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 expensive Figure 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 D E F G C H 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:  The flow from the supply to the valve can be calculated as: (Merritt, 1967) Q L =flow to the load from the valve A 1 =larger valve orifice Q S =flow to the valve from the supply A 2 =smaller valve orifice C d =discharge coefficient ρ=fluid density P S =pressure at the supply P L =pressure at the load (1) (2)

Discharge Coefficient Review for 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. 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: Q L =flow from the valve to the load X v =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: (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: w = the area gradient of the valve Q c = leakage flow at center position μ = 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)  The leakage flow can be derived from equation 5 assuming Q L, P L, and x v 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) C d =discharge coefficient w=area of the orifice ρ=density of the fluid P s =supply pressure (7) (Merritt, 1967)

Critical Center Valve Pressure Sensitivity  Pressure sensitivity for a critical center valve is: (Merritt, 1967)  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: U=Underlap of valve (10) (11)  If P L and x v 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): 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 = underlap (Merritt, 1967) (13)

Closed Center Valve  The metering region has an overlap  Overlap reduces high pressure leakage (Merritt, 1967) Figure Closed Center Spool Valve Diagram

Closed Center Flow  Closed center leakage flow is laminar  It is determined as follows: (14) D=diameter of the valve housing L 0 =overlap ε=eccentricity of the spool (Merritt, 1967)

Closed Center Flow Gain  Constant dead band near the null position Figure Dead band on closed center valve (Wittren 1975)

References  John Deere Corporation, Fundamentals of Service-Hydraulics. John Deere Corporation: Moline, IL.  Merit, H. E., Hydraulic Control Systems. John Wiley & Sons, Inc.: New York, NY.  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.  Wittren, R.A., Power Steering For Agricultural Tractors. ASAE Distinguished Lecture Series No. 1. ASAE: St. Joseph, MI.