Endless belt power transmission systems or components – Pulley with belt-receiving groove formed by drive faces on... – Load responsive
Reexamination Certificate
2000-11-09
2003-02-11
Bucci, David A. (Department: 3682)
Endless belt power transmission systems or components
Pulley with belt-receiving groove formed by drive faces on...
Load responsive
C474S028000
Reexamination Certificate
active
06517455
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a continuously variable transmission or CVT provided with an electro-hydraulic control system and a method for operation thereof. The CVT is known, for example, from EP-A-0.787.927 and is in particular suited for motor vehicles.
DESCRIPTION OF THE RELATED ART
The known CVT comprises a V-type drive belt wound around a first pulley provided on a first shaft and a second pulley provided on a second shaft. Both pulleys have two conical discs, an axial separation of which is adjustable by means of a piston/cylinder assembly of each pulley. During operation of the transmission the drive belt is clamped with a first clamping force between the discs of the first pulley and with a second clamping force between the discs of the second pulley. The clamping forces are effected by exerting a pressure in a piston/cylinder-assembly associated with a pulley for each of said first and second pulleys. The level of the clamping forces determines the maximum amount of torque that can be transmitted between said first and second shaft virtually without relative movement in the tangential direction between the drive belt and the discs of the pulleys, i.e. belt slip, occurring. The (instantaneous) maximum amount of torque is referred to as the (instantaneous) torque transmitting capacity of the CVT. In the CVT a transmission ratio is related to the ratio of the running radii of the drive belt between the discs of the pulleys, which may be influenced by changing the ratio of the pressures in the piston/cylinder assemblies. An electro-hydraulic control system is provided to control said pressures and to achieve an appropriate transmission ratio and torque transmitting capacity of the continuously variable transmission.
It is generally known in the art, that the efficiency of the CVT is inversely proportional to the clamping forces applied by the pulley discs to the drive belt. Therefore, the control system is preferably arranged such that the lowest allowable clamping force for achieving torque transmission without belt slip is applied to the belt. For effecting the clamping forces, the control system is provided with a hydraulic system comprising a source of hydraulic medium in the form of a pump and a line pressure valve for controlling the pressure of the medium delivered by the pump to a main hydraulic line, the so called line pressure. To minimise energy loss, said line pressure is equal to, but not higher than, the highest pressure applied in the cylinders of the piston/cylinder assemblies. The control system is further provided with an electronic system comprising an electronic control unit (ECU) for generating a control current operating said line pressure valve in dependence on one or more variables, such as the transmission ratio, rotational speed of the pulleys and/or the torque to be transmitted by the transmission. Usually said line pressure valve is operated through applying a range of control pressures to said valve, which control pressure is regulated by an electronically operable pressure regulator, which in turn is operated by said control current. The pressure regulator derives said control pressure from an essentially constant pressure regulator feed pressure, which is somewhat higher than the maximum level of the control pressure to be applied. The control current generated by the ECU for operating the pressure regulator typically varies in a range between 0 and 1 Ampere. In dependence on said control current the control pressure regulated by the pressure regulator typically varies in a range between 0 and 6 bar, whereas the feed pressure is typically about 7.5 bar.
The known set-up of the control system is adopted both in conventional CVTs where line pressures are applied ranging from a few bar to 45 bar and in the currently favoured transmission designs, where the maximum line pressure level applied may range from 80 to 90 bar depending on CVT design specifics. Current developments in CVT design indicate that said maximum level might increase even further.
It appears that in the currently favoured CVT designs the accuracy with which the line pressure may be controlled does not meet the accuracy formerly achieved in conventional CVT designs. However, as was mentioned earlier, it is desirable for efficiency considerations to be able to accurately control the line pressure, so that the lowest allowable line pressure is set, while still effectively preventing belt slip.
SUMMARY OF THE INVENTION
It Is an object of the invention to improve the efficiency of currently favoured CVT designs and in general to provide for a CVT with a control system capable of accurate control of the line pressure. According to the invention these objects may be achieved by a CVT having the features defined in the characterising portion of claim
1
. It is also an object of the invention to provide for a method for accurately and efficiently operating the CVT of currently favoured design. According to the invention this object may be achieved by adopting the method according to claim
13
.
The CVT of the invention is characterized by pressure control means that are capable of influencing the line pressure by a further pressure acting on a part of the line pressure valve. In effect this means that the line pressure valve is controllable both by said control pressure and by said further pressure. This has the advantage that a number of options are opened for a considerable improvement of the accuracy of the control system and thereby of the transmission efficiency. According to the invention it is, for example, possible to use the control pressure to operate the line pressure valve so that approximately the required line pressure is set and to use the further pressure to fine tune the setting of the line pressure valve. In this manner it is achieved that the accuracy of the line pressures control and thus also the efficiency of the CVT is improved considerably.
According to an elaboration the invention the pressure control means comprise a further pressure regulator capable of regulating the further control pressure that serves as said further pressure. According to the invention it is advantageous, if in a first part of the range of applied line pressure levels, e.g. the lower halve between a few bar and approximately 45 bar, the line pressure valve is controlled by the control pressure regulated by the known pressure regulator, whereas the a second part of said range, e.g. the upper halve approximately between 45 and 87 bar, the valve is controlled by the further control pressure serving as said further pressure and regulated by the further pressure regulator. In this manner the pressure regulator and the further pressure regulator are effectively adopted in series, so that the total range of control pressure levels available for controlling the line pressure valve is essentially doubled. It was found that the extra cost from incorporating such pressure control means might be overcome by the positive effect they have on the efficiency of the CVT.
According to another elaboration of the invention, which is particularly suited for a CVT having a hydraulic circuit which is arranged such that the pressure in the cylinder of the piston/cylinder associated with the first pulley is said line pressure, whereas the pressure in the cylinder of the piston/cylinder associated with the second pulley is determined by means of a flow control valve capable of controlling a flow of hydraulic medium from said main hydraulic line to said piston/cylinder assembly or a flow from said piston/cylinder assembly to said reservoir. According to the invention the pressure control means comprise a passage connecting the piston/cylinder assembly associated with the second pulley to the line pressure valve, such that the pressure in said piston/cylinder assembly serves as said further pressure capable of influencing the line pressure. The control system, or more in particular the line pressure valve, is arranged such that when said pressure in the piston/cylinder assembly associated
Pelders Hendrikus Adrianus Arnoldus Wilhelmina
Van Wijk Wilhelmus Johannes Maria
Bucci David A.
McAnulty Timothy
Van Doorne's transmissie B.V.
Young & Thompson
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