Hydraulic Steering 1

8/13/2019 Hydraulic Steering 1

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Hydrostatic SteeringSystem

Lecture 2Day 1-Class 2


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Basic System Components

Steering ValveCylinder/Actuator

FilterReservoirSteering PumpRelief Valve

Can be built intopump Figure 2.1 Basic

steering system(Parker-Hannifin)


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Pump

Driven by direct or indirect coupling withthe engine or electric motor

The type depends on pressure anddisplacement requirements, permissiblenoise levels, and circuit type


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Gear Pump

Fixed displacement for open centerTolerates dirt wellSuitable for rugged applicationsCheapSimpleHigh noise levels

Pressure pulses


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GerotorType of internal gearpumpUsed for pressures

less than 1200 psiQuieter than otherinternal or externalgear pumps

Figure 2.3 Gerotor Pump

(John Deere)


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Vane PumpUsually fixeddisplacement foropen center, but canhave variable

displacementQuieter operationthan the gear pumpPressure ripples aresmall, smoothoperationMore expensive

Figure 2.4 Vane pump(John Deere)


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Piston PumpVariabledisplacement, closedcenterFlow is pulsating

Can handle highpressures, highvolumes and highspeeds

High power to weightratioComplex andexpensive

Figure 2.5 PistonPump (John Deere)


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ActuatorsThere are three types of actuators

Rack and pinionCylinderVane

The possible travel of the actuator is limited bythe steering geometry

Figure 2.6.ActuatorTypes(Wittren,1975)


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Cylinders

Between the steered wheels Always double acting

Can be one or two cylindersRecommended that the stroke to boreratio be between 5 and 8 (Whittren)


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Hydrostatic Steering ValveConsists of two sections

Fluid controlFluid metering

Contains the followingLinear spool (A)Drive link (B)Rotor and stator set(C)Manifold (D)Commutator ring (E)Commutator (F)Input shaft (G)Torsion bar (H)

A

B

DE

F

G

CH

Figure 2.7. Parker HGAhydrostatic powersteering valve (Parker)


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Steering Valve CharacteristicsUsually six wayCommonly spool valvesClosed Center, Open Center, or CriticalCenterMust provide an appropriate flow gainMust be sized to achieve suitable pressurelosses at maximum flow

No float or lashNo internal leakage to or from the cylinderMust not be sticky

Wittren (1975)


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Valve FlowsThe flow to the load from the valve can be calculatedas:

)(1

)(1

21 LS d LS d L P P AC P P AC Q

The flow from the supply to the valve can becalculated as:

)(1

)(1

21 L sd L sd s P P AC P P AC Q

(Merritt, 1967)

Q L=flow to the load from the valve A 1=larger valve orificeQ S=flow to the valve from the supply A 2=smaller valve orificeCd=discharge coefficient =fluid densityP S =pressure at the supply P L=pressure at the load

(1)

(2)


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Discharge Coefficient Review

21

21

])(*74.135.1[

DR

LC d 50 L

DR

21

)6428.2( DR

LC d

for

50 L

DRfor

L = length of the orifice

D = diameter of the orifice

R = Reynolds number

Discharge coefficient for a short tube orifice

(Merritt, 1967)


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Reynolds Number

The Reynolds number requires thevelocity of the fluid, so it will be aniterative process to solve for the flow

rate.

VD R

=fluid density

V=fluid velocity

D=diameter of the pipe

= fluid viscosity

(Merritt, 1967)


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Flow GainFlow gain is the ratio of flow increment tovalve travel at a given pressure drop(Wittren, 1975) It is determined by the following equation:

v

Lq x

Q K

Q L=flow from the valve to the load

Xv=displacement from null position

(3)

(Merritt, 1967)


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Flow Gain

Lands ground tochange areagradient

Figure2.8. Valvespool withmodifiedmeteringlands


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Pressure Sensitivity

Pressure sensitivity is an indication of the effectof spool movement on pressureIt is given by the following equation from Merritt:

v

L

p x

P K (4)

(Merritt, 1967)


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Critical Center ValveThere is no underlap or overlap of meteringlandsLinear flow gainVery expensive to manufacture

Leakage flows are minimum

(Merritt, 1967)

Figure 2.9.CriticalCenterValveDiagram


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Flow for Critical Center Assuming all the orifices of a valve are symmetrical,the load flow can be approximated as:

)(1

Lv

v svd L P

x

x P wxC Q

w = the area gradient of the valveQ c= leakage flow at center position = fluid viscosity (typical value is 2 x 10-6 lb-sec/in2)r c= radial clearance between spool and sleeve (typically 2 x 10-4 in)

(Merritt, 1967)

(5)

sc

c P wr

Q

32

2

The leakage flow can be derived fromequation 5 assuming Q L, P L, and x v are 0.

(6)


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Critical Center Flow GainFlow gain of a critical center valve in thenull position can be obtained by thefollowing equation (Merritt, pg. 87)

sd q

P wC K

Cd=discharge coefficient

w=area of the orifice

=density of the fluid

P s=supply pressure

(7)

(Merritt, 1967)


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Critical Center Valve Pressure

SensitivityPressure sensitivity for a critical center valve is:

v

L s p x

P P K

)(2

(Merritt, 1967)

20

32

c

S d

p r

P C

K

For a Practical Critical Center Valve:

(8)

(9)


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Open Center ValveOpen center valves have an underlap atthe metering region allowing maximumflow in the null position.

(Merritt, 1967)

Figure2.10 OpenCenterValveDiagram


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Open Center Valve Flow

The following equation represents the flow to the load foran open center valve:

))1)(1()1)(1(( 2/12/1

S

Lv

S

Lv sd L P P

U x

P P

U x P

wU C Q

U=Underlap of valve

(10)

sd c

P wU C Q 2 (11)

If P L and x v are taken to be 0 then, the leakage flow is:

(Merritt, 1967)


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Open Center Flow GainIn the null position, the flow gain can bedetermined by (Merritt, pg. 97):

sd q P wC K 20

The variables are the same as defined in theprevious slide.

(12)

(Merritt, 1967)


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Open Center Pressure

SensitivityIn the null position, the open center pressuresensitivity is:

U

P K s p

20

U = underlap(Merritt, 1967)

(13)


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Closed Center Valve

The metering region has an overlapOverlap reduces high pressure leakage

(Merritt, 1967)

Figure2.11.ClosedCenterSpoolValveDiagram


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Closed Center FlowClosed center leakage flow is laminarIt is determined as follows:

sc

cc P

r L Dr Q ]

231[

12 2

2

0

3

(14)

D=diameter of the valve housing

L0=overlap

=eccentricity of the spool

(Merritt, 1967)


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Closed Center Flow GainConstant dead bandnear the null position

Figure 2.11. Dead band onclosed center valve (Wittren1975)


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References

John Deere Corporation, 2000. Fundamentals ofService-Hydraulics. John Deere Corporation: Moline, IL.Merit, H. E., 1967. Hydraulic Control Systems. JohnWiley & Sons, Inc.: New York, NY.

Parker-Hannifin Corporation, 1999. Mobile HydraulicTechnology, Bulletin 0274-B1. Motion and ControlTraining Department: Cleveland, OH.Parker-Hannifin Corporation, 2000. Hydraulic Pumps,Motors, and Hydrostatic Steering Products, Catalog1550-001/USA. Hydraulic Pump/Motor Division:

Greenville, TN.Wittren, R.A., 1975. Power Steering For AgriculturalTractors. ASAE Distinguished Lecture Series No. 1.

ASAE: St. Joseph, MI.

Hydraulic Steering 1

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