Hydrostatic auxiliary steering device

A pair of flow restricting orifices regulate flow from a main engine driven pump through a control valve and steering wheel metering device to a servo motor in a vehicle steering system. Under emergency conditions, an additional flow restricting orifice connects the downstream side of one of the aforementioned pair of orifices to a flow dividing valve. The flow dividing valve connects a reserve pump in parallel with the main pump to augment the supply of pressure medium and sustain a minimal operational steering speed under control of the differential pressure across the other of the pair of orifices, which then dominates control of the supply of pressure medium to the servo motor through the additional flow restricting orifice while there is no flow through said one of the pair of orifices.

BACKGROUND OF THE INVENTION 
The present invention relates to a hydrostatic steering system for motor 
vehicles. 
Vehicle steering apparatus of the type to which the present invention 
relates, is generally known as disclosed for example in U.S. Pat. No. 
4,553,389 to Tischer et al. In such steering apparatus, a controlling pump 
and/or a flow dividing valve supplies pressure medium to a steering servo 
motor for meeting instantaneous steering requirements. The demand for 
pressure medium is signaled by a differential pressure across a metering 
orifice that regulates the supply of the pressure medium. The pressure 
supply in such prior art systems is such that the differential pressure at 
the metering orifice not only breaks down when the main pump goes out of 
action, but also when the main pump is intact and the steering speed is so 
high as to require the entire output of the main pump for steering 
purposes, under engine idling conditions for example. Under such an 
operating situation, involving rapid steering movements under engine 
idling conditions, a reserve pump is utilized to augment the main pump 
output. However, under slow vehicle travel conditions, the reserve pump 
customarily driven by the vehicle wheels cannot satisfy the demand for 
flow of pressure medium. Excessive wear of the reserve pump therefore 
results from the pump acceleration effort encountered. 
It is therefore a basic object of the present invention to eliminate the 
aforementioned disadvantages of known hydrostatic auxiliary steering 
apparatus and to improve the same in such a manner that the reserve pump 
is only enabled and loaded when a specific minimal steering speed cannot 
otherwise be achieved with the flow from the main pump. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a metering unit driven by the 
vehicle steering wheel is connected to a control valve for controlling 
pressurization of the steering servo motor. A first metering orifice 
regulates flow from a main engine driven pump to the metering unit through 
the control valve. A second vehicle driven pump may be connected in 
parallel with the first pump to the control valve through a flow dividing 
valve. A second metering orifice is located between the first pump and the 
metering unit, and is interconnected with the first metering orifice 
downstream of the pump by a connecting line having a third or additional 
metering orifice and a check valve therein through which the pressure 
medium may flow in one direction from the downstream side of the second 
metering orifice. The flow dividing valve aforementioned is connected to 
the connecting line between the third metering orifice and the check valve 
so as to introduce pressure medium from the second pump only under 
emergency conditions in order to meet certain minimal steering speed 
demands incapable of being met by the output of the first pump. 
The arrangement of additional orifices in the steering system as 
aforementioned, in accordance with the present invention enables 
satisfactory steering operation wherein the supply of pressure medium 
depends upon load, by introducing the output of the reserve pump only when 
the requirement of the steering servo motor for flow of pressure medium 
thereto in order to attain a minimal operational speed under emergency 
conditions, is not otherwise met.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the embodiment shown in FIG. 1, a first high pressure pump 1 having a 
variable volumetric flow capacity, conveys pressure medium from a 
reservoir 2 through a pressure line 3 to a pressure port 4 of a control 
unit generally referred to by reference numeral 25. From port 4, the 
pressure medium flows through a check valve 26 and pressure supply passage 
5 to a control valve generally referred to by reference numeral 6. The 
control valve 6 has a by-pass passage 7 formed therein by flow 
constricting control slots which are open in the neutral, middle position 
of the control valve as shown in FIG. 1, establishing a flow connection 
with the reservoir 2 through a return port 8 of unit 25 and a return line 
9. A steering servo motor 10 is connected through ports 28 and 30 of 
control unit 25 to one side of the control valve 6. A metering unit 12 
drivingly connected to a vehicle steering wheel 11, is connected through 
passages 32 and 34 to the other side of control valve 6. The steering 
wheel 11 is also mechanically connected to the control valve 6, as shown, 
as is already well known in the art. 
A first metering orifice 13 having a constant flow-restricting cross 
sectional area, is located in the pressure supply passage 5 between the 
port 4 and the by-pass passage 7. A branch passage 14 extends from the 
pressure supply passage 5 upstream of the first metering orifice 13 to the 
control valve 6 and is blocked by the control valve in its neutral, middle 
position. As the control valve is displaced from its neutral position, the 
passage 14 is opened to correspondingly increase its cross sectional flow 
area until the fluid connection between the pressure supply passage 5 and 
one of the opposite ports 32 and 34 of the metering unit 12, acting as an 
intake, is fully opened. 
