Hydrostatic drive for vehicles

The invention provides a hydrostatic drive for vehicles, especially for the drive wheels of automotive heavy processing machines that have a drive for main drive wheels via a multispeed gear transmission and a drive for auxiliary drive wheels, wherein the drives for both the auxiliary drive wheels and the main drive wheels are hydrostatic. These hydrostatic drives each comprise an adjustable pump and an adjustable motor. A multispeed gear transmission is provided between the main drive motor and the main drive wheels and a clutch is provided between the adjustable motor and the auxiliary drive wheels. To obtain a higher travel speed, it is possible to disconnect the auxiliary drive and to switch, via a changeover valve, the delivery of the adjustable pump from the auxiliary drive to the main drive. Thereby is obtained the advantage that through the same control of both main and auxiliary drive wheels there is ensured a functional interplay of the drives. By switching the delivery of the adjustable pump of the auxiliary drive to the main drive, the working capacity of the vehicle's main drive engine is optimally utilized, since the operation of the auxiliary drive wheels at high travelling speed is not used and the drive of the main drive wheels is correspondingly reinforced.

The invention concerns itself with a hydrostatic transmission for vehicles, 
particularly, though not exclusively, for heavy duty vehicles that work on 
unfavorable soil conditions. 
In a typical prior art hydrostatic transmission, a multispeed gear 
mechanism and a hydrostatic transmission for all drive wheels are driven 
directly by the main drive engine of the vehicle, the main drive wheels 
being driven by the multispeed gear mechanism, purely mechanically by 
means of drive shafts, Cardan drives, etc., while the hydrostatic 
transmission has an adjustable pump for driving hydromotors, that drive 
auxiliary drive wheels, connected therewith by means of conduits. At the 
same time there can be provided between the hydromotors and the auxiliary 
drive wheels, clutches, transmission gearing and similar driving elements. 
Known already are transmissions of the above mentioned kind wherein both 
the main drive wheels and the auxiliary drive wheels are driven, in the 
lower speed gears, i.e. while in the highest transmission ratios (e.g. 1st 
and 2nd gears), while in the higher speed gears (e.g. 3rd, 4th, etc. 
gears) only the main drive wheels are driven. To achieve this, the 
hydrostatic auxiliary drive is often coupled to the gear shift and is 
operated only in the 1st and 2nd forward and reverse gears and is 
disconnected in the other gears. 
For satisfactory operation it is necessary that the auxiliary drive wheels 
be driven so that the peripheral speed thereof corresponds as precisely as 
possible to the peripheral speed of the main drive wheels or that in the 
case of certain operations they have a peripheral speed slightly higher 
than the main drive wheels. This relationship of the peripheral speeds 
must be maintained over the whole speed range of all driven wheels. 
In addition, it is desirable to regulate the power of the hydrostatic drive 
circuit for the auxiliary drive wheels for optimal utilization of the 
generated power (to provide adaptation to the soil conditions, change in 
the direction of travel, etc.). In addition, an overload protection of the 
main drive engine is desirable and necessary. 
To meeting these conditions there have already been made a series of 
suggestions, all of which include more or less complicated control systems 
that work electrically or electronically. Thus, for example, West German 
laid-open application No. 29 21 756 (U.S. Pat. No. 4,186,816) has 
disclosed an electronic control for vehicles that has a main drive and a 
hydrostatic auxiliary drive of the above mentioned kind wherein the 
respective speeds are measured by means of sensors in the main drive and 
the auxiliary drive is controlled in accordance with the measured value. 
The coordination of an alternating current generator with the main drive 
(see U.S. Pat. No. 3,561,557) has also been disclosed for the dependent 
control of the drive wheels. 
These systems are expensive and can only be used for the control of the 
hydrostatic drive for the auxiliary drive wheels. 
West German laid-open application No. 24 52 835 (U.S. Pat. No. 3,918,546) 
has disclosed a hydrostatic auxiliary transmission for vehicles, that 
connects and disconnects the auxiliary drive wheels via a control device 
and which comprises a constant speed pump, a control pump, a flow divider 
valve, a throttle and directional valves. This already known control 
apparatus does not make it possible to actuate the auxiliary drive wheels 
in accordance with the torque that appears therein in dependence on the 
load. 
It has been proposed (West German Pat. No. P 30 35 522.7) to construct the 
hydrostatic auxiliary drive in which the hydropump is constructed as an 
adjustable pump that is adjusted by the pressure of fluid flow at one or 
more throttle positions, against spring tension, toward a larger pump 
displacement volume and is adjusted by the working pressure formed in the 
auxiliary drive toward a lesser pump displacement volume. 
This proposed drive refers only to the combination of one mechanical or 
hydrodynamic multispeed gear transmission in the main drive and a 
hydrostatic auxiliary drive. The different drives of main and auxiliary 
drive wheels work functionally together but have different operating 
characteristics. 
