CVT仍未进入商业性的实际使用。其原因如下 :1 .因为在牵引接触区的高压和高温 ,目前牵引传动对滚零件的材料的可靠性不够。2 .牵引油不能满足汽车的所有工况。3.不具有能支承高速和大的轴向负荷的轴承。1

Better drivability and fuel consumption are continuous issues for driving system of automobiles. One of the answers is CVT (Continuously Variable Transmission). It was reported that push steel V Belt CVT improved fuel consumption by more than 15% [1] . More than 700,000 CVT units have been produced and they have obtained good rdxults in the market. In 10 years, CVT has been spreading slowly throuout the world. But so far the cars on the market have been small, that is less than 1300cc cars. Recently, some reports mentioned that a high power V Belt was tested in 3.3L Van or Formula 1 racing cars, but CVT for large engines have not been put on the market yet. In recent years, requests for high power CVT for automobiles have come to light. Traction Drive CVT has the possibility of satisfying this requirement. In traction drive, power is transmitted by the shear force of the EHL (Elastohydrodynamic Lubrication) [2] oil film existing between a pair of rolling elements. The performance of traction drive depends to a large extent on the rheological. properties of the fluid at the EHL contact. This theory was made in 1960's [3] . As is well known, examples of traction drive CVT applied to automobiles in the 1960's were Toric transmission by Hewko [4] and the vehicle test by Kraus [5] . Basic research was performed by Lowenthal [6] and Coy [7] . Traction fluid was developed in 1970's by Monsanto. But a traction drive CVT has not been put into practical use commercially. The reasons were as follows: (1) the materials of the traction drive rolling elements were not sufficiently reliable because of high pressure and high temperature at the traction contact point. (2) there was no traction oil which satisfied all the conditions of automobiles. (3) there were no bearings which could support high speed and large axial load. The authors started research and development of a Half Toroidal traction drive CVT in 1978. From 1982 to 1987, 8 prototypes were made and life tests performed. Some prototypes were installed in cars and road tests were performed. The result of this research was reported in 1990 [8] . Development of the speed ratio control mechanism and development of a double cavity CVT were reported by Nakano [9] . A New design for heavy truck was reported by Lohr and Dawe [10] . In the United Kingdom, Full Toroidal CVT has been developing earnestly [11] . For over 4 years we have been developing high power traction drive CVT by using better materials and heat treatment of rolling elements. Development of a better traction fluid for automobiles has helped this research progress tremendously. Now, the dream of a 5000cc CVT car is coming true. In this paper, the design principles behind high power traction drive CVT are presented.