In a similar manner, displacement of the control valve 6 from its neutral 
position to effect adjustment of operating pressure, correspondingly 
restricts the by-pass passage 7 to finally block flow therethrough. In 
such neutral, middle position of the control valve 6, flow to and from the 
steering servo motor 10 is also blocked. With increased displacement of 
the control valve 6 from its neutral position, the pressurized passage 32 
or 34 from the metering unit 12, is connected with one chamber of the 
steering servo motor 10 to effect movement of the vehicle wheels to be 
steered. The other chamber of the steering servo motor 10, which is 
de-pressurized at that moment, is connected through control valve 6 and 
return line 9 to the reservoir 2. 
In the direction of flow, downstream of the first metering orifice 13, a 
branch control line 15 extends from the pressure supply passage 5 to a 
volumetric flow adjusting device 36 associated with the first high 
pressure pump 1. A second high pressure pump 16 having a constant 
volumetric flow characteristic, is operatively interconnected in parallel 
with the first high pressure pump 1, through a flow dividing valve 17, 
with the pressure supply line 3 for pressurized flow through supply 
passage 5 upstream of the first metering orifice 13 in the embodiment 
shown in FIG. 1. In the neutral position of the control valve 6, flow from 
the second high pressure pump 16 is always conducted through the first 
metering orifice 13 to the by-pass passage 7. The pump 1 is customarily 
regulated to maintain a constant pressure drop across the metering orifice 
13. The pressure downstream of the first metering orifice 13 is then 
communicated through port 38 of the control unit 25 and the control line 
15 to the adjusting device 36 of the first high pressure pump 1 for 
regulation thereof. The first high pressure pump 1 is preferably driven by 
the motor vehicle driving engine. The second high pressure pump 16 is 
customarily driven by the vehicle wheels, so that its output is a function 
of vehicle speed. 
A second metering orifice 18 is located in passage 14 extending from the 
supply passage 5 of the first high pressure pump 1 to the metering unit 12 
through control valve 6. The downstream sides of the first and second 
metering orifices 13 and 18 are connected to each other by a connecting 
line 19. A third or additional metering orifice 20 is located in the 
connecting line 19 at the downstream side of the first metering orifice 
13. Further, a check valve 21 through which the pressure medium is 
conducted from the downstream side of the second metering orifice 18, is 
located in the connecting line 19 between the third metering orifice 20 
and the downstream side of the second metering orifice 18. 
A control line 22 is connected to the connecting line 19 between the third 
metering orifice 20 and the check valve 21. Control line 22 extends to the 
flow dividing valve 17 for adjustment thereof. The pressure downstream of 
the first metering orifice 13 is thereby communicated through the third 
metering orifice 20 and the control line 22 to the flow dividing valve 17 
for said adjustment. 
The flow dividing valve 17 is a 3/3-channel valve, for example. In a first 
position of the flow dividing valve 17, under the bias of a spring 23, the 
second high pressure pump 16 is connected only to the pressure line 3. In 
addition to the force of the spring 23, the pressure from the control line 
22 also urges the flow dividing valve 17 to such first position thereof 
while the pressure from a control line 24 connected to the pressure line 3 
urges the flow dividing valve 17 in the opposite direction. With 
increasing pressure in the pressure line 3 and consequently in the control 
line 24, a second position of the flow dividing valve 17 may be 
established in which the flow from the second high pressure pump 16 is 
divided between pressure line 3 and return line 9. In response to further 
displacement of the flow dividing valve 17, a final position is reached in 
which the second high pressure pump 16 is connected only to the return 
line 9. 
In the embodiment of FIG. 1, a sliding transition to the second high 
pressure pump 16 is effected when the first high pressure pump 1 is 
partially disabled. Furthermore, when the steering system is being 
operated exclusively by the second high pressure pump 16, the supply of 
pressure medium occurs in the same manner as the supply from the first 
high pressure pump 1, since the flow dividing valve 17 acts as a pump 
regulator. 
The three metering orifices 13, 18 and 20, the control valve 6 and the 
metering unit 12 form the structural unit 25 in the embodiment of FIG. 1 
as hereinbefore described. In the embodiment shown in FIG. 2, three 
corresponding metering orifices 113, 118 and 120 are located outside of a 
structural unit 125, which otherwise corresponds to the unit 25 of FIG. 1. 
Thus, unit 125 includes a control valve 106 having a by-pass passage 107 
therein corresponding to the by-pass passage 7 of control valve 6 in FIG. 
1. The first high pressure pump 101 shown in FIG. 2 is a constant-flow 
type, supplying the steering servo motor 100 with pressure medium to meet 
operating demands, with any excess being delivered to another pressure 
operated device 127 through a flow dividing valve 126. The first metering 
orifice 113 is located in the pressure line 103 extending from the flow 
dividing valve 126. The pressure medium upstream of the first metering 
orifice 113, is delivered to a first valve actuating control surface of 
the flow dividing valve 126 through a control line 128. The pressure 
medium at the downstream side of the first metering orifice 113 is 
delivered through the control line 115 to a second valve actuating control 
surface of the flow dividing valve 126 on which a spring 129 also acts. 
