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TSM363 Fluid Power Systems

Pumps and Actuators

Tony Grift

Dept. of Agricultural & Biological EngineeringUniversity of Illinois

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Agenda

• Units, Pumps, Pressure Relief Valve• Cylinders

• Double acting/ Single acting• Single rod, Double rod

• Cylinder construction• Pressure, Flow, Work and Power in cylinders• Pressure, Flow, Torque and Power in

pump/motors• Volumetric and Torque efficiency of

pump/motors• Pump implementations

3

Hydraulic Units (SI)

rad

mD

Nm

s

NmP

NmW

NF

mA

s

mq

m

Np

3

2

3

2

nt Displaceme

T Torque

ow Power

Work

Force

Area

Flow

Pressure

Pascal

Newton

Watt

Pascal 1010 Bar 1 52

5

m

N

4

Control can take place in various ways. Power is pressure times flow rate

• Pressure control• Pressure relief valve• Pressure reducing valve (regulator)• Pressure compensation: Only provide the pressure needed to

move the load(s). In idle reduce energy loss by providing a open center condition

• Pressure compensated pump. Make the pressure independent of the flow required to move the load at a preset speed. Deal with multiple cylinders that need to move simultaneously

• Flow control• Throttle (needle valve, very crude, Orifice equation applies)• Pressure compensated Flow Control Valve: Assure a preset flow

rate independent of the pressure drop across the valve• Pressure and Flow control

• Load sensing systems: combine pressure and flow control to reduce energy losses

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Choice of pump depends on these factors

• Application• Max and working Pressure and Flow rate requirements• Constant rate / Variable rate• Pump efficiency• Leakage• Noise level• Contamination sensitivity

• Price

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Pump ‘family tree’

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Video 8: Power units (3:26)

• Power supply unit• Converts Mechanical energy into hydraulic energy• Hydraulic Fluid is conditioned (cooled, cleaned)

• Components• Drive motor• Safety valve• Oil reservoir • Pump

• External Gear pump function (constant delivery)• Where teeth unmesh, volume increases, oil enters• Where teeth mesh, volume decreases, oil leaves

• Pressure only builds when there is a resistance (load)• Safety valve needed to prevent failure when cylinder stalls

• Pressure Relief Valve diverts flow back to tank when cylinders are stalled

• Reservoir• Cools oil• Cleans oil from suspended particles, water and air which takes time

(Capacity)• Filters trap impurities• 70% of all malfunctions are due to impurities

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• In case of a motor shaft, the Work can be found by multiplying a force through a distance. Suppose we assume a force at a distance . The total work per radian of the shaft is now equal to

Mechanical Torque in a pump/motor

r

F

1 Radian

r

rFWMECH *

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Mechanical Power in a pump/motor

• The power is now equal to this value divided by the time per radian.

• If the shaft is turning at it takes seconds per radian.

• Since for Power we have to divide the Work by time, this leads to:

s

rad1

T

MECH rFrF

t

WPow **

1*

Nms

rad

s

Nm

TPowMECH

*

*

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Hydraulic Power in pump/motor ( )

qpPowHYD *

outin qq

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Pump/Motor flow is proportional to the speed of rotation and the displacement per revolution

*Dq

Pump displacement is a volume per angular displacement (radian).Assuming the volumetric efficiency is 1.0 (no leakage)

10,** VVDq

More realistic, with Volumetric Efficiency (why in numerator?)

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Torque Required to drive a pump is proportional to Pressure

More realistic, with Torque Efficiency (why in the denominator?)

10,*

TT

DpT

(No losses here)

***

*DpPow

Dq

qpPowHYD

HYD

Constant

**

**DpT

TPow

DpPow

MECH

HYD

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Power Efficiency of Pumps

VV

T

qDq

DpT

***

*

From before:

And

TPowMECH *

TVTVMECH

pqDp

D

qTPow

*

***

**

Power Efficiency: Without loss

Efficiencies

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Data sheet Eaton MHT vane pump

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A Pressure Relief Valve (PRV) provides overload protection

• In the symbol there are• Main pressure lines (solid)• Sense lines (dashed)• Spring return• Adjust arrow• Direction arrow• Pressure and Tank connections

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Some questions about a PRV

• Is this valve normal open or normal closed• Closed / open, think of a door

• Where is the pressure sensed, upstream or downstream and why ?

• What is the pressure at the tank port ? • Why is a PRV adjustable ?

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Cylinders

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Video 9: Hydraulic actuators (6:58)

• Cylinders convert hydraulic energy into linear motion• Motors generate rotary motion• Single acting cylinder: One working port

• Can do work in only one direction (extension)• External force retracts the cylinder• No perfect seal, over time oil passes on to unpressurized

side: need for drain• Good for high load single lift (scissor platform)• Return stroke through gravity or spring return• Plunger (ram) cylinders: Cap end only, very powerful and

stiff• Double acting cylinder: Two working ports

• Pressure advances and retracts the cylinder: push and pull• Cylinder retracts faster than it extends due to different

areas of cap end and rod end side

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Video 9: Hydraulic actuators (6:58) cont.

• Example: ratio of cap and rod end side area is 2:1.

• Assume • During extension rod end pressure =0• During retraction cap end pressure =0

2*

* * 2*

*

CAP ROD

CAPEXT CAP EXT RET RET

RODRET ROD

A AA

F p A F F FA

F p A

22

*N

F N p A mm

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Video 9: Hydraulic actuators (6:58) cont.

• Example: ratio of cap and rod end side area is 2:1.

