Remote Linkage SystemPitman Arm StopsHelps to protect against
overheating.Adjusted so that the Pitman arm stops just before the
wheels must stop.Should be used with any system that has a separate
steering valve. (Vickers, 1967)
For part time power steering, or power boost, the steering valve
is only actuated if the force required to steer the vehicle is
greater than the force of the valves centering springs. Lets look
at two different steering situations. First, lets say we are
driving at highway speed and we go through a gentle banked curve.
The force required to steer the vehicle will be less than the force
of the centering springs, so power boost will not be activated. Now
lets say we are making a sharp turn at low speed or parking. The
force required for such a maneuver is greater than the force of the
centering springs, so the springs become compressed. This moves the
steering valve spool and power boost is activated.
The steering valve of a full-time power steering system has no
centering springs. Instead, the valve is attached to the wheel such
that hydraulic boost is always active. In the 1950s tractors
starting becoming too large to steer with man power alone, so
hydraulic assist or hydro-mechanical power steering was added to
make steering effort easier. It is best suited for small and medium
tractors that only require part time power steering. As tractors
become larger and need full time power steering, it is more
economical to use hydrostatic steering. Generally the hydraulic
components are less costly that complicated mechanical gears that
would be necessary for a large machine. Hydrostatic steering is
also more space effcient than hydraulic assisted steering since the
mechanical linkages can be eliminated by hoses and large mechanical
parts can be replaced by smaller hydraulic compnents.We can
separate our power steering systems into four basic layouts. The
first layout has an integral valve and actuator mounted directly to
the pitman arm. In the second layout, the actuator may be coupled
to the steering linkage with the valve mounted at the steering
wheel.Our fist system layout makes use of an integral valve and
actuator coupled directly to the pitman arm and connected to the
steering wheel mechanically. This system type is the easiest to use
when converting an existing mechanical steering linkage to
hydraulic power assist. It is very simple in that it only requires
a supply and return line. Unfortunately, it may not be very
aesthetic and hoses can be easily exposed to the surrounding moving
parts.The next type of steering mounts the valve directly at the
steering wheel and the actuator at the steering linkage. While it
is convienent to have the valve at the wheel and the actuator at
the linkage, it requires four hoses all near the steering wheel.
This can create a space problem, especially when there are several
other control elements at the steering wheel. Of course, power
steering systems create noise, heat, and vibration which can
interfere with driver comfort, so it is favorable to mount the
components farther from the drivers station.It is possible that the
valve and the actuator be mounted separately from each other, but
both within the linkage. While it still requires several hoses, it
is possible to get the hoses and other components away from the
potentially crowded around the steering wheel. It is possible to
keep the valve, actuator, and pump close together so that the hose
length can be kept relatively short. The fourth type of system uses
and integral valve and actuator mounted at the steering wheel. The
actuator operates a rack and pinion or crank arm which drives the
pitman arm. Using this type of design makes converting mechanical
steering to hydraulic assisted steering fairly easy. This design
replaces the entire steering gear on a manual system. It may
require some modifications to create enough space for the hydraulic
assembly an hoses. This can be a high priced investment, so the
financial benefits of replacing the mechanical gear must be
considered carefully.This design works well on small tractors and
passenger vehicles, but it is not suitable for large tractors,
especially articulated tractors. Here we can see a system like type
1, an integral valve and actuator as part of the steering linkage.
In this case, when the wheel is turned, the pitman arm is moved
activating the steering valve through the drag link. The actuator
moves the steering arm for the left wheel and the right wheel is
steered with cross steering arm. You can see from the picture why
this design good for adding hydraulic boost to a mechanical system
as the integral steering unit can be fit into the linkage where it
is convenient.Here we have a combination of types 1 and 3 so that
two cylinders can be used. We have integral cylinder and valve on
one side of the vehicle. The valve is remote linked to the other
cylinder. When the vehicle is steered the valve is used to control
both the cylinders. The remote dual system is like system 3 with
two cylinders. The valve and cylinders are not integral, but are
mounted in the linkage with the valve between the left steering arm
and the pitman arm. This a common system lay out for rear wheel
steered vehicles. The manual steering gear transmits motion of the
steering wheel to the Pitman Arm and provides some reduction of
steering wheel movement.First, in the event of a hydraulic system
failure, the steering gear ratio reduces the heavy manual steering
effort. Second, automotive vehicle drivers just arent ready for a 1
to 1 ratio steering system. One automobile manufacture, in fact,
was criticized for a low-ratio power steering system until the
public became used to it. There is initially a tendency to
over-steer with a low ratio gear.A power steering pump is usually a
vane-type pump or some similar construction of positive
displacement. It is driven by the vehicle engine, usually through a
pulley and V-belt or other type indirect coupling.
The power cylinder is double-acting. It is a differential
cylinder, so that steering response may be slightly different in
left or right turns with a controlled flow rate. The differential
is slight, however, and unnoticeable to most people. When two
cylinders are used, oil is pumped simultaneously to the rod end of
one and the head end of the other, so the differential is
cancelled.