An object of the present invention is to provide a transmission wherein 
both the main drive and the auxiliary drive are hydrostatic and have the 
same control arrangement thereby to facilitate a load-functional 
interplay. The control of both drives being provided in a manner such that 
when a relatively high travelling speed of the vehicle is needed, the 
driving power for the auxiliary drive can be connected to the main drive 
and the power of the main vehicle drive engine can be utilized in as 
favorable a manner as possible. 
According to the invention there is provided a hydrostatic transmission 
arrangement for vehicles comprising a main hydrostatic transmission 
including a multispeed gear transmission for transmitting drive from this 
hydrostatic transmission to main drive wheels and an auxiliary hydrostatic 
transmission for driving auxiliary drive wheels, each transmission 
comprising a hydrostatic pump adapted to be driven by a main vehicle drive 
engine, a fixed displacement control pump adapted to provide a fluid 
output in proportion to the speed of said main vehicle drive engine and a 
hydrostatic motor connected by a hydraulic circuit to be driven by said 
hydrostatic pump and mechanically connected for driving engagement with 
its associated drive wheels, a throttle means connected to intercept and 
control fluid flow from said control pump thereby to regulate operating 
pressure of the associated hydrostatic pump, and adjusting means 
responsive to fluid output pressure from the throttle means to adjust 
displacement of operating fluid in the transmission, said auxiliary 
hydrostatic transmission being connected to the auxiliary drive wheels by 
way of a clutch means operable to disconnect drive to these wheels when 
desired. 
In this arrangement both auxiliary and main drives are hydrostatic and 
equipped with substantially identical controls thereby ensuring a desired 
functional interplay of both drives. 
According to preferred embodiment of the invention, it is possible, in 
order to obtain a higher travelling speed when the auxiliary drive is 
disconnected, to switch, via changeover valves, the delivery of the 
auxiliary adjustable pump from the auxiliary drive to the main drive. By 
this means it is possible that the generated power of the main drive motor 
be optimally utilized. 
The multispeed transmission can advantageously be a mechanical gear 
transmission. In one arrangement the auxiliary drive has two hydromotors 
each connected by a coupling to a different auxiliary drive wheel by way 
of a wheel gear. 
Since the control can be optimally designed using a throttle adjustment for 
only one gear, in an advantageous embodiment, there are provided one or 
more shutters (restrictions or orifices) connectable by control valves to 
provide optimum operation in other gears. 
To be able to coordinate the peripheral speeds of the auxiliary drive 
wheels and the main drive wheels, at least one of the control throttles 
provided according to the invention is adjustable in order to make changes 
in control pressure. The adjustment may be, for example, manual. 
Since the speed of the main vehicle drive engine and the working pressure 
of the hydrostatic drive control in proportion the adjustable pump 
operates so as to provide, at relatively low speeds, high working pressure 
corresponding to a high traction and, at relatively high speeds, a low 
working pressure corresponding to a low traction. In addition, the 
hydrostatic drive arrangement according to the invention will operate even 
when one of the drives has a slippage which would cause a stall if a 
hydrodynamic drive had been used. 
In a further embodiment of the invention, the hydromotors can be 
adjustable. This is especially appropriate when they are to be used over 
several gears, as the stroke-volume ratio of the pump-motor combination 
can be adapted to a greater range of gears. At the same time the 
connection and disconnection of the hydromotors can depend on the ratio of 
each gear so that, for example, they become operative only in the 1st and 
2nd forward gears and in the reverse gear. In addition, the auxiliary 
drive couplings are opened in the neutral position of the gear shift.

With reference to the drawing, a main vehicle drive engine 5, via a power 
distribution gear 6, drives an adjustable hydrostatic pump 7 which in turn 
drives an adjustable hydrostatic motor 8. The adjustable motor 8 drives a 
multispeed mechanical gear transmission 4, a permanent coupling 3, a 
differential gear 2, and thus the main drive wheels 1. From the 
distribution gear 6 there is also driven an adjustable hydrostatic pump 9 
which in turn drives adjustable hydrostatic motors 12 and 13 via a first, 
10, of two changeover valves 10 and 11. The adjustable motors 12 and 13 
are individually connected with auxiliary drive wheels 18 via couplings 14 
and 15 and wheel gears 16, 17. Couplings 14 and 15 are clutches operable 
by a control valve 43 which is common to both couplings. The adjustable 
pumps 7 and 9 for the auxiliary and main drives are connected respectively 
with pumps 19, 20 which are main control and auxiliary control pumps. 
Pumps 19, 20 provide fluid supply at a pressure proportionate to speed of 
the main vehicle drive engine. In the case of the auxiliary control pump 
20 the fluid under pressure is communicated, by way of feed conduit 38, to 
(a) adjustable throttle 22; (b) via one port of directional control valve 
26, to pump displacement adjusting device 28; (c) via throttle 22 and also 
via serially disposed control valve 23 and shutter (restriction or 
oriface) 24, to check valves 33,34, pressure relief valve 40; and (d) by 
way of another port of valve 26 to adjusting device 28 and to sequence 
valve 43 for the operation of clutches 14,15. By way of check valves 33,34 
the auxiliary circuit 36 is charged. 
