Ch. 4 - CHAPTER 4 Creation and Control of Fluid Flow Fluid...

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1 CHAPTER 4 Creation and Control of Fluid Flow Fluid Power Circuits and Controls, John S.Cundiff, 2001 INTRODUCTION z Primary flow control device in any circuit is the pump. It converts mechanical power to fluid power. z Fixed displacement pump delivers a fixed volume of fluid per revolution called the displacement (in 3 / rev).
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2 INTRODUCTION (contd…) z Pump is driven at given rpm. Flow is measured with a flow meter. Pump displacement is defined by in 3 / min = in 3 /rev rev/min z Variable flow pumps: displacement can be changed as the pump is operating. z Leakage through the clearance between moving and stationary parts increases with pressure, while output flow decreases, refer to flow vs. pressure curves. INTRODUCTION (contd…) z Pump volume efficiency is e vp = Q a / Q t Q a = actual output (in 3 / min ) Q t = theoretical output (in 3 / min ) Theoretical flow is Q t = NV pth N = pump speed (rpm) V pth = pump displacement
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3 INTRODUCTION (contd…) z Overall pump efficiency is e op = Output power (hyd)/ Input power (mech) z Output power is P out = PQ / 231 1714 P = pressure (psi) Q = flow (in 3 / min) z Pump volume efficiency and overall efficiency both decrease as pressure increases. The rate of decrease depends on pump design. Fixed Displacement Pumps z Vane and piston pumps are available as fixed or variable displacement pumps. z Gear pumps are available with fixed displacement and are less expensive. z Disadvantages of gear pumps: Leakage in some gear pumps is high. Leakage increases as pump wears over time. Operating cost is higher z Pressure-balanced gear-pumps have volumetric efficiencies that exceed or rival those of piston pumps.
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4 Fixed Displacement Pumps (cont. .) z One gear is driven with input shaft and it drives the second gear. z Fluid is captured by the teeth as they pass the inlet z Oil travels around the housing and exits at the outlet. z Design is simple and inexpensive Fixed Displacement Pumps (cont. .) z The gerotor has an inner drive gear and an outer driven gear. z Inner gear has one tooth less than outer gear. This feature creates chambers of decreasing volume resulting in “plumbing action”. z A port plate ensures that fluid enters the chambers when it is largest and exits when smallest.
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5 Fixed Displacement Pump Circuits z Circuits with fixed displacement pumps are called constant-flow circuits. z Key Concept – Each revolution of a fixed displacement pump delivers a certain volume of fluid to the circuit. This fluid ultimately returns to the reservoir, either as a return flow, or a leakage flow. Fixed Displacement Pump Circuits (cont.) z Simple circuit with fixed displacement. z Fluid passes through the open center DCV back to reservoir. z
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Ch. 4 - CHAPTER 4 Creation and Control of Fluid Flow Fluid...

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