Hydrostatic drive system for vehicles

A hydrostatic drive system for vehicles including a hydromotor with a control unit forming together a drive unit which in the assembled state can be inserted in the seat of the centering device of the drive system, the configuration and arrangement of the wheel brake valve wakes possible to obtain a drive unit with compact dimensions, in axial as well as in radial direction, especially due to the double function of the wheel brake valve which serves simultaneously as a pressure relief valve.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is a national phase application of PCT/EP/00936 filed Aug. 
8, 1989 and based, in turn, upon German National Application P3827205.9 
filed Aug. 11, 1988 under the International Convention. 
FIELD OF THE INVENTION 
The invention relates to a hydrostatic drive system for vehicles, 
particularly crawlers with a hydromotor mounted in the hub of a drive 
wheel, in or on whose control housing, besides control valves, wheel brake 
valves are provided, which are arranged in pressure lines and regulate the 
cross section of the return flow, steering it respectively on and off, 
depending on the operational pressure of the hydromotor. 
BACKGROUND OF THE INVENTION 
Hydrostatic drive units for vehicles are known (DE C 2 32 35 378). Thereby, 
it is required that the drive unit be so compact, especially in the axial 
direction, that it does not project laterally over the width of a 
caterpillar track of a crawler excavator or of the wheel of a vehicle. At 
the same time, the radial dimensions in the area of the seat of the 
centering device have to be selected so that drive system can be inserted 
and removed as a completely preassembled unit. These requirements are 
difficult to meet, when in or on the control housing of the hydromotor 
wheel brake valves are mounted in addition to other control valves. 
However, the arrangement of the valves directly on the control housing is 
advantageous because of increasing the response sensitivity of the drive 
due to short control lines 
OBJECTS OF THE INVENTION 
It is the object of the invention to provide a hydrostatic drive system 
having a particular valves, particularly of the wheel brake valves. 
SUMMARY OF THE INVENTION 
Due to configuration according to the invention, the wheel brake valves 
serve simultaneously as a pressure relief valve, so that additional 
pressure relief valves can be eliminated. This makes possible to keep the 
dimensions of the control lid with the valves small and to integrate the 
wheel brake valves in the control lid, so that in the assembled state they 
can be slid through the seat of the centering device of the drive system. 
In order to increase the cross section of the inflow during traction, 
without increasing the overall construction volume of the wheel brake 
valves, parallel to each of the wheel brakes valve a bypass line is 
provided which is equipped with a return valve opening in the direction of 
the hydromotor. These valves require only a small space and can be lodged 
in the control lid in suitable places. Further, it is possible to use the 
same hydromotor with the same control lid for various flow volumes by 
simply changing the bypass. 
In order to sustain the operation of the hydromotor with two pressure lines 
and one control line, according to the invention the pressure lines on the 
side of the wheel brake valves facing away from the hydromotor are 
interconnected by a control line with a two-way valve and a control 
pressure line for an adjusting mechanism of the hydromotor starting out 
from this valve. 
The wheel brake valve of the drive system of the invention has in the area 
of the valve seat a throttle gap gradually widening with the opening of 
the wheel brake valve. As a result, the cross section of the opening can 
be precisely and continuously controlled over a long control path. 
Oscillations of the control piston in the area of the closing position are 
prevented. Further, according to the invention, the control piston has a 
compensation space with a compensation opening designed as a throttling 
point in the frontal surface facing the hydromotor. The compensation space 
is closed by a supporting piston on the frontal side opposite to the 
compensation opening, the effective cross section of this supporting 
piston being slightly smaller than the pressure-impacted surface enclosed 
by the valve seat. As a result, in spite of high system pressures, the 
compression springs can be designed with small dimensions. Since at 
opening and closing, through the throttling compensation opening an oil 
flows out of the compensation space, or flows into it, the movement of the 
control piston is attenuated, so that no sudden changes occur. A further 
attenuation of the control piston movements is achieved when the pressure 
medium for the control of the control piston is supplied or evacuated 
through a throttling point. Suitably, the connection of the pressure 
control line is designed as a throttling point.

SPECIFIC DESCRIPTION 
A hydromotor 1 is mounted together with a control housing 2 by means of 
screws to a vehicle frame 4. It drives via a two-stage planetary gear 7 a 
hub 6 of a drive wheel 5 of a caterpillar track 3. The diameter of the 
control housing 2 is smaller than the diameter of the seat of a centering 
device of the drive, so that the assembled drive unit can be inserted from 
the outside through the seat of the centering device. Two pressure-medium 
lines 11, 12 (shown in FIG. 3) end in two openings 9 and 10 of the control 
housing 2. 
The shifting diagram of FIG. 2 shows clearly the functional interconnection 
of the pressure-medium lines 11, 12, of the control elements and the 
hydromotor 1. 
