Motor vehicles – Manually actuated controlling devices – With transmission control
Reexamination Certificate
1999-03-03
2001-10-09
Culbreth, Eric (Department: 3611)
Motor vehicles
Manually actuated controlling devices
With transmission control
C060S494000, C180S307000
Reexamination Certificate
active
06298939
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a system for controlling the traction power of a utility vehicle having an infinitely variable transmission. It is particularly suited to a vehicle with an infinitely variable transmission in which power is split between hydrostatic and mechanical branches.
Vehicles which lave an infinitely variable, hydrostatic-mechanical transmission can be accelerated from standstill to a maximum speed in both directions of travel by adjusting the transmission and therefore do not require a conventional clutch between the transmission and the engine. However, in certain situations, e.g. for changing the speed of an additional mechanical transmission offering several speed ranges, it is necessary to reduce the traction power built up at a specific speed without at the same time reducing the speed. A device which meets this requirement is described in DE 195 23 963 A1. So that the operator controls of this vehicle and their layout as well as the vehicle's response to their use conform as far as possible to that of a utility vehicle with a conventional gear shift system, the traction power is controlled by a pedal, which, because of the way it operates, will be referred to as a clutch pedal hereafter. Depending on the degree to which the clutch pedal is opened, orifice plates associated with respective directions of travel and controlled by the clutch pedal each affect the closing power of a valve, the latter being arranged one after the other in a hydraulic line extending between the two sides of the hydraulic motor forming part of the hydrostatic branch of the transmission. Operating the clutch pedal causes the valves to open in proportion thereto, which leaks pressure to a greater or lesser degree between the two sides of the motor. This causes a reduction in the available traction power in proportion to the reduction in pressure. Pressure will be leaked in different directions according to whether the motor is operating in forward or reverse modes.
In a high-power utility vehicle, the known control device has lived up to the expectations placed on it. However, when it comes to substantially lower-power vehicles, it tends to lack sensitivity in attenuating the transmitted traction power in the lower power range. This inadequate sensitivity causes a perceptibly rough driving behaviour during “meshing”, making it more difficult to carry out front loading work, for example.
One objective of the invention is to provide a device of the type outlined above, which will permit the transmitted traction power to be sensitively proportioned irrespective of the vehicle drive output.
SUMMARY OF THE INVENTION
According to the invention, a system for controlling the traction power of a utility vehicle comprises:
(a) an operator-actuated control member;
(b) an indefinitely variable transmission;
(c) the said infinitely variable transmission comprising a pump and a motor in a hydraulic circuit arranged such that the motor may be driven in either of two directions;
(d) the said circuit including two main proportional valves arranged to leak hydraulic pressure between the two sides of the motor;
(e) the degree of opening of the said main proportional valves being controlled by the said operator-actuated control member;
characterised in that:
(f) each said main proportional valve is located in a respective hydraulic line bridging the two sides of the motor and is arranged to leak hydraulic pressure in a respective direction between the two sides of the motor.
It has been found that the use of separate lines bridging the motor provides greatly increased sensitivity in the control of the pressure drop across the motor, and thus the control of available power.
Preferably, the opening of each of the main proportional valves is controlled via a respective hydraulic control port, the hydraulic pressure at each said port being controlled by an intermediate proportional valve, the said intermediate valve being resiliently biased into the closed position, with the closing bias force decreasing as the operator actuated control member is operated so as to open the said intermediate valve.
The use of a further valve of this type has been found to further increase the sensitivity of the system. This is because at any given open position of the clutch pedal there will be a specific, precisely set closing force on the intermediate valve, which in turn means that the valve will respond sensitively to changes in the position of the clutch pedal.
Because this type of intermediate valve is quite costly, it is preferred to use only one such valve connected to the control ports of both the main proportional valves. In this case, the intermediate valve is connected to the control ports via automatic selector valve means which automatically connect the port of one of the said main proportional valves to the intermediate valve and disconnect the other of the two main proportional valves from the intermediate valve or vice versa, according to the direction in which hydraulic pressure is to be leaked.
Such automatic selector valve means can be readily provided, for example by a “shift valve” essentially comprising two one-way valves back to back. The two lines to the control ports of the main valves are in this case connected to the two inputs of the shift valve and the output of the shift valve connected to the intermediate valve. Flow will occur to the output of the shift valve from whichever of the inputs is at the higher pressure.
An additional advantage of using only one intermediate valve, together with the automatic selector valve means, is that the overall reaction speed for the system is further increased over the prior art discussed above, with consequent increase in sensitivity of the system.
It is of practical use if the resilient closing bias is provided by a resilient closing member, normally a compression spring, which bears on a valve control member, the control member in turn being acted upon by a resilient return member whose return force increases as the valve is opened This arrangement is therefore preferred.
A spring with a relatively flat characteristic curve may be used for the resilient closing member, which insures a very sensitive response. Preferably, the said valve control member is provided by the piston of a hydraulic slave cylinder connected (via a hydraulic line) to a master cylinder actuated by the operator-actuated control member.
Advantageously, the main proportional valves are connected directly across the two sides of the motor without the interposition of other hydraulic components. The result is that the pressure drop between the two sides of the motor is minimized when the clutch pedal is fully depressed, and may be arranged to be substantially zero. This arrangement is therefore preferred.
Further preferable features are set out in the dependent claims and others will be apparent from the following description of one specific embodiment of the invention, which is given by way of example only with reference to the following drawings:
REFERENCES:
patent: 5653107 (1997-08-01), Beck
patent: 19523963 (1997-01-01), None
patent: 1269141 (1972-04-01), None
patent: 1789801 (1993-01-01), None
Bosl Theodor
Groger Gunther
Heindl Richard
Honzek Robert
Agco GmbH
Culbreth Eric
MacMillan Sobanski & Todd LLC
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