外文文獻(xiàn)翻譯機(jī)械設(shè)計(jì)制造及其自動(dòng)化液壓泵中英文對(duì)照(存儲(chǔ)版)
【正文】 Control of pump delivery can be acplished either manually or automatically, using handwheel,levels, or variablespeed motors or transmission actuated by loadsensing controls. Pumps often are rated at the monly available electric motor speed of 1200 or 1800 also may be rated at speeds other than motor speeds. For instance, higher speeds occur in mobile hydraulic pumps driven from internal bustion engines. These engines usually operate at a constant speed and include speeds of 2021 rpm and higher. Some industrial hydraulic pumps are rated at speeds of up to 4000 rpm. The maximum safe speed for a rotating pump is limited by the pump’s ability to avoid cavitation and high outlet pressures. Most rotating pumps also require a minimum operating speed. Although these speeds usually are not critical, pumps operating at high pressures require a minimum speed in order to prevent overheating or internal slippage. Maximum speed and pressure ratings for pumps often are given for both intermittent and continuous operation. Continuous ratings describe the maximum speed and pressure at which a pump can be operated for a normal design life (about 10000 hours). Intermittent ratings are maximum speed and pressure at which a pump can be operated safely for short times and still have a satisfactory service life. Operating a hydraulic pump beyond its drive speed ratings usually reduces its service life. As pointed out earlier, the pressure a system exerts on the hydraulic pump directly affects the delivery rate of the pump. As the pressure increases, the flow rate of the pump decreases. The amount of decrease varies depending on the type of pump used. This change in flow affects the pump’s efficiency. Pump efficiency is described in two ways: Volumetric efficiency the ratio of the actual delivery rate to the theoretical displacement Overall efficiencythe ratio of the hydraulic power output to the mechanical power input. The reliability of a pump is determined by how well the characteristics of the pump are matched to the requirements of the system. Reliability can also be measured in maintenance time. Items such a how much fluid is required, how well the system is designed and maintained, wh
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