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)