Hydraulic auxiliary power steering for motor vehicles

A booster steering system has two pressure pumps for the booster cylinder, one of which is driven by the vehicle, with the wheels when the vehicle is rolling, and the other pump is driven by the vehicle engine. The vehicle driven pump is an emergency pump of non-variable displacement and can provide booster pressure flow to steer a vehicle off the road in an emergency should the engine driven pump fail. The engine driven pump is of variable displacement, normally maintained by an actuating means at zero stroke responsive to a pilot pressure from a steering control valve in neutral position with no steering pressure output. The vehicle driven pump normally provides the pressure output for steering. Accordingly, since most of the time the engine driven pump operates virtually pressurelessly there is no energy loss. When the vehicle is moving slowly in a tight curve a greater volume of output to the booster cylinder is needed and is provided by the engine driven variable displacement pump. By operation of the steering control valve there occurs a differential pressure on the actuating means of the variable displacement pump to effect a pumping stroke. Upon no further need after the steering is completed, an opposite differential pressure is effected with the steering control valve back to neutral position which restores the zero stroke condition. Further, should the engine driven pump output fail, the vehicle driven emergency pump will provide a booster pressure output to at least steer the moving vehicle out of traffic.

BACKGROUND OF THE INVENTION 
An auxiliary power boost steering system having an emergency pump which 
operates with a first and a second pump has been known, for example, in 
the German Pat. No. 27 31 975 (U.S. Pat. No. 4,237,993). A two-pump 
operation is always desirable since at times heavy trucks become 
unsteerable because of the failure of the engine driven first pump. In 
this case, a vehicle may be safely pulled out of traffic while the vehicle 
is rolling, for example, with the second pump (emergency steering pump) 
driven by the gearing in vehicle movement. The known auxiliary power 
steering arrangement shows a so-called emergency valve which, in the case 
of a defect of the engine driven first pump, separates the latter from the 
hydraulic steering circuit and connects in the emergency steering pump. 
Whenever the engine driven first pump is fully operational, the emergency 
steering pump in the standby mode circulates a volume of oil with a 
certain idling pressure required for the maintenance of the hydraulic 
circuit. Since a failure of the engine driven first pump occurs only very 
rarely during operation time of a vehicle, the continuing standby 
operation of the vehicle driven emergency steering pump means a steady 
loss of power. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the invention, the engine driven pump is a variable 
displacement adjustable output pump, but the emergency steering pump is 
vehicle driven by a shaft or axle of the vehicle. A steering flow control 
valve has, in the neutral position, a construction so that output flow of 
the emergency steering pump regulates the variable displacement engine 
driven pump. Moreover, for the regulation of the variable displacement 
pump, a comparator valve connected between the pressure line connections 
to the booster cylinder has been provided connecting the effective 
pressure side of the booster cylinder with the pressure operated actuator 
of the variable displacement pump to set the extent of the stroke. At a 
low speed of travel, the emergency steering pump has an output flow 
sufficient for slow steering movements for usual steering in the 
directions of travel. In this case, the opened area of the inlet flow 
control gaps of a slidable steering control valve effect pressures for the 
actuator means of the variable displacement pump for zero stroke. Thus, 
when the vehicle travels at medium or higher speeds, in the normal 
operating range of the auxiliary power steering, the vehicle driven 
emergency steering pump supplies booster oil for steering purposes while 
the variable displacement pump remains in zero stroke position thereby 
absorbing no engine power, and being in a standby mode. 
The vehicle driven emergency steering pump is the main pump relied on for 
steering at or above a predetermined vehicle speed, e.g., 10 KM/h. 
However, the term "emergency" is used to indicate that if the variable 
displacement pump flow fails when needed, the vehicle driven pump can at 
least provide pressure flow to steer the vehicle off the road. 
However, for low speeds of travel, e.g., in sharp curves, the flow required 
in the booster cylinder can no longer be produced by the slowed speed of 
the vehicle driven steering pump alone. Due to the wide opening of an 
inlet valving gap, the control edges 12 or 13, respectively, the pressure 
difference decreases toward zero. As a result of the small or no pressure 
difference in the control lines 26 and 28, an actuator means swashplate is 
acted on by a spring force in the direction of increased volume discharge. 
The variable displacement pump therefore provides a pressure flow of oil 
whenever the travel conditions require supplemental feed to the booster 
cylinder. When the pressure in a chamber of the booster cylinder rises, 
such pressure chamber being connected through a comparator valve with the 
actuator means regulates the stroke of the pump to provide additional 
flow. Of course, should that pump fail, e.g., due to engine trouble or 
other reason, the vehicle driven emergency pump can at least function to 
steer the vehicle out of traffic just so long as the vehicle is moving. 
The variable displacement pump driven by the motor of the vehicle supplies 
the necessary output flow for a stationary vehicle or in event of very low 
speed of travel. This occurs when the vehicle driven pump, dependent on 
the speed of the vehicle, may not be capable of supplying any or any 
sufficiently large flow. This causes a pressure decrease in the booster 
cylinder, thus to effect output of the variable displacement pump. 
