Failsafe brake system for a motor vehicle, in particular for a hydrostatic construction machine

The invention relates to a drive system for a motor vehicle, in particular for a construction machine. A hydrostatic transmission unit comprising a variable displacement pump (25), a variable displacement motor (1) and a feed pump (26), is connected downstream of a diesel motor (24). In order to change gears in a powershiftable planetary transmission unit (2), gear-shift valves (34, 40) are provided. A brake (14) can be operated by a brake switching valve (30). The brake switching valve (30) is connected via line sections (33, 39) to the gearshift valves. When the brake switching valve (30) is in one switching position, a reservoir (37) is filled with hydraulic fluid. When in another switching position, a connection is made between the reservoir and a brake pressure chamber (20). During normal running of the machine, the reservoir is filled constantly with hydraulic fluid. When the brakes are operated, an adjustable residual pressure always acts on the brake. The brake is designed for dynamic deceleration. Deceleration always takes place in higher gear, thereby preventing unacceptably high values.

In a drive system of the above mentioned kind such as particularly used in 
construction machinery, for example, loaders, excavators, or crawlers, a 
prime mover, especially a diesel engine, drives a hydrostatic variable 
displacement pump. The variable displacement pump, together with a 
variable displacement motor, form a hydrostatic transmission whose 
reduction ratio is preferably adjusted by a speed-dependent control 
pressure (automotive drive regulation). Together with the hydrostatic 
transmission, a powershiftable gear (often a two-ratio planetary 
transmission) belongs to said drive system. The utilizable reduction range 
of the drive system can be increased with said powershiftable 
transmission. 
A construction machine, such as a loader, drives its working cycle in the 
first gear. The cruising speed is changed by the variable speed of the 
diesel engine and by means of the hydrostatic transmission. The 
powershiftable transmission makes it possible to change the mechanical 
reduction even while traveling under load. 
For construction machines which can travel quicker than a predetermined 
cruising speed such as 25 km/h, different legal injunctions or regulations 
are to be observed in relation to the brakes. In a single-circuit braking 
system, there has to be an auxiliary operating brake together with the 
normal operating brake. A locking or parking brake is also required. In 
EP-A 0 408 592 was disclosed a powershiftable two-ratio planetary 
transmission unit where two friction clutches (one clutch and one brake) 
can be actuated independently of each other. The friction clutch situated 
inside acts between the inner and outer central gears of the planetary 
transmission unit. Both central gears can thus be optionally braced with 
each other (variable displacement and/or auxiliary braking function). 
Pursuant to the above mentioned legal regulations, when the brake valve is 
actuated in order to introduce a dynamic emergency braking or in case of 
failure of the control pressure or of the power supply, a certain 
deceleration must be obtained. The brake torque needed for this is clearly 
less than the maximum design torque transmissible by the friction 
clutches. Said circumstance causes inadmissibly strong brakings or a 
blockage of the driving gears. In such a case the driver could no longer 
control the vehicle. 
The problem on which this invention is based is to provide a drive system 
in which the braking action is adapted to the cruising speed and the 
vehicle weight both on the road and off-road. 
The problem on which the invention is based is solved by the fact that the 
brake switching valve is connected by line sections with the gearshift 
valves, that in one switching position of the brake switching valve, a 
reservoir is filled with hydraulic fluid and that in another switching 
position of the brake switching valve, the reservoir is connected to a 
pressure chamber of a brake. The proposed solution is particularly 
advantageous since the reservoir in normal operation (brake switching 
valve open) is constantly filled with hydraulic fluid. In the operation of 
the brake the reservoir delivers the volume of fluid into a 
hydraulic-fluid line which discharges in the pressure chamber of the 
clutch or brake. In this manner an adjustable residual pressure always 
acts upon the brake. The brake is designed for dynamic decelerations. The 
deceleration always takes place in the higher gear and inadmissibly great 
decelerations values, which could constitute a possible source of danger, 
are reliably prevented. 
The brake switching valve is advantageously front-mounted on the gearshift 
valves so that a connecting line leads from the feed pump to an input of 
the brake switching valve. The reservoir is attached by one line to the 
line section which follows the brake switching valve. 
An advantageous design of the drive system consists in that the brake 
switching valve is attached by a line section to a hydraulic-fluid line 
which is connected, on one side, with a brake valve and, on the other, 
with the pressure chamber of the brake (or clutch). In the operation of 
the brake, the brake valve can maintain the adjusted pressure as long as 
the reservoir supplies hydraulic fluid. The reservoir feeds into the 
hydraulic-fluid line so that an adjusted residual pressure always acts 
upon the brake. 
