CHAPTER 4 - University of Florida...5 Fixed Displacement Pump Circuits zCircuits with fixed...

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CHAPTER 4

Creation and Control of Fluid Flow

Fluid Power Circuits and Controls, John S.Cundiff, 2001

INTRODUCTION

Primary flow control device in any circuit is the pump. It converts mechanical power to fluid power.

Fixed displacement pump delivers a fixed volume of fluid per revolution called the displacement (in3/rev).

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INTRODUCTION (contd…)

Pump is driven at given rpm. Flow is measured with a flow meter. Pump displacement is defined by

in3/ min = in3/rev rev/min

Variable flow pumps: displacement can be changed as the pump is operating.Leakage through the clearance between moving and stationary parts increases with pressure, while output flow decreases, refer to flow vs. pressure curves.


Pump volume efficiency is evp = Qa / Qt

Qa = actual output (in3/ min)Qt = theoretical output (in3/ min)

– Theoretical flow is Qt = NVpth

N = pump speed (rpm)Vpth = pump displacement

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Overall pump efficiency is

eop = Output power (hyd)/ Input power (mech)

Output power is Pout = PQ / 231

1714P = pressure (psi)Q = flow (in3 / min)

Pump volume efficiency and overall efficiency both decrease as pressure increases. The rate of decrease depends on pump design.

Fixed Displacement Pumps

Vane and piston pumps are available as fixed or variable displacement pumps.Gear pumps are available with fixed displacement and are less expensive.Disadvantages of gear pumps:

– Leakage in some gear pumps is high.– Leakage increases as pump wears over time.– Operating cost is higher

Pressure-balanced gear-pumps have volumetric efficiencies that exceed or rival those of piston pumps.

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Fixed Displacement Pumps (cont..)

One gear is driven with input shaft and it drives the second gear.Fluid is captured by the teeth as they pass the inlet Oil travels around the housing and exits at the outlet.Design is simple and inexpensive

Fixed Displacement Pumps (cont..)

The gerotor has an inner drive gear and an outer driven gear.Inner gear has one tooth less than outer gear. This feature creates chambers of decreasing volume resulting in “plumbing action”.A port plate ensures that fluid enters the chambers when it is largest and exits when smallest.

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Fixed Displacement Pump Circuits

Circuits with fixed displacement pumps are called constant-flow circuits.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.)

Simple circuit with fixed displacement.Fluid passes through the open center DCV back to reservoir.Pressure required to overcome pressure drop in the lines and through DCV develops.When an operator shifts the DCV, flow is diverted to the cylinder. Now the pressure equals all drops.A key disadvantage of a constant flow circuit is that pressure must build from very low level to the level required to accelerate the load.Cycle time is important !

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Fixed Displacement Pump Circuits (cont.)

When load is such that operating pressure is close to the cracking pressure of the relief valve, the pump flow can be divided into three flows.

– Flow through relief valve back to the reservoir.– Flow to the load.– Leakage flow through DCV back to the reservoir

(smallest of the three flows)

Variable Displacement Pump Circuits

Variable displacement pump circuits are called demand flow circuits.Circuit has no relief valve. Although not shown, relief valve should always be used to ensure that pressure can never reach unsafe levels.A dangerous pressure spike can be produced by rapid deceleration of a large load.

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Variable Displacement Pump Circuits (cont..)

Vane pump have a series of spring loaded vanes that slide back and forth in slots. The chamber formed between adjacent vanes and cam ring decreases in size as the rotor turns. Fluid flows into chamber when it is maximum size and exits when it is minimum size.Change in chamber size provides the pumping action.


The cam ring is held in position with a threaded rod turned with hand wheel. The ring slides to the left when the wheel is turned.

Vane pump can be converted to pressure compensated pump by replacing the hand-wheel adjustment with a spring.