Suitable design of the grooves and lands of the steering control valve 
effects opening and closing of flow control gaps proportionally to ensure 
a smooth regulation of the variable displacement pump for a continuous 
output flow, when needed. 
The arrangement described has the advantage that both pumps are used only 
when needed. The variable displacement pump operates only to supplement 
peak requirements or when the vehicle is standing still, and may be small 
in size. In contrast, the prior art supplemental flow pumps run constantly 
and waste energy; standby zero stroke operation saves energy in the 
present invention. It is thus possible to supervise continuous readiness 
for operation or condition of wear of the vehicle driven steering pump for 
pressure and quantity by instruments. Further, the invention is 
constructed simply as compared to the prior art and requires fewer 
components in addition to power saving in operation of heavy vehicles 
requiring large hydraulic flow steering. 
The arrangement provides for return to zero stroke position of the variable 
displacement pump as the booster cylinder pressure increases to full 
operational status.

In FIG. 1, a hand steering wheel, now shown, is connected with a steering 
control valve 1, for example, an axial sliding valve, in a known manner 
synchronizable by the steering movement. A first pump 3 driven by the 
vehicle engine 2 and a second pump 4 driven by vehicle movement, are 
connected to the steering control valve 1. The second pump 4, subsequently 
also referred to as emergency steering pump, is coupled with a gear shaft 
or, as indicated by the wheel 5, with an axle drive and supplies a 
constant flow of oil or a flow of oil rising linearly with the speed of 
travel. Both pumps 3 and 4 draw pressure oil from a tank 6 and convey it 
by line branches 7 and 8 and a common pressure line 10 into an inlet 
groove 11 of the steering control valve 1. In the neutral position of the 
steering control valve 1, as shown, the inlet groove 11 communicates via 
open inlet control edges 12, 13 and open return control edges 14, 15 with 
the tank 6. Connections to tank are omitted for simplicity, it being 
understood that in neutral position the system is open circuit. Between 
the inlet and return control edges are cylinder feed grooves 16, 17 which 
are connected with pressure chambers 21, 22 of a booster cylinder 23 via 
cylinder lines 18,20. In the line branches 7 and 8, non-return vllves 24 
or 25 are disposed, respectively, the function of which will be explained 
in more detail later on. 
According to the invention, the engine driven pump 3 is constructed as a 
variable stroke displacement pump. Variable displacement pump delivers 
steering flow responsive to a control signal, e.g., a pressure change, 
when additional flow is needed for operation of an actuator means A which 
changes the extent of pumping stroke of the variable displacement pump 
from zero to some setting that produces needed output flow. 
This occurs when the vehicle driven pump 4 is unable to fill a pressure 
chamber 21 or 22 because the vehicle is not moving or is executing a sharp 
turn at slow speed. 
The engine driven pump may be of the kind shown in U.S. Patent to Pedersen, 
3,748,329, which has variable output from zero, when no additional 
pressure flow is needed, to some value of output as required when an 
additional flow is needed. Thus, by virtue of a pressure differential 
acting on an actuating means A of pump 3 caused by a need for additional 
pressure flow, the normal zero output of that pump is changed to 
contribute additional flow. 
At the end of such steering operation, no additional output being needed, 
the variable pump is restored to zero stroke operation by pilot pressure 
operated actuator means acting against the force of a spring (for example, 
in U.S. Pat. 3,748,329 the piston 28 and spring 30). The losses then are 
only leakage of the system balanced out and the pilot pressure (standby 
pressure) is kept constant for zero stroke. Features concerning the 
construction and the mode of operation of such variable displacement pumps 
are also shown in a counterpart patent, German Pat. No. 23 22 890. The 
engine operated pump 3,viz., variable displacement pump 3, is connected by 
a control line 26 with a comparator valve 27 which is connected between 
the two cylinder lines 18 and 20, thus connecting the pressurized chamber 
21 or 22 of the booster cylinder 23 with the stroke controlling actuator 
means of the variable displacement pump to so initiate a pumping action. 
The flow control areas at the inlet lands and grooves effected by flow 
control edges 12 and 13 of the steering control valve 1, are areas through 
which, in the neutral position shown, a control pressure is created in the 
lines 7, 8 and 10 effected by the emergency steering vehicle driven pump 
4. This control pressure for the variable displacement actuator means is 
predetermined so that between the line 10 and the control line 26, a 
pressure differential of about 12 to 15 bar occurs, which acts as a 
control force on the actuator means that varies the pumping stroke. The 
pumping stroke of the variable displacement pump 3 is adjusted so that the 
output booster flow maintains that pressure differential at 12 to 15 bar. 
By way of the control line 26 and an additional control line 28, it can be 
mentioned that the pressure difference serves as control pressure for the 
regulation of the conveying volume of the variable displacement pump 3. 