For an economical manufacture it is advantageous if the gearshift valves, 
the brake switching valve and the brake valve are comprised in a common 
valve block. With regard to the changing of the gears, it is advantageous 
if the hydraulic-fluid lines leading away from the gearshift valves are 
attached by recoil valves having throttles parallel therewith to the 
pressure chambers of the clutch and of the brake. By virtue of the recoil 
valves with the throttles parallel therewith, it is possible by relatively 
simple means to produce an effective pressure modulation. In this manner 
it is possible to adapt the transition behavior of the clutch or brake 
from the engaged to the disengaged state and vice versa. 
A structurally simple solution is obtained if the gearshift valves are 
designed as 3/2 directional valves connected via signal flow lines with an 
electronic transmission control device. All valves can be designed with 
the same structure. 
The brake switching valve and the gearshift valves are in de-energized 
state in a switching position that locks the flow of hydraulic fluid on 
the side of the feed pump. In this manner it is possible, in case of power 
failure, to effect, together with the normal braking operation, an 
emergency braking. 
To monitor the control pressure, it is advantageous if a pressure sensor 
detects the pressure prevalent in the hydraulic-fluid lines. If said 
pressure is not sufficient to aerate the clutch or the brake, a driving 
operation by the electronic control device can be eliminated. When the 
hydraulic-fluid lines leading to the clutch or brake are pressureless 
while the brake is not active, the transmission control device can react 
in a manner such that one gearshift valve is actuated. 
It is optionally possible to use a pressure switch instead of a pressure 
sensor.

One part of a drive system is diagrammatically reproduced in FIG. 1 with a 
prime mover for a power-shiftable two-ratio transmission. The latter is 
preferably designed as a planetary transmission unit 2 while the prime 
mover is preferably a hydraulic variable displacement motor 1. The 
powershiftable transmission can also have more than two ratios. 
The variable displacement motor 1 can work in both directions of rotation 
with continuous speed and torque conversion starting from an initial zero 
speed. The variable displacement motor drives an input shaft 3 of the 
planetary transmission unit 2. 
The planetary transmission unit 2 is composed, in detail, of one inner 
central gear 4, one outer central gear 5, several planetary gears 6 and 
one planetary carrier 7. The planetary carrier 7 is supported on an output 
shaft 8 and can be non-rotatably connected therewith by a switching 
mechanism 9. The output shaft 8 carries one pinion 10 which is in meshed 
driving connection with another pinion 11 of an output shaft 12. The 
gears, such as those of a loader, are driven by the output shaft 12. 
Both gear ratios of the powershiftable planetary transmission unit 2 are 
hydraulically shiftable by one clutch 13 and one brake 14. A common disc 
carrier 15 of the clutch 13 and the brake 14, with the outer central gear 
5, are made as a one-piece part. 
To engage the first gear, the inner clutch 13 is disengaged by feeding 
hydraulic fluid to a pressure chamber 16 via an indicated hydraulic-fluid 
line 17. A piston 18 is then moved to the left against the force of 
springs, preferably plate springs 19. The brake 14 situated outside is 
engaged. 
To engage the second gear, the clutch 13 is engaged while the brake 14 is 
disengaged, preferably after a cross-over gearshift with a pressure 
modulation. For this purpose, hydraulic fluid is fed to a pressure chamber 
20 via a hydraulic-fluid line 21. A piston 22 is moved to the 
left-referring to the drawing-against the force of plate springs 23. 
The functional groups all together required for the driving and actuation 
of the powershiftable planetary transmission unit 2 are reproduced in FIG. 
2. By actuation of the planetary transmission unit it is to be understood 
here the interaction of the clutch 13 and the brake 14 to change the gears 
and actuate the brake during the operation. 
A prime mover, in particular a diesel motor 24, drives a hydrostatic 
variable displacement pump 25. The latter forms, together with the 
variable displacement motor 1, a hydrostatic transmission the reduction 
ratio of which is electronically regulated by an automotive drive 
regulation. 
The hydraulic fluid needed for the electro-hydraulic transmission control 
is fed to an input of a brake switching valve 30 from a tank 27 by a feed 
pump via a connecting line 28 where a one way valve 29 is inserted. The 
brake switching valve 30 is an electromagnetically shiftable 3/2 
directional valve controlled by an electronic transmission control device 
31 via a control signal line 32. In the switching position shown, the 
brake switching valve 30 is disconnected (de-energized). One line section 
33 leads from the brake switching valve 30 to a gearshift valve 34. Said 
gearshift valve is likewise designed as 3/2 directional valve and can be 
controlled by the electronic transmission control device 31 via a control 
signal line 35. 
