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)