摘要:中间轴式变速器多用于前置后驱的汽车。该变速器具有两个特点：一是其直接挡传动效率高，磨损及噪声也很小；二是在齿轮中心距较小的情况下仍可以获得较大的一挡传动比。

本文主要在参考已有变速器基础上，对平头牵引车中间轴式变速器进行优化设计。首先，要结合整车数据，进行相关参数的选择与计算，其次选择主要的尺寸和性能参数。然后使用CAD绘制零件图及装配图，同时校核各种参数并进行布置方案。最后，对变速器改进优化，使其符合“标准化、系列化、通用化”的原则，同时零部件要有足够的可靠性及耐久性。

它功用是：（1）改变传动比，扩大驱动轮转矩和转速的变化范围，以适应经常变化的行驶条件，如起步、加速、上坡等，同时使发动机在有利的工况下工作；（2）在发动机旋转方向不变的前提下，使汽车能倒退行驶；（3）利用空档，中断动力传递，以使发动机能够起动、怠速，并便于发动机换档或进行动力输出。这台变速器具有五个前进档（包括一个超速档五档）和一个倒档，并通过锁环式同步器来实现换档。

关键词：牵引车，中间轴，参考设计

Abstract:The duty of this design is to design a FR type manual transmission used in the saloon.It’s the contershaft-type transmission gearbox,this transmission has two prominent merits.Firstly,the transmission efficiency of the direct drive keeps off;Secondly ,it’s all allowed to obrtain in the biger gear tatio of the first gear when the center distance is smaller.

This article has been transmission in the reference basis, middle of the tractor transmission shaft to optimize the design. Firstly, in conjunction with vehicle data, the choice of parameters and calculation, followed by selected major dimensions and performance parameters. CAD drawing and then use part drawings and assembly drawings, and check the parameters and make layouts. Finally, improved transmission is optimized to meet the "standardization, serialization, universal" principle, while parts have sufficient reliability and durability.

Its function is:（1）Changing gear ratio, expanding the torque of the driving wheel and the range of the rotational speed, to adapt the travel condition which frequently changes, like start, acceleration, climbing and so on, simultaneously causes the engine to work under the advantageous operating mode;（2）Under the premise of the invariable rotation, enables the automobile to travel back;（3）Using neutral, severances the power transmission, to make the engine start, idle, and is advantageous for the engine to shift gears or to carry on the dynamic output. This gearbox has five (including over drive—fifth gear) and a reverse gear, and through the inertial type of synchronizer to realize shift gears.

Keyword: tractor,countershaft,reference&design