Hydraulic control device

A hydraulic control device is disclosed, having a pressure connection and a tank connection and having a metering pump unit (4) that comprises at least two metering pumps (5, 6) connected hydraulically in parallel and operable mechanically in parallel and also a cut-off valve (8) in hydraulic connection between the two metering pumps (5, 6), the cut-off valve (8) being flange-mounted on a metering pump (6) and having in a housing (13) a slider member (12) which, under pressure form the pressure connection (P), is movable axially towards the metering pump (6). It is desirable to keep wear and tear to a low level such a control device. For that purpose, a displacement-limiting device (26, 28) for the slider member (12) is provided, which prevents the slider member from bearing on the metering pump (6).

BACKGROUND OF THE INVENTION 
The invention relates to a hydraulic control device having a pressure 
connection and a tank connection and having a metering pump unit that 
comprises at least two metering pumps connected hydraulically in parallel 
and operable mechanically in parallel and also a cut-off valve in a 
hydraulic connection between the two metering pumps, the cut-off valve 
being flange-mounted on a metering pump and having in a housing a slider 
member which, under pressure from the pressure connection, is movable 
axially towards the metering pump. 
Such a control device is preferably used to steer vehicles. The metering 
pump unit is in this case connected by way of a directional valve to a 
steering motor. Both the metering pumps and the directional valve are 
connected to a steering handwheel or to a comparable device. When the 
steering handwheel is turned, the directional valve is displaced in the 
desired direction and the metering pump unit continues to convey hydraulic 
fluid until the steering motor has reached the desired position. In normal 
undisturbed operation, both metering pumps are operative. They can convey 
a correspondingly large volume of hydraulic fluid, which allows the 
steering motor to respond rapidly to movements of the steering handwheel. 
When the pressure at the pump connection drops or fails for any reason, for 
example, because the supply pump responsible is defective, the metering 
pumps can be used in a so-called "emergency operation" also as auxiliary 
pumps. In that case the metering pumps are then used to generate the 
pressure of the hydraulic fluid. For this, the necessary energy has to be 
introduced by way of the steering handwheel, that is, generally by human 
muscle power. To relieve the operator of the effort involved in this, it 
is known from DE 22 28 531 C2 to provide a cut-off valve, by means of 
which, when the pressure at the pump connection fails, the second metering 
pump is switched off. The operator then has to operate only one metering 
pump. For the same movement of the steering motor he then has to turn the 
steering handwheel further, but the effort involved is less. 
It is precisely with vehicle steering systems that it is desirable always 
to keep the necessary installation space as small as possible, and also to 
make the parts required for the steering as light-weight as possible. For 
that reason, in the control device according to DE 22 28 531 C2, the two 
metering pumps are flanged to one another. In addition, the cut-off valve 
is also flange-mounted on one of the metering pumps. In normal operation, 
when the pressure connection is supplied with pump pressure of the supply 
pump, the slider member of the cut-off valve is pressed by this pressure 
from the pressure connection against the adjacent metering pump. Since it 
is desirable to use no more material than necessary for the housing of the 
metering pump, this may possibly result in deformation of the housing of 
that metering pump under the pressure of the slider member. Although this 
does not impair the ability of the metering pump to function, the wear and 
tear that occurs is in some cases considerable, and can shorten the 
service life of the control device. This wear and tear occurs not only 
through the axial wear in the gear assembly adjacent to the cut-off valve. 
Because of the increased friction, both pumps have to be acted upon with 
an increased pressure and, because of the relatively high pressure 
difference over the gear wheels, this leads to greater wear in the two 
gear assemblies. 
SUMMARY OF THE INVENTION 
The invention is therefore based on the problem of reducing the wear and 
tear in such a control device. 
That problem is solved in a hydraulic control device of the kind mentioned 
in the introduction in that a displacement-limiting device is provided for 
the slider member, which prevents the slider member from bearing on the 
metering pump. 
The displacement-limiting device therefore stops the slider member in the 
housing before it comes to bear on the metering pump. At the same time, 
the slider member is therefore prevented from being able to exert 
corresponding forces on the metering pump or the housing of the metering 
pump. Accordingly, deformation of the housing of the metering pump is also 
avoided. The friction inside the housing of the metering pump is then not 
increased further, so that wear and tear can be kept at a lower level. An 
advantage of this construction is that the housing of the metering pump 
requires no change. In particular, it needs no reinforcement, which would 
normally give rise to an increase in weight and/or to an enlargement of 
the installation space, both of which are undesirable. 