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A small cylinder called the compensator, is placed on the opposite side.Outlet pressure acting on the

compensator piston creates a hydraulic force that opposes spring force. When outlet pressure rises to certain point, hydraulic force becomes greater than spring force and cam ring shifts to the left.As pressure continues to rise, ring shifts more to the left until it is centered on the axis of rotation.Pump displacement is almost zero.


Maximum pressure is limited by the compensator spring in the pressure-compensated pump.

Pressure compensated valve can maintain deadhead pressure with very little energy input. Hydraulic power output is proportional to pressure times flow. If flow is zero, hydraulic power output is zero.

Advantage of demand flow circuit over constant flow circuit:

– Pressure is available from the instant the DCV is shifted, it does not build from zero.

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Flow vs. Pressure characteristics for pressure-compensated variable displacement vane pump When pressure reaches 2900 psi (cutoff pressure), the cam ring begins to shift and pump flow decreases.Rate of decrease (slope of the curve) is set by the spring constant of the compensator spring.


Exploded view of pressure compensated variable displacement pump.

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There are two piston pump designs:

– Axial Piston Pump.

– Radial Piston Pump.


Axial piston pump has a series of cylinders mounted parallel to the axis of rotation.Arrangement is similar to shell chambers in a revolver.Each piston installed in the cylinder has a spherical end that mounts in a shoe. The shoe is held against a swashplate by a spring in the cylinder block.Swashplate remains stationary as the cylinder block rotates with the input shaft.

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When the swashplate is at an angle to the shaft, it moves the piston back and forth in the cylinders as the cylinder block rotates.

This movement provides the “pumping action”.

It is helpful to follow the motion of one piston as the cylinder block makes one revolution.


Schematic illustrating the motion of one piston during a single rotation of the cylinder block.

Axis of rotation is in the plane of the paper.

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Implementation of actual axial piston pump designThree key components:

– Cylinder block spring.Holds block in the position so the piston shoes are always in contact with swashplate. This spring rotates with the cylinder block.

– Yoke spring assembly.This spring holds the swashplate against the actuator piston .

– Actuator Piston.When fluid flows into the cylinder the actuator piston extends and reduces the angle of the swashplate.


Axial piston pump can be configured as a pressure-compensated pump. Outlet pressure (high pressure), Ps, is incident on the end of the compensator valve spool. Ps multiplied by the area of the spool gives a hydraulic force, Fh, which is opposed by the spring force Fs, produced by compensator valve spring.

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When Ps increases to the point where Fh equals Fs, the spool shifts downwards and the fluid flows to the actuator piston.

The pressure at the actuator piston is Pc = Ps - , where is the pressure drop across the orifice formed when the compensator valve cracks open.

As Ps increases, the compensator valve opens more, decreases, and Pc approaches Ps .

Increase in Pc increases the hydraulic force produced by the actuating piston, and it rotates the yoke until it is perpendicular to the shaft and the pump displacement is zero.

The pump will hold this pressure and deliver no flow until something is done to lower the pump outlet pressure.

Pressure- compensated axial piston pump can be used in a circuit without relief valve.

PΔ

PΔ

PΔ


Radial Piston Pump:Principles

Cylinders are positioned radially around the axis of rotation. As the shaft rotates, the connecting rods push the pistons back and forth in the cylinders to develop the pumping action.

Commonly used for pneumatic systems in the fluid power industry.

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Improvement in Efficiency with Load Sensing

Load Sensing was developed to improve the efficiency of a circuit. It requires a variable displacement pump.

An example: – An application requires max. 20 GPM at max. 2500 psi. Select a pump to supply

20 GPM at 2500 psi. However, application requires mostly less than 20 GPM at 2500 psi. Percentage of total operating time the system operates under reduced load, and the way the system responds to this condition, is a key issue. The specific reduced load situation considered is the activation of a cylinder requiring metered flow rate of 9 GPM at 1300 psi.

Consider the operation of an open-centered and close-centered circuit that meets the functional objective and then consider a closed-center circuit with load sensing.


