外文翻译--变速液压装置.doc

Variable Speed Hydraulic System It is particularly important on many hydraulic system, as on machine tools, to be able to vary the speed of operation at will .This can be carried out in the following ways, sometimes more than one way being combined: By varying the pump output manually; By using several pumps in combinations; By restricting or throttling the output of a automatically variable delivery pump, or a pump accumulator system, or by throttling the inlet; By by-passing part of the pump output with a flow dividing valve; By varying the volume of the operating jack. 1、Variation in Pump Delivery. Pump delivery can be varied by Alteration in its speed; Alteration of its stroke in a variable stroke type of pump; Using two or more pumps of different delivery in parallel so that by stopping and starting the pumps in various combinations different total deliveries can obtained. The first system is an easy one when the pump is electrically driven, although the electric motor involved is comparatively complicated dor normal requirements. Mechanical variable speed gear boxes have been used successfully with constant speed electric drive. Several of the pump mechanisms previously described can readily be adapted to five a varying output by reducing the working strok manually by means of a control wheel, etc. The third system is simple enough, but varies the output in fixed steps. Two pumps n parallel can give three ranges of output corresponding to Pump A, Pump B, Pump A Plus B. Three pumps in parallel can give seven steps corresponding to Pump A, Pump A Plus B, Pump B Plus C, Pump B, Pump A Plus C, Pump A Plus B Puls C. Pump C, Since, however, variable stroke pumps are readily available,such a complication as three pumps in parallel hardly seems worthwhile although the two-pump system is probably excellent for such duties as presses, etc. , where a great part of the working stroke is at low pressure, where a relatively cheap type of pump can be used, cutting out in f