In an advantageous construction, the displacement-limiting device is in the 
form of a mechanical stop member in the housing. The forces that position 
the slider member so that it unblocks the connection between the two 
metering pumps are then absorbed by the housing, and are not therefore 
passed on to the metering pump. Without great difficulty, however, the 
housing can made strong enough to be able to absorb these pressures or 
forces, without being deformed. 
In this connection it is preferred for the slider member to have at least 
one radially extending projection which co-operates with the stop member. 
The displacement-limiting device can in the process be removed from the 
actual movement area of the slider member, so that neither the movement of 
the slider member in the region provided for that purpose nor any flows of 
hydraulic fluid that ought possibly to be controlled by the slider member 
are disrupted. The fact that the projection extends radially moreover 
makes a larger area available for the application of force, so that parts 
of smaller dimensions are sufficient for absorbing the forces. 
In this connection the projection is preferably arranged in the region of 
the end of the slider member remote from the metering pump. The necessary 
installation space is generally available there. Manufacture is 
consequently simplified. Moreover, virtually the entire housing thickness 
is available for absorbing the retaining forces. 
It is especially advantageous for the stop member to be formed by a 
diametral enlargement of a housing bore receiving the slider member. A 
diametral enlargement of this kind can be made very easily. At the moment 
at which the slider member is inserted with its radially projecting 
projection into the housing bore, the stop member with which the 
projection can co-operate is also already present. 
The projection is preferably circumferential. There is therefore a uniform 
distribution of force in the circumferential direction. There is little 
danger that the slider member will tilt and therefore become canted and 
jammed. 
In this connection it is an advantage for the projection to be interrupted 
by an anti-rotation groove in which a securing element built into the 
housing engages. It is simple to make such a groove in the projection, for 
example by sawing or milling. Since this groove is located in a radially 
outer position, the anti-rotation means acts with a correspondingly large 
leverage, so that the necessary securing elements can be made somewhat 
less strong. 
In addition or as an alternative thereto, the slider member can have a 
connecting channel between its side wall and its end face remote from the 
metering pump. The pressure connection then opens into the circumferential 
wall of the housing bore in which the slider member is arranged in such a 
way that the mouth of the pressure connection and the mouth of the channel 
coincide in a predetermined position range of the slider member which is 
shorter in the axial direction than the path traversed by the slider 
member until it comes to bear on the metering pump, and a throttle is 
arranged parallel to the slider member. With this construction the drive 
for the slider member is, as it were, interrupted before the slider member 
comes to bear on the pump. The pressure from the pressure connection is 
able to propagate as far as the end face of the slider member only through 
the channel. In this case, however, it is requirement that the pressure 
from the pressure connection can reach the channel in the first place. 
That is possible only for such time as the channel and the pressure 
connection coincide with one another. As soon as the overlap ends, no 
hydraulic fluid can continue to flow. The hydraulic fluid that has built 
up the pressure on the end face of the slider member, reduces its pressure 
by way of the throttle that is arranged parallel with the slider member. 
In this manner a relatively stable positioning of the slider member in the 
housing can be achieved at a position which still has the required spacing 
from the metering pump housing. 
The throttle is preferably arranged in the slider member. This facilitates 
manufacture. It is sufficient for the slider member to be provided with a 
through-bore which then either itself forms the throttle or is provided 
for receiving a throttling unit. 
It is also preferred for the pressure connection and/or the channel to open 
by way of an annular groove into the side wall or circumferential wall. 
With this construction, it no longer matters whether the slider member is 
aligned in the correct rotated position relative to the pressure 
connection. On the contrary, the hydraulic fluid is able to spread by way 
of the annular groove uniformly over the circumference of the slider 
member and the housing bore. It is important merely that the annular 
groove or the annular grooves coincide suitably in the axial direction. 
The distance between the mouth of the pressure connection into the 
circumferential wall and a mechanical stop member for the slider member on 
the side remote from the metering pump is preferably larger than the path 
traversed by the slider member until it comes to bear on the metering 
pump. Since the mouth of the channel, for example, the annular groove, has 
a smaller axial dimension than the path traversed by the slider member 
until it comes to bear on the metering pump, but the distance between the 
mouth of the pressure connection and the stop member is still larger than 
that path, it is possible in this way to ensure that the sealing face of 
the slider member in the circumferential direction is large enough to 
protect the pressure chamber at its end face with the necessary 
reliability against the admission of hydraulic fluid under pressure.