Open-Center Circuit with fixed displacement pump delivers 20 GPM continuously. The relief valve is set on 2500 psi. When the DCV is manually shifted, the pump builds pressure to 1300 psi, and the load begins to move. The operator partly closes the DCV, thus creating a restriction, until the restricted flow gives the desired load speed. The flow is dumped at some pressure less than 2500 psi, the full open position of the relief valve.

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To produce the graph at bottom of Fig 4.17, assume 11 GPM is dumped at 2500 psi.

The power loss for the open circuit isPloss oc = 2500(11) = 16 hp

1714

Alternate calculation:Max. Power loss=Total Pump power – power utilized Power loss = 2500(20) – 1300(9) = 22.34 hp

1714 1714


Closed center circuit has a variable displacement pump. Compensator valve spring is a 2500 psi spring. The pump builds 2500 psi before the compensator valve moves to open a pathway for fluid to flow to the yoke actuating piston. The piston extends and forces the swashplate to the zero displacement position. The pump then maintains the 2500 psi and supplies only enough flow to replace leakage.

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The operator cracks open the DCV to start the load. Pressure drops slightly and the compensator valve moves slightly to partly close the pathway to the yoke actuating piston. Piston retracts slightly and the swashplate tilts slightly so that the pump is now delivering fluid to the DCV and thus the load. This sequence of events continues until the operator has opened valve to the position where 9 GPM is flowing to the load. Pressure is less than 2500 psi, depending on the characteristics of the compressor valve spring.Load pressure is 1300 psi, thus pressure drop across the DCV is 2500 – 1300 = 1200 psi. Pump is delivering 9 GPM, thus the power loss is

Ploss cc = 1200(9) = 6.3 hp1714


Closed Center Circuit with Load Sensing is achieved with a special compensator mounted on a variable displacement pressure compensated pump.Assume special compensator is set to destroke the pump at 200 psi. As the DCV is shifted to extend the cylinder, a pilot line senses the 1300 psi required to move the load. Work port pressure is added to the “destroke” pressure, and the pump delivers the required 9 GPM at 1300 + 200 = 1500 psi.

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Actual power loss in this case isPloss ls = 200(9) = 1 hp

1714

The three-way, two-position DCV is shifted with pilot pressure. The pump is destrokedwhen this valve is in the position shown.There is no pilot pressure on the left side. The pump pressure applied on the right side shifts the DCV to the position shown.There is no hydraulic pressure to add to the spring force (200 psi) in the compensator. Thus pump builds only 200 psi pressure. When the operator shifts the three-position DCV, the pilot line is connected to supply pressure, which is 1300 psi in this example. This pilot pressure acts on the left side of the two-position DCV, causing it to shift to the right.Supply pressure is added to the spring force in the compensator and the pump builds 1300 + 200 = 1500 psi.

Comparison of Pump Performance Characteristics for Three Main Designs

Data was selected from manufacturers’ literature for a fixed displacement pump of each design: – Gerotor (gear)– Vane– Piston

Pumps with similar displacements were selected from three different manufacturers

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Gerotor Pump– The Hydreco Model 1919 gerotor pump has a displacement

of 4.53 in3/ rev. The manufacturer gives performance data up to a max. speed of 3000 rpm. The max. pressure curve is 2500 psi.

– When driven at 1200 rpm, the volumetric efficiency drops from 92% at 500 psi to 78% at 2500 psi. (Fig 4.21a)

– At 1800 rpm, the volumetric efficiency drops from 93% at 500 psi to 84% at 2500 psi. (Fig 4.21b)


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The reason for higher volumetric efficiency at the higher speed can be explained as follows.– The leakage flow is given by

Q1 = Qt - Qa

Where Q1 = leakage flow (GPM)Qt = theoretical flow (GPM)Qa = actual flow


Theoretical flow at 1200 rpm is given by Qt = 1200(4.53) = 23.53 GPM

231Volumetric efficiency at 1500 psi is evp = 0.85, so

Qa = Qt evp = 23.53(0.85) = 20.0 GPM

Substitution into previous equation gives, Q1 = 23.53 – 20 = 3.53 GPM

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Leakage flow is primarily a function of pressure, with rotational speed being a much less significant factor. Suppose the leakage flow is same for performance test run at 1800 rpm, 1500 psi.