The gearshift valve 34 is de-energized (through flow locked) in the 
switching position shown. 
From the line section 33, one line 36 branches off. The latter leads to a 
hydropneumatic reservoir or accumlation 37. An orifice 38 is situated in 
the line 36. 
Another line section 39 branches off from line section 33 and leads to an 
input of another gearshift valve 40. Said gearshift valve has the same 
structure as the gearshift valve 34 and is connected, via a control signal 
line 41, with the transmission control device 31. 
The clutch 13 of the planetary transmission unit 2 is engaged by the 
gearshift valve 34. The hydraulic-fluid line 17 provided for this leads 
from the gearshift valve 34 to the planetary transmission unit 2 (FIG. 1, 
pressure chamber 16). 
The brake 14 is actuated by the gearshift valve 40. The hydraulic-fluid 
line 21 provided for this leads from the gearshift valve 40 to the 
planetary transmission unit 2 (FIG. 1, pressure chamber 20). 
Over a control signal line 42, a pressure sensor 43 is attached to the 
hydraulic-fluid lines 17 and 21, via a shuttle valve 44. The pressure 
prevalent in said hydraulic-fluid lines is continuously monitored by the 
pressure sensor 43. If said pressure is not enough to aerate the clutch 13 
and the brake 14, the transmission control device eliminates one 
operation. If the brake 14 is not engaged, but the hydraulic-fluid lines 
17 and 21 are de-energized, the transmission control device can react in a 
manner such that, for example, one of the gearshift valves 34 or 40 is 
actuated. With said engagement, a blockage of the driving gears of the 
vehicle is prevented. A pressure switch can be used, instead of a pressure 
sensor, with which the different pressure ranges can be detected. Three 
pressure sensors can be used instead of the combination of pressure sensor 
43/shuttle valve 44. 
A return line 49 connects the gearshift valve 40 with the tank 27. A return 
line 50, of the gearshift valve 34, discharges in the return line 49. The 
hydraulic fluid returns, via the brake switching valve 30, by a line 
section 51 which discharges in another hydraulic-fluid line 52. The 
hydraulic-fluid line 52 is attached to a brake valve 53 on one side. On 
the other side it leads to a planetary transmission unit 2 and, via a line 
node 54, is connected with the hydraulic-fluid line 21. The line node 54 
ensures that the pressure prevalent in the pressure chamber 20 of the 
brake 14 is dependent both on the pressure of the hydraulic fluid in the 
hydraulic-fluid line 21 and on the pressure of the hydraulic fluid in the 
hydraulic-fluid line 52. 
The second gear of the planetary transmission unit 2 is engaged as follows: 
the right panel of the gearshift valve 40 is engaged. The hydraulic fluid 
delivered by the feed pump 26 reaches, via the open brake switching valve 
30 and the hydraulic-fluid line 21, into a cylindrical space 55 (FIG. 3). 
A piston 56 is axially movably supported, in the cylindrical space 55, 
under the action of the force of a spiral compression spring 57. The 
hydraulic fluid reaches, via a spring-loaded recoil valve 58 and the 
continuation of the hydraulic-fluid line 21, into the pressure chamber 20 
of the brake 14. The piston 22 is moved to the left. The brake 14 
disengages. 
In a downshift from the second to the first gear, the gearshift valve 40 
changes its position and assumes the switching position shown in FIG. 1. 
The hydraulic-fluid line 21 is connected with the tank 27 by the return 
line 49. Due to the change of switching position of the gearshift valve 
40, the piston 56 is moved to the left, against the force of the spiral 
compression spring 57, by the hydraulic fluid escaping from the pressure 
chamber 20-when the recoil valve 58 is closed. To prevent the brake 14 
from suddenly transmitting the full torque when the free play of the 
friction members has been reduced to a value of zero, the hydraulic fluid 
enclosed in the cylindrical space 55, and now to the right of the piston 
56, flows off via the hydraulic-fluid line 21 to the tank over a throttle 
59 and radially extending holes 60 in the piston 56. In addition, the 
periodic cycle can be affected by the characteristic line of the spiral 
compression spring 57 and the design of the throttle 59. Due to the steady 
reduction of the pressure prevailing before the piston 56, the brake 14 
finally becomes completely engaged with a steady transition by the blade 
springs 23. 
