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2025-01-15 09:24本頁(yè)面
【正文】 produced by the pressure regulating valve, the system work load ,and flow losses in the hydraulic tubing A pump is classified on the basis of the physical arrangement of its pumping mechanism and its basic principle of operating. Pumps classified by principle of operation include positive displacement and nonpositivedisplacementtypes. Positivedisplacementpumps ， are equipped with a mechanical separation(gears,vanes,or impellers) between the inlet and outlet, which minimizes internal leakage or slippage. Therefore,the output of positivedisplacement pumps is almost unaffected by variations in system pressure Nopositivedisplacement pumps(such as centrifugal pumps) do not have a positive internal separation against leakage or slippage. Because of this slippage,the delivery of these pump is reduced as the working pressure of a system is increased. However, nonpositivedisplacement pumps deliver a continuous flow, while positivedisplacement pumps deliver an intermittent(pulsating) flow. These pulsations are small and can be smooth out by the accumulator or the system piping. Most hydraulic pumps are positivedisplacement pumps of the rotary type. Positivedisplacement pumps have either a fixed or variable displacement. The volume of delivery ,or gpm, of a fixeddisplacement pump can be changed only by changing the speed of the pump, because the physical arrangement of the pumping mechanism cannot be changed, (This does not mean that the flow in other portions of the system cannot be adjusted by valves.) The flow of a variabledisplacement pump can be changed by changing the physical arrangement of the pumping mechanism with a builtin controlling device. This device often functions in response to system pressure or other signals. Variabledisplacement pumps are more plex than fixeddisplacement pumps and,therefore,cost more. In addition, the efficiency of a variabledisplacement pump is lower than that of a fixeddisplacement pump. This is offset somewhat by the higher overall efficiency of a system powered by a variabledisplacement pump. Most positivedisplacement pumps are classified as rotary pumps. This is because the assembly that transfers the fluid from the pump inlet to the pump outlet has a rotating motion. Rotary pumps are further classified according to the mechanism that transfers the fluidsuch as gears,vanes,or screws. A different kind of positivedisplacement pump is piston pump. This pump uses a reciprocating (backandforth) motion of the piston, alternately to receive fluid on the inlet side, and to discharge fluid on the outlet side. A radialpiston pump has a revolving assembly with several piston assemblies built into it, and can be classified as a rotary pump. Several types of piston pumps will be
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