機(jī)械設(shè)計(jì)專業(yè)畢業(yè)設(shè)計(jì)文獻(xiàn)翻譯--履帶牽引裝置的液壓傳動(dòng)-機(jī)械設(shè)計(jì)-全文預(yù)覽
【正文】 d 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 are 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 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 (gal/mm). For purposes of paring 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, the 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 mechanicallydriven 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, the 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 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, ., depth of plowing or number of blades. To improve their adaptability further, these specialized machines are usually equipped with many 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 horsepower use, is left pletely to the operator,s judgement. 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 nonpositive power train ponents, and run at their maximum efficiency at only one ratio of output rpm (n2) to input rpm (n1). Whenever this ratio changes due to varying drawbar p
環(huán)評公示相關(guān)推薦
鄂ICP備17016276號-1