Hydrostatic Steering Part 2 Lecture 3 Day 1-Class 3

References  Parker-Hannifin Corporation, Mobile Hydraulic Technology, Bulletin 0274-B1. Motion and Control Training Department: Cleveland, OH.  Parker-Hannifin Corporation, Hydraulic Pumps, Motors, and Hydrostatic Steering Products, Catalog /USA. Hydraulic Pump/Motor Division: Greenville, TN.  Whittren, R.A., 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 (T k )  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 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) I o =Polar moment of inertia of tire print A=area of tire print (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.  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 (F c ) C F = Cylinder Force (lbs) R = Minimum Radius Arm (4) (Parker, 2000) Figure 3.11 Geometry Diagram (Parker)

Calculate Cylinder Area (A c )  F c =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 (V s ) of Cylinder  Swept Volume (in 3 ) One Balanced Cylinder D B =Diameter of bore D R =Diameter of rod (6) (Parker, 2000)

Swept Volume of Cylinder  Two Unbalanced Cylinders  One Unbalanced Cylinder Head Side Rod Side -Same as one balanced (7) (8) (Parker, 2000)

Displacement (D) n=number of steering wheel turns lock to lock (9) (Parker, 2000)

Minimum Pump Flow (Q) N s = steering speed in revolutions per minute Pump 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)