The control lines 26 and 28 are connected in a known manner to operate the 
pistons that control the angle of the swashplate to vary the volume 
displacement of pump 3. For example, pistons 28 and 56 in U.S. Pat. 
3,784,328. to Pedersen on both sides of swashplate 22, wherein the lines 
4' and 50 correspond to lines 28 and 26, respectively, of applicants' 
invention. 
In the neutral position of the steering valve 6 therefore the emergency 
steering pump delivers a pilot stream which, after passing the constricted 
opening cross section of the inlet control edges 12, 13 can flow out by 
way of the return control edges 14, 15 and a return groove 30 to the 
container 6. 
In one steering position, the emergency steering pump 4 is capable of 
providing of itself in the normal operating range of the auxiliary power 
steering, the required booster flow as long as the control pressure lies 
above the mentioned 12 to 15 bar. If however, at relatively low speeds of 
travel, tight curves are travelled with rapid steering the control 
pressure drops due to the high requirement for booster flow. In order to 
meet this high requirement the output of the variable displacement pump 3 
added. 
OPERATION OF VARIABLE DISPLACEMENT PUMP 
Assume the vehicle driven pump 4 is not operating or is operating too 
slowly to expeditiously fill the pressure chambers at a high steering 
speed. 
If the land section (wider hatching) of steering valve 1 is shifted, e.g., 
to the right at this time the opening cross section of the inlet flow gap 
12 is enlarged with simultaneous constrictions of the area at the return 
flow control gap 14. At this time, a pressure drop between the inlet 
groove 11 and the cylinder groove 16 becomes less. Line 26 is always open 
to the interior of the comparator valve 27; groove 16 connects to line 18 
to the comparator valve as well as to pressure chamber 21. Accordingly, 
the pressure difference between the line 10 and the control line 26 is 
lower. This pressure drop is a control signal to the actuator means A of 
variable displacement pump 3 by way of the line branch 7 and the control 
line 28 on the one hand, as well as by way of the line 18, the comparator 
valve 27 and the control line 26, on the other hand. The actuating means 
swashplate (22 in Pedersen) is acted on by a spring (30 in Pedersen) to 
increase volume discharge. The variable displacement pump 3 together with 
the emergency steering pump 4 now deliver a booster flow via the increased 
area of gap 12 and the cylinder line 18 into the pressure chamber 21 to 
fill the momentary increased need for oil flow. However, during filling, 
the pressure in the pressure chamber 21 increases and need for increased 
booster flow decreases, at the end of a steering operation. This causes an 
increase of the pressure difference between lines 10 and 26 which is 
transferred to the variable displacement pump 3 via control line 28, 
comparator valve 27, control line 26 to the actuating means A of variable 
displacement pump 3. Depending on the level of the pressure, the actuator 
means of the variable displacement pump is thus motivated to a smaller 
output volume back to the zero stroke position. The return oil from 
pressure chamber 22 then flows to the tank 6 via the cylinder line 20, the 
cylinder groove 17, the wide flow area at gap 15, a bore 31, another bore 
32 and by way of the return groove 30. 
When a vehicle is stationary, booster flow required for steering is from 
the variable displacement pump 3 alone. The check or non-return valve 25 
is inserted into the line 8 to prevent loss by flow of oil from pump 3 
through the emergency steering pump 4. 
In case of a sudden failure of the variable displacement pump 3 during 
travel, the check or non-return valve 24 is inserted into the line branch 
7 and prevents loss of oil through that pump. In such case, the emergency 
steering pump 4 serves for emergency steering to move the vehicle out of 
traffic. 
In retrospect, when making tight turns at low speeds a shift of control 
valve 1 (the wider hatched section as shown in FIG. 1) to the right, opens 
gap 12 to a greater extent than shown while closing gap 13 thus causing 
the pressurizing of chamber 21 and exhaust of chamber 22. However, the 
need for filling the volume of chamber 21 results in a drop of the pilot 
pressure which maintained pump 3 in zero stroke condition with no output. 
Thus, there is a drop in pressure differential between control lines 10 
and 26 effective to cause actuator means A to permit tilt of a swashplate 
therein by virtue of a spring acting thereon. In other words, as viewed in 
Pedersen U.S. Pat. No. 3,784,328, the drop in differential pressure 
between the lines 58 and 40 results in a pump 3 discharge as will be 
apparent. Such discharge in combination with the discharge of pump 4, or 
without pump 4 if the vehicle is stationary, serves to fill up chamber 21 
thereby building up the pressure therein. This produces a shift of the 
comparator valve 27 and pressure from chamber 21 acts in line 26 causing a 
rise in differential pressure between lines 10 and 26. Such rise in 
pressure differential is conducted via line 28 and comparator valve 27 as 
well as control line 26 to the actuator means A rendering it once more 
conditioned to restore zero stroke operation of pump 3.