Hydrostatic Steering Part 2
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Transcript Hydrostatic Steering Part 2
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.
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, NonMetering
Reversing
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
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 Auxiliary
Port
utility vehicles
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, NonReversing
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
Figure 3.8
(Wittren,
1975)
metering unit as
one linked to
steering wheel
Metering unit at
Figure 3.9
(Wittren,
1975)
steering wheel,
steering valve
remote linked
(Wittren, 1975)
Design CalculationsHydraguide
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
Figure 3.10.
Coefficient of
Friction Chart
and Kingpin
Diagram
(Parker)
(Parker, 2000)
Kingpin Torque
Information about the tire is
needed. If we assume a uniform
tire pressure then the following
equation can be used.
Io
T W * *
E2
A
(1)
W=Weight on steered axle (lbs)
Io=Polar moment of inertia of tire
print
A=area of tire print
(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.
k
2
Io
A
(2)
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
B
2
Tk W*μ
E
8
(3)
(Parker, 2000)
Calculate Approximate
Cylinder Force (Fc)
TK
FC
R
(4)
CF= Cylinder Force (lbs)
R = Minimum Radius Arm
Figure 3.11 Geometry
Diagram (Parker)
(Parker, 2000)
Calculate Cylinder Area (Ac)
Fc
Ac
P
(5)
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)
Figure 3.11 Geometry
Diagram (Parker)
Repeated
(Parker, 2000)
Swept Volume (Vs) of Cylinder
Swept Volume (in3) One
Balanced Cylinder
2
2
VS * ( DB DR ) * S
4
(6)
DB=Diameter of bore
DR=Diameter of rod
(Parker, 2000)
Swept Volume of Cylinder
One Unbalanced Cylinder
Head Side
Vs
*D
2
B
4
*S
(7)
Rod Side
-Same as one balanced
Two Unbalanced Cylinders
*S
2
2
Vs
(2 * DB DR )
4
(8)
(Parker, 2000)
Displacement (D)
Vs
D
n
(9)
n=number of steering wheel turns lock to lock
(Parker, 2000)
Minimum Pump Flow (Q)
D * Ns
Q
231
(10)
Ns = steering speed in revolutions per minute
Pump Flow is in gpm per revolution
(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)