Upstream of the first metering orifice 113, a passage 114 branches off 
from the pressure line 103 and has a second metering orifice 118 located 
therein. A third metering orifice 120 and a check valve 121 are located in 
a connecting line 119 which extends between the downstream sides of the 
first and second metering orifices 113 and 118, as in the case of the 
embodiment shown in FIG. 1 hereinbefore described. 
A flow dividing valve 117 associated with the second high pressure pump 
116, is a 2/3 channel valve by virtue of which the steering servo motor 
100 is supplied by the second high pressure pump 116 either partially to a 
limited extent or exclusively through the pressure supply line 103. Thus, 
under emergency steering conditions, there is a loss of operational ease 
as a sacrifice for the use of a simpler flow dividing valve 117. The 
piston surfaces and springs of the valves 126 and 117 are synchronized 
with each other in such a way that the second high pressure pump 116 draws 
pressure medium from the reservoir 102 when the supply from the high 
pressure pump 101 is such that the pressure drop across the first metering 
orifice 113 results in displacement of the flow dividing valve 126 against 
the bias of spring 129. 
A check valve 130 located in the connecting line 119 between the pressure 
line 103 and the line 114 blocks flow of pressure medium from the line 114 
to the pressure line 103 and to by-pass passage 107 of control valve 106 
of the unit 125. 
The embodiment shown in FIG. 3 varies essentially from the first two 
embodiments hereinbefore described with respect to FIGS. 1 and 2, in the 
provision of a first metering orifice 213 having a variable cross 
sectional flow area disposed in the control valve 206. In the neutral, 
middle position of the control valve 206, the cross sectional flow area of 
the first metering orifice 213 approaches zero so that by-pass passage 207 
is almost completely blocked. In the final positions of the control valve 
206, to which it is displaced from the neutral position, the first 
metering orifice 213 is fully opened in order to establish maximum 
operational steering speed. As a result, the pressure medium conveyed to 
the pressure supply passage 205 flows through the first metering orifice 
213 without any drop in pressure. The pressure is only influenced by the 
second metering orifice 218. Otherwise, the arrangement of the FIG. 3 
embodiment, including the third metering orifice 220 and the check valve 
221, corresponds to the arrangement of orifices 20 and 21 of the 
embodiment shown in FIG. 1. 
Operation of the auxiliary steering apparatus of the present invention will 
now be explained in greater detail with reference to the embodiment of 
FIG. 1. When the vehicle steering wheel 11 is turned so rapidly as to 
require maximum output flow from the first main pump 1 to the steering 
servo motor 10, the valve actuating piston of the control valve 6 is 
displaced from the neutral, middle position shown in FIG. 1 in a left or 
right hand direction to a final position. The by-pass passage 7 is thereby 
blocked so that no more pressure medium flows through the first metering 
orifice 13 and the differential pressure thereacross becomes zero. As a 
result, the pressure medium is then conveyed exclusively through the 
second metering orifice 18 to develop a pressure differential as an 
instantaneous function of steering speed. When the pressure differential 
across the first metering orifice 13 approaches zero, the pressure in line 
3 becomes equal to the pressure in the control line 15. Under that 
condition, pressure medium flows through the second metering orifice 18 
with the pressure in connecting line 19 lower than the pressure in lines 3 
and 15 by an amount equal to the differential pressure across the second 
metering orifice 18. The control line 22 for controlling the connection of 
the second high pressure pump 16 through flow dividing valve 17, is 
connected to the control line 15 through the third metering orifice 20 and 
the connecting line 19 through the check valve 21. Since no pressure 
medium is then flowing through the first metering orifice 13, the 
differential pressure of the second metering orifice 18 prevails through 
the third metering orifice 20. In the foregoing situation, a small stream 
of pressure medium flows through the third metering orifice 20 and the 
check valve 21 to the connecting line 19. Consequently, the differential 
across the second metering orifice 18 dominates the pressure in supply 
passage 5, the pressure line 3 and the control line 22. 
Utilizing the differential pressure across orifice 18 to control the flow 
dividing valve 17, the second high pressure pump 16 is connected only to 
the steering servo motor 10 in the absence of flow through the first 
metering orifice 13, corresponding to the operating condition in which 
total flow from the first high pressure pump 1 is required while the 
differential pressure across the second metering orifice 18 is 
insufficient to develop the necessary steering speed. The steering speed, 
below which the second high pressure pump 16 is connected to the steering 
servo motor when there is insufficient flow from the first high pressure 
pump 1, may be determined from the size of the second metering orifice 18. 
The check valve 21 serves the purpose of blocking flow between the 
pressure line 3 and the control line 22 when the differential pressure 
across the second metering orifice 18 is zero. 
The embodiments shown in FIGS. 2 and 3 basically function in the same 
manner as hereinbefore described in connection with FIG. 1. 
The foregoing is considered as illustrative only of the principles of the 
invention. Further since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact arrangement and operation shown and described, and, 
accordingly, all suitable modifications and equivalents may be resorted 
to, falling within the scope of the invention.