2*

1* *

2

CAP ROD

RODEXT EXT RET RET

CAP CAP

RETROD

A A

Aqv v v v

A A

qv

A

* m NmF N v P Watt

s s

• Linear Power: *Pow F v

3

2

mq

smv

s A m

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Hydraulic Cylinder

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Cylinder construction

1 2 3 4 5 6

7

8 9 10 11 12 13

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1 2 3 4 5 6

7

8 9 10 11 12 13

1. Tie rod ( keeps cylinder assembly together )2. Rod end head ( mounting point )3. Rod end port ( fluid entrance/exit point )4. Piston seals ( dynamic, seals cap end from rod end

pressure)5. Cap end head ( mounting point)6. Cap end port ( fluid entrance/exit point )7. Rod bearing ( lateral support of the rod )8. Rod wiper ( keeps dirt out )9. Rod seal (dynamic, seals fluid from

environment )10. Barrel ( cylinder )11. Piston rod ( mechanical force output )12. Piston ( pressure to force converter )13. Static seal ( seals fluid from environment )

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Cylinders are perfect for linear motion

• Single rod (most common)

• Dual rod (power steering)

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Telescopic cylinder

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Basic Circuit with Double Acting Cylinder

Pump

Actuator

OverloadProtection

Reservoir

Pressuregage

Electric Motor

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Question: Can the cylinder be moved ?

1) Cylinder:• Double acting, differential area

2) Fluid is incompressible3) Rod and cap end connected

Check in FluidSim

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Hydraulic Work (rod end pressure = 0)

AR AC

F

pR pC

s Cq

0

***

R

CC

p

sApsFW

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AR AC

Cq Rq

F

FR FC

pR pC

s

R

R

C

C

RR

R

CC

C

A

q

A

q

vAt

sAq

vAt

sAq

Differential area cylinder: Flow and displacement

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Differential area cylinder: Force equilibrium

RC F

RR

F

CC ApApF

AR AC

Cq Rq

F

FR FC

pR pC

s

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Differential area cylinder: Work

RRCC ApApsFsW

AR AC

Cq Rq

F

FR FC

pR pC

s

0Rp

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Differential area cylinder: Power

RRCC ApApsFsW

RRCC

q

RR

q

CC qpqp

t

sAp

t

sAp

t

sF

t

WPow

RC

AR AC

Cq Rq

F

FR FC

pR pC

s

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Dual rod cylinder: Power

CRCC ApApsFsW

p

RCC

q

CR

q

CC ppq

t

sAp

t

sAp

t

sF

t

WPow

CC

AR AC

Cq Rq

F FR FC

pR pC

s

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Power if (as in motors and pumps)

pqPow *

AR AC

Cq Rq

F FR FC

pR pC

s

qqq outin

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Question: Will the cylinder extend ?

Check in FluidSim

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Video 5: Pressure transmission (0:53)

• Pressure intensification in hydraulic systems:• Differential area cylinders cause this effect• Max pressure in the system is NOT PRV setting !

*

* * *CAP CAP CAP

CAPROD ROD ROD CAP CAP ROD ROD ROD CAP

RODCAP ROD

F p AA

F p A p A p A p pA

F F

PRVp

Check in FluidSim

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Question in ‘Customary Units’

• Given• PRV Setting 15 MPa• pump displacement of 10.54 cm3/rev• the speed of the pump is 1800 rpm

• Required• Torque needed to drive the pump • Power needed to drive the pump

• Neglect friction

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Answer in ‘Customary Units’

s

Nm 4743188.495*Nm25.163*

Nm 25.163615*66775.1*

188.495min

60

1*2*

min1800

66775.110

1*

2

1*54.10

61515

2

3

3

3

3

6

3

2

s

radTPow

m

NE

rad

mEpDT

s

rad

srev

radrev

rad

mE

cm

m

rad

rev

rev

cmD

m

NEMPap

PRV

PRV

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Question in ‘Customary Units’

• Given• PRV Setting• pump displacement of • the speed of the pump

• Required• Torque needed to drive the pump • Power needed to drive the pump .

• Neglect friction

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Answer in SI Units

*

*

TPow

pDT

MECH

PRVMECH

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Types of Pumps and Motors

• External Gear• Internal Gear• Vane• Axial Piston• Radial Piston

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External Gear pump

• Poorly sealing• Low flow rates• Low pressures• Fixed displacement• Low cost

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Internal Gear pump

• Poorly sealing• Low flow rates• Medium pressures• Fixed displacement• Medium cost

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Vane pump

• Medium sealing• Higher pressures• Inexpensive• Fixed or variable

flow rate

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Axial piston pump with swash plate. In this case the swash plate angle is variable, which results in a variable delivery pump

• Excellent sealing• High pressures• Relatively simple design• Variable flow rate• No need for valves• Expensive

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Axial piston pump with swash plate. In this case the swash plate angle is constant, which results in a constant delivery pump

• Here fixed flow rate

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Radial piston pump with variable rate through changing eccentricity between shaft and ‘pintle’

• Excellent sealing• High pressures• Relatively expensive• Variable flow rate• Valves are needed• Complex design

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Radial piston pump with variable rate through changing eccentricity between shaft and ‘pintle’

• Excellent sealing• High pressures• Relatively expensive• Variable flow rate

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External Gear

Fixed Displacement

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Pump ‘family tree’

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TSM363 Fluid Power Systems

Pumps and Actuators

The End

Dept. of Agricultural & Biological EngineeringUniversity of Illinois