机械设计专业毕业设计文献翻译履带牵引装置的液压传动.docx

1、机械设计专业毕业设计文献翻译履带牵引装置的液压传动Hydrostatic drive for crawlers:Some design advantagesHydrostatic operation of vehicles such as dozers and loaders implies use of in engine to operate a hydraulic pump or pumps. High pressure hoses are used to transmit the fluid pressure and flow to separate drive motors whic

2、h can be located some distance from engine and pump. The engine can be run at its most efficient conditions with a reversible, variable swash plate pump. The hydraulic motors can also make use of variable swash plates, simplifying both hydraulic designs. Only the design advantages of such a system a

3、re covered here, though service procedures, life expectancy, and operators all can benefit from other aspects of such drives. A major design objective in the construction equipment manufacturing business since the 1960s has been more efficient use of installed horsepower. Low efficiency means energy

4、 wasted as heat which, in its turn, usually requires extra power to drive fans and maintain temperature. The more inefficient the machine, the hotter it gets, so heat serves as a measuring scale for overall efficiency. Within a machine, engine horsepower is transformed into force (psi) and speed (ga

5、l/mm). For purposes of comparing different systems one can use the expression: Engine hp Force x Speed where force means drawbar pull (lbs) and speed means travel speed (mph). Since all machines have internal losses, the ideal force/speed hyperbola cannot be achieved. However, the more efficient, th

6、e closer a machine system approaches the ideal curve, and the more work it can produce from a given amount of installed engine horsepower (Fig. 1). To appreciate the efficiency of a hydrostatic drive, it is necessary to look at the power utilization of conventional drives. Mechanical drive In a mech

7、anically-driven crawler dozer, gears must be switched for different speeds at varying resistances (drawbar pull). Once in gear, there is a positive, non- variable drive connection between track and engine. In fact, speed in a certain gear can only be varied by changing the engine speed. Therefore, t

8、he drawbar pull and efficiency in every gear speed parallel that of the engine. The advantage of a mechanical drive is that, at a certain travel speed in each gear, its efficiency will be higher than that of a hydrostatic drive. Thus if a crawler tractor could work at those speeds only, there would

9、be no need for other drive systems. In fact, agricultural and forest uses sometimes offer a possibility for adjusting the resistance to fit the mechanical drive, i.e., depth of plowing or number of blades. To improve their adaptability further, these specialized machines are usually equipped with ma

10、ny gear speeds which are close together. Disadvantages of these drives are: The machines are not very well suited for normal tractor applications with constantly varying resistances. Their efficiency drops considerably as soon as the travel speed changes. Speed selection, and with it engine horsepow

11、er use, is left completely to the operator,sjudgement. As shown in Fig. 2, at a certain resistance, many of several speeds may be chosen influencing efficiency and horsepower usage. Torque converter Torque converters (liquid clutches) are non-positive power train components, and run at their maximum

12、 efficiency at only one ratio of output rpm (n2) to input rpm (n1). Whenever this ratio changes due to varying drawbar pull requirements, efficiency drops rapidly (Fig. 3). This figure also shows the efficiency curve of a hydrostatic drive for comparison. At the peak of the torque converter curve it

13、 is also possible that its efficiency is actually higher than that of the hydrostatic drive. To use this, torque converters work in combination with a power-shift transmission and the indicated efficiency curve of the torque converter repeats itself with every gearspeed step. The indicated hydrostat

14、ic drive curve covers the machine s complete travel speed range. It should also be noted that during machine acceleration from a standstill or directional change, the hydrostatic drive becomes much more efficient more rapidly. Powershift transmission Combination of a torque converter with a powershi

15、ft transmission improves engine horsepower use considerably compared to a mechanical transmission. But due to the efficiency behavior of the torque converter, powerspeed curves will still not parallel the ideal curve. Again, use of the installed engine horsepower (efficiency) depends a great deal on

16、 the operator who, at a given resistance (drawbar pull), may select any of several possible speeds (Fig. 4). Torque divider transmission (lock. up converter) - To improve its engine horsepower use, the torque divider transmission uses the advantages of both the powershift and mechanical transmission. Since, in certain speed brackets, the mechanical (direct) drive offers a little higher efficiency, the torque converter will loc