Hydrostatic Steering System

Lecture 2

Day 1-Class 2

Basic System Components

Steering Valve Cylinder/Actuator Filter Reservoir Steering Pump Relief Valve

Can be built into pump Figure 2.1 Basic

steering system (Parker-Hannifin)

Pump

Driven by direct or indirect coupling with the engine or electric motor

The type depends on pressure and displacement requirements, permissible noise levels, and circuit type

Gear Pump

Fixed displacement for open center Tolerates dirt well Suitable for rugged applications Cheap Simple High noise levels Pressure pulses

Gerotor Type of internal gear

pump Used for pressures

less than 1200 psi Quieter than other

internal or external gear pumps

Figure 2.3 Gerotor Pump

(John Deere)

Vane Pump Usually fixed

displacement for open center, but can have variable displacement

Quieter operation than the gear pump

Pressure ripples are small, smooth operation

More expensiveFigure 2.4 Vane pump (John Deere)

Piston Pump Variable displacement,

closed center Flow is pulsating Can handle high

pressures, high volumes and high speeds

High power to weight ratio

Complex and expensive

Figure 2.5 Piston Pump (John Deere)

Actuators There are three types of actuators

Rack and pinion Cylinder Vane

The possible travel of the actuator is limited by the steering geometry

Figure 2.6. Actuator Types (Wittren, 1975)

Cylinders

Between the steered wheels Always double acting Can be one or two cylinders Recommended that the stroke to bore

ratio be between 5 and 8 (Whittren)

Hydrostatic Steering Valve Consists of two sections

Fluid control Fluid metering

Contains the following Linear 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 HGA hydrostatic power steering valve (Parker)

Steering Valve Characteristics Usually six way Commonly spool valves Closed Center, Open Center, or Critical

Center Must provide an appropriate flow gain Must be sized to achieve suitable pressure

losses at maximum flow No float or lash No internal leakage to or from the cylinder Must not be sticky

Wittren (1975)

Valve Flows The flow to the load from the valve can be calculated

as:

)(1

)(1

21 LSdLSdL PPACPPACQ

The flow from the supply to the valve can be calculated as:

)(1

)(1

21 LsdLsds PPACPPACQ

(Merritt, 1967)

QL=flow to the load from the valve A1=larger valve orifice

QS=flow to the valve from the supply A2=smaller valve orifice

Cd=discharge coefficient ρ=fluid density

PS=pressure at the supply PL=pressure at the load

(1)

(2)

Discharge Coefficient Review

21

21

])(*74.135.1[

DR

LCd 50

L

DR

21

)6428.2(

DR

LCd

for

50L

DRfor

L = length of the orifice

D = diameter of the orifice

R = Reynolds number

Discharge coefficient for a short tube orifice

(Merritt, 1967)

Reynolds Number

The Reynolds number requires the velocity of the fluid, so it will be an iterative process to solve for the flow rate.

VD

R

ρ=fluid density

V=fluid velocity

D=diameter of the pipe

μ= fluid viscosity

(Merritt, 1967)

Flow Gain Flow gain is the ratio of flow increment to

valve travel at a given pressure drop (Wittren, 1975)

It is determined by the following equation:

v

Lq x

QK

QL=flow from the valve to the load

Xv=displacement from null position

(3)

(Merritt, 1967)

Flow Gain

Lands ground to change area gradient

Figure 2.8. Valve spool with modified metering lands

Pressure Sensitivity

Pressure sensitivity is an indication of the effect of spool movement on pressure

It is given by the following equation from Merritt:

v

Lp x

PK

(4)

(Merritt, 1967)

Critical Center Valve There is no underlap or overlap of metering

lands Linear flow gain Very expensive to manufacture Leakage flows are minimum

(Merritt, 1967)

Figure 2.9. Critical Center Valve Diagram

Flow for Critical Center Assuming all the orifices of a valve are symmetrical,

the load flow can be approximated as:

)(1

Lv

vsvdL Px

xPwxCQ

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

(Merritt, 1967)

(5)

sc

c Pwr

Q

32

2

The leakage flow can be derived from equation 5 assuming QL, PL, and xv are 0.

(6)

Critical Center Flow Gain Flow gain of a critical center valve in the

null position can be obtained by the following equation (Merritt, pg. 87)

s

dq

PwCK

Cd=discharge coefficient

w=area of the orifice

ρ=density of the fluid

Ps=supply pressure

(7)

(Merritt, 1967)

Critical Center Valve Pressure Sensitivity Pressure sensitivity for a critical center valve is:

v

Lsp x

PPK

)(2

(Merritt, 1967)

20

32

c

Sdp r

PCK

For a Practical Critical Center Valve:

(8)

(9)

Open Center Valve Open center valves have an underlap at

the metering region allowing maximum flow in the null position.

(Merritt, 1967)

Figure 2.10 Open Center Valve Diagram

Open Center Valve Flow

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

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

S

Lv

S

LvsdL P

P

U

x

P

P

U

xPwUCQ

U=Underlap of valve

(10)

s

dc

PwUCQ 2 (11)

If PL and xv are taken to be 0 then, the leakage flow is:

(Merritt, 1967)

Open Center Flow Gain In the null position, the flow gain can be

determined by (Merritt, pg. 97):

s

dq

PwCK 20

The variables are the same as defined in the previous slide.

(12)

(Merritt, 1967)

Open Center Pressure Sensitivity In the null position, the open center pressure

sensitivity is:

U

PK sp

20

U = underlap

(Merritt, 1967)

(13)

Closed Center Valve

The metering region has an overlap Overlap reduces high pressure leakage

(Merritt, 1967)

Figure 2.11. Closed Center Spool Valve Diagram

Closed Center Flow Closed center leakage flow is laminar It is determined as follows:

sc

cc P

rL

DrQ ]

2

31[

12 2

2

0

3

(14)

D=diameter of the valve housing

L0=overlap

ε=eccentricity of the spool

(Merritt, 1967)

Closed Center Flow Gain Constant dead band

near the null position

Figure 2.11. Dead band on closed center valve (Wittren 1975)

References

John Deere Corporation, 2000. Fundamentals of Service-Hydraulics. John Deere Corporation: Moline, IL.

Merit, H. E., 1967. Hydraulic Control Systems. John Wiley & Sons, Inc.: New York, NY.

Parker-Hannifin Corporation, 1999. Mobile Hydraulic Technology, Bulletin 0274-B1. Motion and Control Training Department: Cleveland, OH.

Parker-Hannifin Corporation, 2000. Hydraulic Pumps, Motors, and Hydrostatic Steering Products, Catalog 1550-001/USA. Hydraulic Pump/Motor Division: Greenville, TN.

Wittren, R.A., 1975. Power Steering For Agricultural Tractors. ASAE Distinguished Lecture Series No. 1. ASAE: St. Joseph, MI.