Q1 = 1800(4.53) = 35.3 GPM231

= (Qt - Q1) = 31.77Volumetric Efficiency is

evp = Qa = 31.77 x 100 = 90 %Qt 35.3


Measured volumetric efficiency was evp = 88.7%,

slightly less than efficiency calculated by assuming constant leakage flow. There is some increase in leakage at higher speeds due to increased turbulence of the fluid.Good design practice : Select a smaller pump and operate it at higher speed to achieve a higher volumetric efficiency.

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Volumetric efficiency in the 85% to 90% range is achievable with a gerotor pump (pressure < 1500 psi). Volumetric efficiency decreases as operating temperature increases. Viscosity of the fluid decreases and more leakage occurs throughclearances in the pump.Overall efficiency accounts for the loss of mechanical energy due to friction and loss of hydraulic energy due to leakage flow.Clearances between moving parts are thought of as orifice. Fluid on one side of the orifice has a high pressure and fluid at the other side has pressure equal to that in the pump housing.Overall efficiency for Hydreco Model 1919 gerotor pump is plotted in Fig 4.22a (1200 rpm) and Fig 4.22b (1800 rpm). Efficiency is near maximum at 1000 psi. Increase in overall efficiency at higher rpm is smaller


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Key comparisons between volumetric and overall efficiencies for the gerotor pump.


Vane Pump

The Vickers Model 25V (T) vane pump has a displacement (cam ring fixed for maximum displacement) of 4.81 in3/rev.

The specifications:– Maximum Speed 1800 rpm– Maximum Pressure 2500 psi

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Volumetric efficiency at 1200 rpm decreases from 97.3% at 500 psi to 84% at 2500 psi (Refer Fig 4.24a).

The vane pump volumetric efficiency is 5 to 6% higher than the gerotor pump. The volumetric efficiency is higher because leakage flow is less.

Volumetric efficiency at 1800 rpm decreased from 97.8% at 500 psi to 88% at 2500 psi (Refer Fig 4.24b). With the gerotor pump, it is higher at higher shaft speed across the whole pressure range.


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Overall efficiency of vane pump is quite low at low pressures.

Overall efficiency at both 1200 and 1800 rpm increases to a maximum of 1000 psi. (Refer Fig 4.25a and 4.25b)

Decrease in overall efficiency as pressure increases from 1000 psi to 2500 psi is less for the vane pump than the gerotor pump.

The vane pump efficiency curve is “flatter”, indicating that vane pump performance will change less as load pressure changes in 1000 to 2500 psi range.


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Comparison of Volumetric and Overall Efficiency for a Vane Pump operated at 1200 and 1800 rpm.


Axial Piston Pump

The Sauer-Danfoss Series 90-075 variable displacement axial piston pump has a displacement of 4.57 in3/rev.

The specifications : – Maximum speed : 3950 rpm– Maximum pressure : 7000 psi

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The pump has a higher maximum speed than the gerotor pump (3000 rpm) or the vane pump (1800 rpm). It is rated for much higher pressure (7000 psi vs. 2500 psi).

Volumetric efficiency at 1200 rpm decreases from 99% at 1000 psi to 89% at 6000 psi (Refer Fig 4.26a)

Volumetric efficiency at 1800 rpm decreased from 98.8% at 1000 psi to 90.3% at 6000 psi. (Refer Fig 4.26b)

Increase in volumetric efficiency at higher speed was less for the piston pump than the gerotor or the vane pump.


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Overall efficiency of the piston pump is quite high (>90%) for the pressures < 4000 psi.