An engine drives via a transmission gearing a hydropump (not shown) which 
supplies a pressure medium via a pressure-medium line 13 to a hydromotor 
1. The pressure-medium lines 11, 12, 13 and a return-flow line 15 leading 
to a tank 16 are regulated by a 4/3 four port, three position valve 14, 
which in its neutral position closes the pressure-medium line 13 and 
connects the remaining lines 11, 12 with the return-flow line 15. However, 
other embodiments are also possible, e.g. wherein in the neutral position 
the pressure-medium lines 11, 12 are only connected with each other or are 
selectively closed. In the neighboring positions, the valve 14 actuates 
one of the two pressure-medium lines 11, 12 with the high pressure source 
and connects the respective other one with the return-flow line 15. By 
changing from one neighboring position to the other, the flow through the 
hydromotor 1 is modified, so that it is driven forward or backward 
corresponding to the respective open circuit. 
The outline of the control housing 2 is symbolized by a dash-dot line 17. 
It can be seen that after the connection openings 9, 10, the 
pressure-medium lines 11, 12 run inside the control housing 2 and are at 
least partially formed by built-in channels. In the control housing 2 a 
two-position valve 18 is mounted and connected via pressure-medium lines 
19, 20 to the pressure-medium lines 11, 12. It also connects the 
respective pressure-carrying pressure medium line with a control pressure 
line 22 leading to the two-position adjuster 25 of the hydromotor 1. 
In the pressure-medium lines 11, 12, wheel brake valves 28, 29 are mounted. 
Parallel thereto bypass lines 26, 27 with return valves 30, 31 opening 
towards the hydromotor 1 are provided. Between the valve 14 and the wheel 
brake valve 28, a pressure control line 32 leads to the wheel brake valve 
29 in the pressure-medium line 12 and from the pressure-medium line 12 a 
pressure control line 33 leads to the wheel brake valve 28 in the 
pressure-medium line 11 The wheel brake valves 28, 29 are prestressed in 
closed direction by springs 47, 52, 59. Shown in FIG. 3. 
The pressure-medium lines 11, 12 are alternately supply the medium or 
discharge the latter from respective ducts of the hydromotor. An output 
shaft 24 (FIG. 3) of the hydromotor acts upon a parking brake 23, which is 
prestressed by spring force and is vented when the hydrostatic 
transmission starts operating through the pressure medium by means of a 
separate control. 
At the control housing 2, there is a connection 34 for a pressure control 
line coming from outside. From there, a pressure control line 35 leads 
inside the control housing 2 to an adjusting element of a three parts, two 
position valve 36, to which are connected the control pressure line 22 via 
a branch pressure control line 37 and further the two-position adjustment 
device 25 of the hydromotor 1 via a line 38 and a throttle 39. To an oil 
sup connection 40 are connected the valve 36 via an oil duct 41, as well 
as the hydromotor 1 via another oil duct 42 and a collection pipe 43. 
A valve housing of the wheel brake valve 28, 29 (FIG. 3) is marked 45. It 
is closed at one end by a lid 46 which can be screwed on. A thereagainst 
supported spring 47, can be prestressed to a desired extent by shim plates 
21 of various thickness. With its side facing away from the lid 46, the 
spring 47 rests against a control piston 48. Inside the control piston 48, 
in a compensation space 51, a supporting piston 49 with a sealing element 
50 is contained, which comes to lie against the lid 46 via a cylindrical 
extension. A compression spring 52 is provided between a frontal surface 
53 at the supporting piston 49 and a frontal surface 54 on the control 
piston 48. The frontal surface 54 of the control piston 48 contains a 
compensation bore 55 designed as a throttling point and is located between 
the compensation space 51, wherein the spring 52 is lodged, and the 
opening 61 towards the hydromotor 1. The housing space, wherein the 
compression spring 47 is located, is connected via a connection opening 56 
to a supply oil duct (not shown) in the control housing 2, so that no 
additional ducts are necessary outside the control housing. 
The end of the control piston 48 facing away from the lid 46 is built as a 
valve seat 57. The counterpiece 58 of the valve seat 57 is a piston which 
is guided in the valve housing 45 and presses against a spring 59. The 
wheel brake valve has two openings 60 and 61 for the passage of the 
pressure medium. The throughflow is controlled through the pistons 48, 58, 
which cooperate as a valve. Next to the valve seat 57, a throttle gap 63 
is provided, whose cross section gradually grows in size with the increase 
of the opening, which prevents a pendulation of the control piston between 
the closed position and the open position. A connection for the pressure 
control line 32, respectively 33 is marked 62. 
DESCRIPTION OF OPERATION 
Over two pedals (not shown) the travel speed and the forward or reverse 
drive ranges are selected. Depending on the drive range, the valve 14 
steers the pressure of the adjustable hydropump towards one of the 
pressure-medium lines 11 or 12. The respective other one of the 
pressure-medium lines 11 or 12 is connected with the pressure-free return 
flow line 15. In the neutral position (median position) of the valve 14, 
both pressure-medium lines 11, 12 are connected with the return-flow line 
15. 