A relief valve is required in the pressure line to protect the
pump from overloads.
The flow control valve maintains a constant rate of flow to
operate the power cylinders. Without this valve, variations in
engine speed would affect the sensitivity of the steering unit by
causing variations in pump flow. For safety, its best to have the
unit respond with exactly the same sensitivity at all speeds.
The steering valve is a four way valve that functions as a
positioning servo valve. It must direct fluid to either end of the
power cylinder. Most steering valves are the open-center type. When
the valve is in neutral, oil from the pump is recirculated freely
through the valve back to the reservoir.
The filter is preferably installed in the return line and should
have a bypass valve to prevent blocking flow if the element is
clogged. A 10-micron or smaller filter is recommended in power
steering systems to prevent damage to the pump and steering valve
from metal particles and dirt.
Power steering systems operate with vented reservoirs. The
breather or vent in the reservoir should be equipped with a
3-micron filter element.The reservoir must hold all the oil
required by system during operation, plus a sufficient level to
avoid a vortex at the suction line. It should be capable of
dissipating heat generated in the steering system.
Working hydraulic lines for the most part are flexible hoses,
since the steering components move during operation. Long lines may
be partly flexible hoses and partly tubing where flexibility isnt
required.
Integral steering unit circuitThe hydraulic circuit for an
integral unit system can be as simple as shown in the pictorial
diagram to the upper right. Controlled flow originates in the pump
and is routed through the pressure line to the steering valve. The
valve directs the flow to the power cylinder and returns it through
the return line to the tank.
Remote Linkage SystemIn a remote installation, the valve and
cylinder are mounted separately. Each is equipped with an end cap
threaded to accommodate a mounting ball stud. The caps also contain
two ports apiece to make the hydraulic connections between the
valve and cylinder. Otherwise, the cylinder and valve are the same
construction as in the integral steering unitIn the figure to the
right, there is cut-away view of a typical S20 series steering
valve. It consists of a power cylinder bolted to a steering valve.
The rod end (anchor) ball stud mounts the unit to the vehicle
frame. The head end ball stud is connected to the steering linkage.
The center or control ball stud is connected to the Pitman arm drag
link to actuate the steering valve. A single centering spring is
mounted between the ball stud and valve, and is flanked by
centering washers. This arrangement gives the driver road feel in
both directions.Oil flow through the steering unit is shown in the
figure to the right.
View A is the neutral flow condition. There is no relative
movement between the spool and valve body; in other words, the
spool is centered. Oil from the pump is directed back out to the
tank.
View B shows the control ball stud actuated to retract the
cylinder. The spool, as we see it, has been pushed to the left. Oil
from the pump is directed to the rod end of the cylinder. Since the
rod is anchored, pressure pushes against the rod packing to move
the entire steering unit to the left. At the same time, oil pushed
out the head end is returned to the tank. Flow continues as shown
until the control ball stud stops. The valve body then immediately
catches up with the spool and the flow condition again is as in
View A.
When the ball stud moves to the right (View C), flow reverses.
Pump delivery is routed to the head end of the cylinder and oil
pushed out of the rod end is returned to the tank. Pressure in the
cylinder pushes on the head and moves the steering unit to the
right to follow the control ball stud.The small check ball in the
valve body is normally held seated by pressure at the valve inlet
(pressure port). If there is a hydraulic failure or a loss of
power, pressure drops and lets this valve unseat. Then, oil can
circulate freely between the head and rod ends of the cylinder.
This avoids a hydrostatic lock and permits manual steering. The
control ball stud then simply moves the anchored rod. The steering
unit thus acts as a drag link to steer the vehicle.
An optional relief valve (upper figure) can be incorporated in
the steering valve if the flow control and relief valve is not
used. This is actually a compound relief valve. It doubles as a
check valve in case of power loss.
As shown in the bottom figure, the control ball valve ball stud
can be mounted in any of four positions relative to the port
connections in the valve.When two cylinders are operated from the
same valve, a special body is available which incorporates
auxiliary ports on the side. The figure on top shows the hydraulic
connections when a separate cylinder is used together with an
integral steering unit.that is, a combination integral-remote
circuit. Connections to the remote circuit are shown from the
valves side ports.
In the figure to the right, you see a dual remote circuit. This
circuit can use the special side-ported body, but as shown the
second remote cylinder can be teed into the lines to the opposite
cylinder. Notice that the port connection to the cylinders are
opposite, so that one will retract as the other extends.Pitman arm
stops are used in many power steering systems to protect against
overheating when the steering wheel is hard over. Without Pitman
stops, the steering valve continues to supply some boost when the
wheels are turned as far as possible and the driver is still
pulling the steering wheel. The only place the oil can go is over
the relief valve which will generate excessive heat. The Pitman arm
stops are adjusted to stop the arm just before the wheel stops,
thus giving the steering valve room to center.Pitman arm stops can
and should be incorporated with any system that has a separate
steering valve. They cannot be incorporated in valve-on-gear or
integral power gear systems.