Overall efficiency at 1800 rpm is slightly higher than 1200 rpm. (Refer Fig 4.27a and Fig 4.27b). Since leakage flow is predominantly a function of pressure, not speed.


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Comparing power loss (expressed as percent of input power) for the three pump designs, calculated at 1200 rpm and 1500 psi.

The trend in loss is such that the vane pump is about midway between the gerotor and the piston pumps.


Losses due to friction are higher in the vane pump than piston pumps. Both pumps have relatively high number of moving parts as compared to the gerotor pump.

Vane pump has a larger lubricating film area than the piston pump, so the friction power loss is 11% for the vane pump and only 5% for the piston pump.

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Multiple Pump Circuits

Multiple Pump Circuits.

Multiple pump design places two or more pumps in the same housing and drives them with a single input shaft.

Sometimes only one inlet is provided, but each pump has is own outlet.

Advantage – Isolation of circuits.


The pump supplies flow to a hydraulic motor and cylinder.When solenoid activated, two position DCV is shifted, flow is directed to the motor.If the manually activated three-position DCV is shifted, flow is diverted to the cylinder. Pressure in the motor and cylinder decides how the flow will divide. If motor pressure is higher, flow will go to the cylinderBy this the motor will stop until the three position DC is recentered or cylinder reaches its full extension and the pressure builds to the pressure required by the motor circuit.

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It is undesirable to interrupt the motor operation to activate the cylinder.

An efficient solution to this is to use a multiple pump, the same electric motor and the same hydraulic components.

An exception is that the three-position DCV now has a open center rather than a closed center.

Each pump has a relief valve to protect the circuit.


Gear pumps are available with more than two pumps in the same housing, commonly referred to as a tandem pump.

Many manufacturers of all three pump designs supply models with a pad for mounting a second pump on the primary one.

Different pump designs can be mixed – Ex : Multiple gear pump can be mounted on the auxiliary mount

of a piston pump.

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Pump Mounts

Pump Mounts are gear boxes that are designed to power hydraulic pumps.

They have provision for mounting one to four pumps on the output side.

Pump Mounts

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Basics of Directional Valves

The valves in the next figures control the direction of the oil flow and are called directional control valves (DCVs).Each of the valves shown is a four-port device (4 connections to the hydraulic circuit).Ports P and T provide pressure and return, while A & B are for connection to the actuator or circuit to be controlled by the valve.

Directional Control Valve

Goering, 2003, Off-Road Vehicle Engineering Principles

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4 Way – 3 Position ValveCenter Conditions

• Closed Center: has no flow when valve centered•Open Center: relieves pressure to tank when centered•Float Center: relieves pressure from load and pump


Valve Basics, con’t

These DCV’s are three-position valves with the valve spool shown in the centered position.The valve spool can be slid to the right to align the left-most box with the external ports, or to the left to align with the right-most box with external ports.

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4 Way – 3 Position Valve

Lab-Volt, Hydraulic Trainer Manual

4 Way – 3 Position Valve

Lab-Volt, Hydraulic Trainer Manual

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Flow Control Valves

Flow control valves.

Used in constant-flow (fixed displacement pump) circuits to control actuator speed.

Simplest type – needle valve also called non-pressure-compensated flow control valve.

Turning manual adjustment on needle valve causes needle to move down intothe orifice, thus reducing orifice area.

Pressure drop across valve ( ) is increased by continuously restricting the orifice until enough pressure is produced.

Extra turn will further reduce the orifice, increase the , increase the pressure relief valve, dump more fluid to the reservoir.

fcPΔ

fcPΔ

Flow Control Valves

Flow across the valve represents an energy loss.

Needle valve is inexpensive, but the operating cost is high because of energy loss.

Pressure-compensated flow control valve has the provision for changing the as the load pressure changes.

Total pressure at the relief valve is Pr = + (maintained constant)

As increases , decreases, and vice versa.