DETAILED DESCRIPTION OF THE INVENTION 
A hydraulic control device 1 illustrated diagrammatically in FIG. 1 
comprises a directional valve 2, which is connected to two working 
connections L, R that are arranged to be connected to a steering motor 3. 
Further, a pump or pressure connection P and a tank connection T are 
provided. 
The other side of the directional valve 2 is connected to a metering pump 
unit 4, which comprises a first metering pump 5 and a second metering pump 
6 which are connected hydraulically in parallel and are operable 
mechanically in parallel by way of a common shaft 7, which also operates 
the directional valve 2. 
The concept that the first and the second metering pumps 5, 6 are connected 
in parallel means that the input connection of the metering pump 5 is 
connected or can be connected to the corresponding input connection of the 
second metering pump 6 and the output connection of the first metering 
pump 5 is connected or can be connected to the corresponding output 
connection of the second metering pump 6. The metering pump unit 4 in fact 
comprises also a cut-off valve 8, which is able to interrupt this 
connection between the first metering pump 5 and the second metering pump 
6. In the position shown in FIG. 1, the connection is interrupted and the 
second metering pump 6 is short-circuited, so that on rotation of the 
shaft 7 only the first metering pump 5 is able to convey hydraulic fluid 
towards the directional valve 2. 
Operation of such a control device 1 per se is known. A pump 9 conveys 
hydraulic fluid from a tank 10 to the directional valve 2. At the same 
time the cut-off valve 8 is displaced against the force of a spring 11 
into a position in which the two metering pumps 5, 6 are connected 
hydraulically in parallel. 
If the shaft 7 is now operated, hydraulic fluid passes by way of the 
directional valve 2 to the steering motor 3. The amount of hydraulic fluid 
is here determined by the two metering pumps 5, 6. 
If the pump 9 fails, that is, if there is no pressure at the pump 
connection P of the directional valve 2, the cut-off valve 8 cuts out, 
that is, it interrupts the connection between the two metering pumps 5, 6. 
Only the metering pump 5 is still able to work together with the 
directional valve 2. When the shaft 7 is turned, the metering pump 5 
conveys the hydraulic fluid required for operation of the steering motor 
3. 
Normally, yet further connections and lines are provided, in particular a 
load pressure control connection LS, but this is not shown here for 
reasons of clarity. 
FIG. 2 shows in more detail the construction of a metering pump unit 4 in 
which the two metering pumps 5, 6 are flanged together with the cut-off 
valve 8. Here, the first metering pump 5 has a gear assembly 5A, 5B and 
the second metering pump has a gear assembly 6A, 6B. Both gear assemblies 
have the same diameter and the same orientation with respect to one 
another. The gear assembly 5A, 5B has a smaller axial length than the gear 
assembly 6A, 6B of the second pump, however. The first metering pump 5 
accordingly also has a smaller capacity than the second metering pump, 
that is, for the same angle of rotation it conveys less fluid than the 
second metering pump 6. The mechanical connection between the two gear 
assemblies is effected by way of the shaft 7. 
The cut-off valve 8 flange-mounted on the second metering pump 6 comprises 
a slider member 12 which is mounted so as to be axially displaceable in a 
housing 13 having a housing bore 14. The housing 13 is closed by a cover 
15. 
A control pressure line 16, which is connected to the pump connection P of 
the directional valve 2 (indicated purely diagrammatically) enables the 
end face of the slider member 12 remote from the metering pump 6 to be 
acted on by pressure. When the pressure P acts on the control pressure 
line 16, the slider member 12 is displaced towards the second metering 
pump 6. For this, the slider member 12 has grooves 17 running in an axial 
direction, by means of which it is able to interconnect grooves 18, 19 
that are provided in the inner wall of the housing bore 14. The grooves 
18, 19 have a predetermined length circumferentially. They are in turn 
connected to lines 20, 21 which enable the first metering pump 5 to be 
connected to the second metering pump. 
Furthermore, an annular groove 22 is provided in the slider member 12, by 
means of which groove the lines 21 can be short-circuited. 
The end face of the slider member 12 adjacent to the metering pump 6 is 
subjected to the pressure in a pressure chamber 23 which is connected to a 
control connection 24, indicated purely diagrammatically. The control 
connection 24 is in its turn connected to the tank connection T of the 
directional valve 2. 
If the full pump pressure P were allowed to act on the slider member 12, 
the slider member 12 would bear with considerable force on a cover plate 
25 of the second metering pump 6, which force could deform this cover 
plate 25, indeed so that considerable wear and tear of the gear assembly 
6A, 6B of the second metering pump would ensue. 