In the hydraulic-fluid line 17, a cylindrical space 61 having a 
spring-loaded piston 62 and a spring-loaded recoil valve 63 with a 
parallel throttle 64 is situated integrated in the input shaft 3. The 
operation for filling and emptying the pressure chamber 16 of the clutch 
13 develops in the manner explained with regard to the pressure chamber 
20. For disengaging the clutch 13, the right panel of the gearshift valve 
34 is engaged. When the clutch is engaging or disengaged, the 
hydraulic-fluid line 17 is open toward the tank 27 via the line 50 with 
the return line 49 (see FIG. 2). 
Since separate gearshift valves 34 and 40 are respectively provided for the 
clutch 13 and the brake 14, different cross-over engagements can be 
accomplished with relative ease. 
While running, the hydropneumatic reservoir 37 is constantly filled with 
hydraulic fluid (the brake switching valve 30 is engaged for 
through-flow). 
During the braking operation, the brake switching valve 30 is de-energized, 
that is, the left panel according to FIG. 2 is engaged. The hydropneumatic 
reservoir 37 relays the stored volume of hydraulic fluid through the 
orifice 38, via the line 51, to the line 52 and an open return valve 65 
and the hydraulic-fluid line 21 (line node 54) to the pressure chamber 20. 
The brake valve 53 can maintain the adjusted pressure as long as the 
hydropneumatic reservoir 37 feeds it. Said reservoir hereby generates more 
hydraulic fluid than flows off via the throttle 59 and leakage. During the 
brake operation, the pressure chamber 20 is always loaded with a residual 
pressure via the hydraulic-fluid line 52. The brake 14 is thus suited for 
dynamic decelerations, for example, to a full braking from maximum speed. 
The deceleration always takes place in the second gear (clutch without 
residual pressure, brake with residual pressure). The brake torque needed 
for this is clearly less than the maximum design torque transmissible by 
the brake 14 (or clutch 13). The residual pressure superimposed on the 
pressure chamber 20 in the operation of the brake prevents inadmissibly 
high deceleration values or a blockage of the vehicle gears. The driver in 
each case can control the vehicle. The residual pressure is advantageously 
adjustable in dependence on the weight of the vehicle so that an optimally 
synchronized brake operation is obtained at maximum vehicle speed on the 
road and off-road. 
It is advantageous to comprise the valves: brake switching valve 30, 
gearshift valve 34, gearshift valve 40, shuttle valve 44 and brake valve 
53 in a common valve block 66 (marked in the drawing with a thick dotted 
line). 
A switch 45 is attached to the transmission control device 31 via a control 
signal line 46. If the switch 45 delivers a signal for actuating the 
parking brake, the brake switching valve 30 and the gearshift valves 34 
and 40 are engaged without power; a parking brake light 70 is 
simultaneously activated. 
The electronic transmission control device 31 communicates, via a line 47, 
with a selector switch 48. 
The selector switch 48 can assume the changing positions forward, neutral 
and reverse (V, N, R). Together with the selecting position for the first 
and the second gear, the selector switch can be converted to the position 
D in which both gears of the planetary transmission unit are automatically 
engaged. 
To obtain a regulated deceleration, the brake valve 53 can be controlled by 
a manual brake valve 71. 
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Reference numerals 
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1 variable displacement 
29 recoil valve 
motor 30 brake switching valve 
2 planetary transmission 
31 transmission control 
unit device 
3 input shaft 32 control signal line 
4 inner central gear 
33 line section 
5 outer central gear 
34 gearshift valve 
6 planetary gear 
35 control signal line 
7 planetary carrier 
36 line 
8 output shaft 37 reservoir 
9 gearshift place 
38 orifice 
10 pinion 39 line section 
11 pinion 40 gearshift valve 
12 output shaft 41 control signal line 
13 clutch 42 control signal line 
14 brake 43 pressure sensor 
15 disc carrier 44 shuttle valve 
16 pressure chamber 
34 switch 
17 hydraulic-fluid line 
46 control signal line 
18 piston 47 line 
19 plate springs 48 selector switch 
20 pressure chamber 
49 return line 
21 hydraulic-fluid line 
50 return line 
22 piston 51 line section 
23 plate springs 52 hydraulic-fluid line 
24 diesel engine 53 brake valve 
25 variable displacement 
54 line node 
pump 55 cylindrical space 
26 feed pump 56 piston 
27 tank 57 spiral compression 
28 connecting line spring 
58 recoil valve 63 recoil valve 
59 throttle 64 throttle 
60 hole 65 recoil valve 
61 cylindrical space 
66 valve block 
62 piston 
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