Constant Pr means constant load on pump and constant flow on relief valve.

fcPΔ LPΔ

LPΔ fcPΔ

fcPΔ

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Flow Control Valves

Schematic of a pressure-compensated flow control valve.A force balance on the spool of the valve shown. A spring is rated at 100 psi, i.e it produces force equivalent to a 100 psi pressure.Force balance on the spool is

PcAc – PcAr – 100 Ac ) = 0

Ac = area of spool cap (in2)Ar = area of spool rod end (in2)Pc = pressure in cavity between

the two spool ends.

Flow Control Valves

Pressure Pc must equal 100 psi for spool to be in force balance. Spool finds the position that maintains 100 psi in the center cavity. If inlet pressure is 500 psi, pressure drop across the orifice in Fig 4.31a is = 400 psi.The pressure drop between the center cavity and the outlet valve is 100 psi.This pressure drop sets the flow through the orifice created by the position of the handwheel adjustment.

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Flow Control Valves

Refer Fig 4.31b

Complete schematic for the flow control valve is shown.

The spring cavity opens to the downstream pressure.

The downstream pressure adds to the spring pressure to give the total pressure in the center cavity.

Flow Control Valves

Orifice equation (from chapter 2)

If is constant, then Q will be constant.Using the pressures shown in Fig 4.31b, Cavity pressure = Downstream pressure - Spring pressure

= 200 + 100 = 300 psiThe spool finds a position where the orifice is

= 500 – 300 = 200 psi

Pk Q Δ=

PΔ

PΔ

oPΔ

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Flow Control Valves

If the downstream pressure goes as high as 400 psi, Pc = 400 +100 = 500 psi

Then = 500 – 500 = 0 psiDenoting the downstream pressure as PL and the pressure drop across handwheel orifice as , total pressure at the inlet is

Pfc = + + PL

Three cases from this equation are as follows.

oPΔ

hwPΔ

oPΔ hwPΔ

Flow Control Valves

Case 1 PL = 0Pfc = 400 + 100 = 500 psi

Case 2 PL = 200Pfc = 200 + 100 + 200 = 500 psi

Case 2 PL = 400Pfc = 0 + 100 + 400 = 500 psi

For all the three cases , the pressure at inlet valve is held constant. Pump and relief valve “see” the same pressure, thus flow is held constant.

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Flow Control Valves

Flow dividers valves split the flow from a single pump to supply two circuits that operate at different pressures.Orifices can give splits anywhere from 50:50

to a 90:10 split.

Flow Divider Valves

The flow divider valve shown can be used with a fixed-displacement pump.

The flow divider valve, or priority valve, is also pressure compensated .


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Flow Control Valves

Cross section of flow divider is shown, where flow enters port 1 and exits port 6.Key component is the spool (component 2)Spool has passage drilled down the center.Fluid enters spool at 3 and splits to flow along passage 4 in both directions.Orifices at both ends of passage 4 are identified as component 5If they are same size, flow divider is designed for 50:50 split.

Flow Control Valves

When flow through both orifices is same, pressure drop is same at both ends, and spool is in force balance.

If pressure at left port is lower than at right port, fluid entering 1 takes the path of least resistance and flows to left port.

Higher flow at left orifice produces higher pressure drop at the orifice.

Greater pressure on the upstream side of left orifice creates a force imbalance on spool and shifts it to the left.

Spool moves closer to end plate and partially blocks the orifice. End of the spool moves towards the end plate.

Spool moves until it finds the position where flow is equal in both directions.

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Circuits using Flow Control Valves

Flow control valve to meter into the actuator (Fig 4.34)A flow control valve with a built-in check valve is used for the meter-in and meter-out circuits so that the cylinder can be retracted at full speed.


Flow control valve to meter flow out of the actuator (Fig 4.35)

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No check valve is needed for the bleed-off circuit, because a return flow path through the DCV is provided.Flow will take the path of least resistance, therefore, it will go through the DCV rather than through the flow control valve.

END OF CHAPTER 4

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