Further, the description of the operation relates to the position of the 
valve 14, wherein pressure from the hydropump is transmitted to the 
pressure-medium line 11 and the pressure-medium line 12 is connected with 
the pressure free return flow line 15. In the reverse situation of 
pressure distribution in the pressure-medium lines 11 and 12 in the 
opposite switch position of the valve 14, the description of the function 
of the control elements applies correspondingly to the other side. 
The pressure medium--hydraulic oil--flows under pressure through 
pressure-medium lines 11 and the connection opening 9 into the control 
housing 2. The pressure acts upon the two-way valve 18, which closes the 
duct 20, for instance by means of a ball, and this allows the pressure to 
work via control pressure line 22 on the two-position adjusting unit 25, 
36 of the hydromotor 1. Further, via the opening 60, the pressure medium 
acts upon the wheel brake valve 28 and displaces the counterpiece 58 of 
the control piston 48, against the force of spring 59 of the valve seat 
57, thereby opening the wheel brake valve 28, so that through the opening 
61 pressure medium flows to the hydromotor 1, actuating the same. A 
fraction of the flow volume bypasses the wheel brake valve 28 through the 
bypass line 26 and the return valve 30 to the hydromotor 1. 
From the hydromotor 1, the pressure medium flows out over the wheel brake 
valve 29 and the valve 14 through the pressure-medium line 12 and the 
return-flow duct 15. The return valve 31 prevents the return flow through 
the bypass line 27, so that the total flow volume flows through the wheel 
brake valve 29, which is kept open by the pressure control line 32, as 
long as the pressure in the pressure-medium line 11 does not drop below a 
certain determined pressure level. This is due to the fact that the 
pressure over the pressure control line 32 and over the connection 62 acts 
upon a recessed piston surface 65 of the control piston 48 of the valve 
29. Thereby, the control piston 48 is displaced against the force of 
springs 47 and 52 and this way separates at the valve seat 57 the control 
piston 48 from its counterpart 58. The freed valve cross section allows 
the pressure medium to flow through from the opening 61 on the side of the 
hydromotor 1 to the opening 60, and from there to the valve 14. 
The pressure in the pressure-medium line 11 is sufficient to keep open the 
wheel brake valve 29 in the pressure-medium line 12, even at very reduced 
drive outputs of the hydropump. 
When during thrust motion the hydromotor 1 is driven by the drive wheel 5 
and the pressure drops below a certain value in the pressure-medium line 
11, and thereby also in the pressure control line 32, the wheel brake 
valve 29 moves correspondingly to the pressure drop in closing direction, 
since the recessed piston surface 65 of the control piston 48 is no longer 
sufficiently impacted by pressure, so that the control piston 48, under 
the action of springs 47, 52, throttles the return flow from hydromotor 1 
against the counterpiece 58 which also returns towards the closed 
position. The hydromotor 1 works at this point against the closing wheel 
brake valve. In case of lacking control pressure (supply pressure) the 
vehicle is fully stopped. If the return flow continues to rise, the 
control piston 48 opens in the manner of a pressure relief valve. 
Through connection 34, the pressure coming via line 35 from a controllable 
source outside the control housing 2 acts upon adjustment element of the 
valve 36. This connects the pressure space of a two-position adjustment 
unit 25 via a throttle 39 and lines 38 and 41 with a pressure-free 
discharge oil duct 43. 
At a control pressure impact of for instance 30 bar, in the line 35 the 
positioning device of the value 36 connects the pressure space of the 
two-position adjusting unit 25 via the lines 37, 38 with the control 
pressure line 22, whereby the hydromotor 1 is set to a smaller 
displacement volume. Leak oil from the hydromotor 1 is evacuated through 
the ducts 42 and 43. 
Via a pressure control line 44, the parking brake 23 is vented, namely 
before the hydrostatic actuation takes effect. 
Reference numerals 
1 hydromotor 
2 control housing 
3 caterpillar track 
4 vehicle frame 
5 drive wheel 
6 hub 
7 planetary gear 
8 seat of centering device 
9 connection opening 
10 connection opening 
11 pressure-medium line 
12 pressure-medium line 
13 pressure-medium line 
14 4/3-way valve 
15 return valve 
16 tank 
17 housing outline 
18 two-way valve 
19 pressure-medium line 
20 pressure-medium line 
21 shim plates 
22 control pressure line 
23 parking brake 
24 output shaft 
25 two-position adjuster 
26 bypass line 
27 bypass line 
28 wheel brake valve 
29 wheel brake valve 
30 return valve 
31 return valve 
32 pressure control line 
33 pressure control line 
34 connection 
35 line 
36 3/2-valve 
37 line 
38 line 
39 throttle 
40 leak oil connection 
41 discharge duct 
42 discharge duct 
43 discharge duct 
44 pressure control line 
45 valve housing 
46 lid 
47 spring 
48 control piston 
49 supporting piston 
50 seal 
51 compensation space 
52 spring 
53 frontal surface 
54 frontal surface 
55 compensation bore 
56 connection bore 
57 valve seat 
58 counterpiece 
59 spring 
60 opening 
61 opening 
62 connection 
63 throttle gap 
64 frontal side 
65 recessed piston surface