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Hydrostatic Steering Part 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.

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hydrostatic steering part 2

Hydrostatic SteeringPart 2

Lecture 3

Day 1-Class 3

  • 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
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
Open Center Circuit, Non-Reversing

Metering Section

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

Figure 3.1. Open Center Non-Reversing Circuit

open center circuit reversing
Open Center Circuit, Reversing
  • Reversing – Wheels automatically return to straight

Figure 3.2. Open Center Circuit, Reversing (Parker)

open center circuit power beyond
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
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
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
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
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
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)

  • Steering valve and metering unit as one linked to steering wheel

Figure 3.8 (Wittren, 1975)

  • Metering unit at steering wheel, steering valve remote linked

Figure 3.9 (Wittren, 1975)

(Wittren, 1975)

design calculations hydraguide
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 t k
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

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

(Parker, 2000)

kingpin torque
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

(Parker, 2000)

kingpin torque16
Kingpin Torque
  • If the pressure distribution is known then the radius of gyration (k) can be computed. The following relationship can be applied.


  • 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


(Parker, 2000)

calculate approximate cylinder force f c
Calculate Approximate Cylinder Force (Fc)


CF= Cylinder Force (lbs)

R = Minimum Radius Arm

Figure 3.11 Geometry Diagram (Parker)

(Parker, 2000)

calculate cylinder area a c
Calculate Cylinder Area (Ac)


  • 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

(Parker, 2000)

determine cylinder stroke s
Determine Cylinder Stroke (S)

Figure 3.11 Geometry Diagram (Parker) Repeated

(Parker, 2000)

swept volume v s of cylinder
Swept Volume (Vs) of Cylinder
  • Swept Volume (in3) One Balanced Cylinder


DB=Diameter of bore

DR=Diameter of rod

(Parker, 2000)

swept volume of cylinder
Swept Volume of Cylinder
  • One Unbalanced Cylinder
    • Head Side
    • Rod Side

-Same as one balanced

  • Two Unbalanced Cylinders



(Parker, 2000)

displacement d
Displacement (D)


n=number of steering wheel turns lock to lock

(Parker, 2000)

minimum pump flow q
Minimum Pump Flow (Q)


Ns = steering speed in revolutions per minute

Pump Flow is in gpm per revolution

(Parker, 2000)

steering speed
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)