Hydrostatic SteeringPart 2

Lecture 3

Day 1-Class 3

References

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.

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

Open Center System

Fixed Displacement Pump Continuously supplies flow to the

steering valve Gear or Vane

Simple and economical Works the best on smaller

vehicles

Open Center Circuit, Non-Reversing

Non-Reversing-Cylinder ports are blocked in neutral valve position, the operator must steer the wheel back to straight

Metering Section

Figure 3.1. Open Center Non-Reversing Circuit

Open Center Circuit, Reversing Reversing –

Wheels automatically return to straight

Figure 3.2. Open Center Circuit, Reversing (Parker)

Open Center Circuit, Power Beyond

Any flow not used by steering goes to secondary function

Good for lawn and garden equipment and utility vehicles

Auxiliary Port

Figure 3.3. Open Center Circuit, Power Beyond (Parker)

Open Center Demand Circuit Contains closed center

load sensing valve and open center auxiliary circuit valve

When vehicle is steered, steering valve lets pressure to priority demand valve, increasing pressure at priority valve causes flow to shift

Uses fixed displacement pump

Figure 3.4. Open Center Demand Circuit (Parker)

Closed Center System Pump-variable delivery, constant

pressure Commonly an axial piston pump with

variable swash plate A compensator controls output flow

maintaining constant pressure at the steering unit

Possible to share the pump with other hydraulic functions Must have a priority valve for the steering

system

(Parker, 1999)

Closed Center Circuit, Non-Reversing Variable

displacement pump All valve ports

blocked when vehicle is not being steered

Amount of flow dependent on steering speed and displacement of steering valve

Figure 3.5. Closed Center Circuit, Non-Reversing (Parker)

Closed Center Circuit with priority valve With steering

priority valve Variable volume,

pressure compensating pump

Priority valve ensures adequate flow to steering valve

Figure 3.6. Closed Center Circuit with priority valve (Parker)

Closed Center Load Sensing Circuit A special load

sensing valve is used to operate the actuator

Load variations in the steering circuit do not affect axle response or steering rate

Only the flow required by the steering circuit is sent to it

Priority valve ensures the steering circuit has adequate flow and pressure

Figure 3.7. Closed Center Load Sensing Circuit (Parker)

Arrangements Steering valve and metering

unit as one linked to steering wheel

Metering unit at steering wheel, steering valve remote linked

Figure 3.8 (Wittren, 1975)

Figure 3.9 (Wittren, 1975)

(Wittren, 1975)

Design Calculations-Hydraguide

Calculate Kingpin Torque Determine Cylinder Force Calculate Cylinder Area Determine Cylinder Stroke Calculate Swept Volume Calculate Displacement Calculate Minimum Pump Flow Decide if pressure is suitable Select Relief Valve Setting

(Parker, 2000)

Kingpin Torque (Tk)

First determine the coefficient of friction (μ) using the chart. E (in) is the Kingpin offset and B (in) is the nominal tire width

(Parker, 2000)

Figure 3.10. Coefficient of Friction Chart and Kingpin Diagram (Parker)

Kingpin Torque Information about the tire is needed.

If we assume a uniform tire pressure then the following equation can be used.

W=Weight on steered axle (lbs)

Io=Polar moment of inertia of tire print

A=area of tire print

2** EA

IWT o (1)

(Parker, 2000)

Kingpin Torque If the pressure distribution is known then the

radius of gyration (k) can be computed. The following relationship can be applied.

A

Ik o2

22

8E

BW*μTk

If there is no information available about the tire print, then a circular tire print can be assumed using the nominal tire width as the diameter

(2)

(3)

(Parker, 2000)

Calculate Approximate Cylinder Force (Fc)

R

TF KC

CF= Cylinder Force (lbs)

R = Minimum Radius Arm

(4)

(Parker, 2000)

Figure 3.11 Geometry Diagram (Parker)

Calculate Cylinder Area (Ac)

P

FA cc

Fc=Cylinder Force (lbs) P=Pressure rating of steering valve Select the next larger cylinder size

-For a single cylinder use only the rod area-For a double cylinder use the rod end area plus the bore area

(5)

(Parker, 2000)

Determine Cylinder Stroke (S)

(Parker, 2000)

Figure 3.11 Geometry Diagram (Parker) Repeated

Swept Volume (Vs) of Cylinder

Swept Volume (in3) One Balanced Cylinder

SDDV RBS *)(*4

22

DB=Diameter of boreDR=Diameter of rod

(6)

(Parker, 2000)

Swept Volume of Cylinder

Two Unbalanced Cylinders

)*2(4

* 22RBs DD

SV

One Unbalanced Cylinder Head Side

Rod Side

-Same as one balanced

SD

V Bs *

4

* 2 (7)

(8)

(Parker, 2000)

Displacement (D)

n

VD s

n=number of steering wheel turns lock to lock

(9)

(Parker, 2000)

Minimum Pump Flow (Q)

231

* sNDQ

Ns = steering speed in revolutions per minutePump Flow is in gpm per revolution

(10)

(Parker, 2000)

Steering Speed

The ideal steering speed is 120 rpm, which is considered the maximum input achievable by an average person

The minimum normally considered is usually 60 rpm

90 rpm is common

(Parker, 2000)