In the case of the main control pump 19, the fluid under pressure is 
communicated, by way of feed conduit 37, to (a) adjustable throttle 21; 
via one port of directional control valve 25, to pump displacement 
adjusting device 27; (b) via throttle 21, by way of another port of valve 
25 to adjusting device 27, and (c) via throttle 21, to check valves 31,32 
and pressure relief valve 39. By way of check valves 31,32 main circuit 35 
is charged. 
Directional valves 25,26 provide manual selection of forward, neutral and 
reverse operation of the hydrostatic transmission. 
Valve 10 as illustrated connects pump 9 with motors 12,13 while valve 11 is 
closed. In the alternative position of these valves 10,11, valve 10 is 
closed and valve 11 connects pump 9 in parallel with pump 7 to drive motor 
8. 
The adjusting devices 27,28 adjust the output of pumps 7,9 respectively in 
response to a pressure differential across the devices acting against 
springs 41,42 which act to bias the devices to a central operational 
state, and the restoring force produced by the pumps 7,9 operation. 
Pressure-relief valves 39,40 in the respective control and feed circuits 
37,38 prevent over pressure from occurring in the transmission. 
The hydrostatic drive according to the invention functions as follows: 
In operation in the lower gears (e.g. 1st and 2nd gears) of the multispeed 
gear transmission 4, both the main drive and the auxiliary drive are 
operational. To control the adjustable pumps 7 and 9, pumps 19 and 20 
deliver, proportionally to speed of the main vehicle drive engine, an oil 
flow that is fed to the throttles 21 and 22 wherein there is caused in 
proportion to the delivery rate (the speed), a velocity head that is 
communicated via directional valves 25,26 to the adjusting devices 27 and 
28. This velocity head causes an adjustment of the output of pumps 7,8 as 
the pressure acts against springs 41,42 and the restoring force of the 
adjustable pumps 7,9 which depends on the working pressure in the main 
fluid circuits 35,36. 
This mode of operation, which in the case of low torque requirements, that 
is, low working pressure, fully adjusts the adjustable pumps to produce a 
relatively high travel speed and, in case of a high torque requirement 
corresponding to high working pressure, adjusts the adjustable pumps to 
produce a high pressure low flow rate to produce a relatively low travel 
speed. This results in a power regulation that protects the main engine 5 
from overloading and allows for the requirements of the vehicle 
utilization. 
When the multispeed gear transmission 4 is shifted from one working gear to 
another, the transmission ratio of the main drive changes. In order that 
the operation of auxiliary drive and main drive will agree when this other 
gear is selected, there is connected or disconnected in the auxiliary 
drive via the directional valve 23, the shutter 24 (an orifice of desired 
size). By means of this can be changed the pressure level in the control 
and feed circuit 38 of the auxiliary drive so that the operating 
characteristics of the auxiliary and main drive match. 
In the higher gears predominant in road travel, the auxiliary drive, as a 
rule, is not needed and can be obstructive. For this purpose the auxiliary 
drive is disconnected by operation of directional valves 10,11, the 
couplings 14,15 are opened by operation of valve 43, and the oil flow from 
circuit 36 is communicated to the main drive circuit 35. An increase of 
the travel speed results from operation of circuits 35,36 in parallel in 
this manner. This connection and disconnection can be either manual or 
linked with a control mechanism in the multispeed gear transmission, that 
is, with the introduction of a certain speed there results a changeover of 
the adjustable pump 9 from the auxiliary drive supplementation of the main 
drive. 
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LIST OF REFERENCE NUMERALS 
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1 main drive wheels 
23 auxiliary circuit mode 
control valve 
2 differential gear 
24 shutter (orifice) 
3 coupling 25 directional valve 
4 multispeed gear 26 directional valve 
transmission 
5 main drive engine 
27 pump adjusting device 
6 power distribution gear 
28 pump adjusting device 
7 adjustable pump 29 pipe 
8 adjustable motor 
30 pipe 
9 adjustable pump 31 check valve 
10 changeover valve 
32 check valve 
11 changeover valve 
33 check valve 
12 adjustable motor 
34 check valve 
13 adjustable motor 
35 main circuit 
14 clutch coupling 36 auxiliary circuit 
15 clutch coupling 37 control and feed conduit 
16 wheel gear 38 control and feed conduit 
17 wheel gear 39 pressure-relief valve 
18 auxiliary drive wheels 
40 pressure-relief valve 
19 control pump 41 spring element 
20 control pump 42 spring element 
21 adjusting throttle 
43 clutch coupling operation 
valve 
22 adjusting throttle 
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