In order to prevent the slider member 12 from bearing on the cover plate 25 
of the metering pump 6, the slider member 12 is formed with a 
circumferential and radially projecting projection 26 which is arranged in 
a diametral enlargement 27 of the housing bore 14. The distance between 
the projection 26 and a stop face 28, which is formed by this diametral 
enlargement 27, is less than the distance between the end face of the 
slider member facing the second metering pump 6 and the cover plate 25. 
Thus, before the slider member 12 comes to bear on the cover plate 25 of 
the second metering pump 6, the projection 26 comes to bear on the stop 
face 28. The forces which displace the slider member 12 towards the 
metering pump 6 are thus already absorbed in the housing 13, so that they 
are unable to reach the metering pump 6. In this connection the stop face 
28 is arranged at the end of the housing 13 remote from the metering pump 
6. Virtually the full thickness of the housing is therefore available for 
absorption of forces. 
As can be seen from FIG. 3, a groove 29 can be provided in the projection 
26 as well, in order to provide an anti-rotation means there. 
The projection 26 need not be circumferential. It would be sufficient to 
provide individual, preferably symmetrically distributed, radially 
projecting projections. The circumferential projection 26 has the 
advantage, however, that it can be made in a simple manner so that is 
absorbs the necessary forces. It need not be unduly thick. An axial extent 
of from 3 to 5 mm has proved satisfactory in most cases. 
The projection 26 and the stop face 28 therefore form here a 
displacement-limiting means for the slider member. Movement of the slider 
member 12 beyond a certain extent is reliably prevented. 
FIG. 4 shows an alternative construction, in which identical parts have 
been provided with the same reference numbers and corresponding parts have 
been provided with primed reference numbers. 
The slider member 12' has no circumferential projection in this embodiment. 
The displacement-limiting means is here formed by a combination of the 
following features: the slider member 12' has a channel 30 which is 
connected to a circumferential groove 31 and leads to the end face 32 of 
the slider member 12'. In the housing bore 14 there is provided a further 
circumferential groove 33 which is connected to the pressure connection P. 
Provided that the circumferential groove 31 coincides with the groove 33, 
hydraulic fluid is able to flow under the pressure P through the channel 
30 to the end face 32 of the slider member 12'. 
Between the end face 32 and the circumferential groove 31 there is provided 
an apron 34 which, when the slider member 12' has been displaced far 
enough towards the metering pump 6, closes the groove 33. 
Furthermore, in the slider member 12 there is a throttle 35 which lies 
hydraulically parallel to the slider member, that is, between the pump 
connection P and the tank connection T that is connected to the control 
line 24. 
In this connection the axial distance A, which is necessary for the apron 
34 to cover over the groove 33, is less than the distance B which the 
slider member 12' has to traverse before it comes to bear on the cover 
plate 25 of the metering pump 6. Even larger is a distance C between the 
groove 33 and the cover 15, which here serves as stop member for the 
slider member 12'. Thus, A&lt;B&lt;C. 
The cut-off valve 8 according to FIG. 4 operates as follows: when there is 
pressure at the pressure connection P, this passes by way of the groove 
33, the circumferential groove 31 and the channel 30 to the end face 32 of 
the slider member 12' and displaces the slider member 12' against the 
force of the spring 11 towards the metering pump 6. Before the slider 
member 12' comes to bear on the cover plate 25 (for which it would be 
necessary for it to traverse the distance B), the apron 34 (after 
traversing the distance A) closes the groove 33, so that further supply of 
hydraulic fluid under pressure to the end face 32 of the slide valve 12' 
is prevented. The pressure at the end face 32 then reduces by way of the 
throttle 35, and the slider member 12' is pushed by the force of the 
spring 11 away from the metering pump 6 again, so that the groove 33 is 
again uncovered. The slider member 12' can therefore never come to bear on 
the cover plate 25. In fact, sooner or later a state of equilibrium will 
occur, in that an additional throttle is formed between the grooves 31, 33 
so that the pressure drop across the two throttles 31, 33 and 35 is of the 
same magnitude as the counter-force exerted by the spring 11. 
This last solution can also be used in conjunction with a mechanical stop 
member which can assume, for example, an additional securing function. 
In each case an excessively large pressure force on the cover plate 25 is 
avoided, so that wear and tear, in particular of the metering pump 6, can 
be reduced.