Continuously variable transmission

Details Continuously variable transmission Continuously variable transmission

1999-04-14

2001-08-28

Bucci, David A. (Department: 3682)

Endless belt power transmission systems or components

Pulley with belt-receiving groove formed by drive faces on...

Speed responsive

C474S028000

Reexamination Certificate

active

06280357

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a continuously variable transmission or CVT, in particular for motor vehicles, provided with an adjustable pulley assembly comprising two conical sheaves on a pulley shaft and at least one sheave being axially movable with respect to the pulley shaft by means of hydraulically operated movement means. The movement means comprise a piston/cylinder assembly for generating a first force acting on the movable sheave in the direction of the other sheave and centrifugal pressure compensating means for generating a second force acting on the movable sheave in a direction opposite with respect to said first force.
DESCRIPTION OF THE RELATED ART
Such a CVT is known from the European patent publication EP-A-0.777.069 and comprises a driving primary pulley assembly and a driven secondary pulley assembly, each provided with an axially movable and an fixed sheave. A tapered drive belt is trained around said pulley assemblies. The ratio of the radial positions of the drive belt between the sheaves of the primary and of the secondary pulley assembly determines the speed ratio between a primary shaft drivingly connected to the primary pulley assembly and a secondary shaft drivingly connected to the secondary pulley assembly. The piston/cylinder assemblies define hydraulic chambers, the so called pressure chambers, that are used to set said radial positions of as well as the tension in the driving belt. By increasing or decreasing the fluid pressure in a pressure chamber, said first force acting on the movable sheave in the direction of the fixed sheave of pulley assembly may be increased or decreased respectively. Said movable sheave may thus e.g. be urged towards the other sheave of a primary pulley assembly, so that the circular section of contact between the tapered drive belt and the conical sheaves of said primary pulley assembly is urged radially outward, whereas the section of contact between the drive belt and the conical sheaves of the secondary pulley assembly is urged radially inward. Depending on whether fluid is allowed to flow away from the pressure chamber of the secondary pulley assembly, the transmission ratio or the tension in the drive belt changes.
In order to accurately control a CVT, accurate control of the pressure levels inside the pressure chamber(s) of both pulley assemblies is essential. However, by the rotation of the pulley assemblies during operation of the CVT, the centrifugal force builds up a centrifugal pressure in the fluid in a pressure chamber, which centrifugal pressure is dependent of the rotational speed of a pulley assembly and of the radial dimension of the pressure chamber. Due to the centrifugal pressure the tension in the driving belt can become undesirably high or it can become impossible to exploit the full speed ratio coverage of the CVT at high rotational speeds. To compensate for force acting on the movable sheave due to the centrifugal pressure in the pressure chamber, the known construction is provided with centrifugal pressure compensating means as an integral part of the piston/cylinder assembly. In the known construction the cylinder of the piston/cylinder assembly extends in axial direction beyond the piston of the piston/cylinder assembly and is provided with an annular wall part extending radially inward, thereby defining a further hydraulic chamber, the so called compensation chamber. The centrifugal pressure compensating means exert a second force acting on the movable sheave in a direction opposite with respect to said first force. The magnitude of the second force is dependent of the fluid pressure in the compensation chamber, which in turn is dependent of the centrifugal force. It is thus achieved, that the cylinder pressure may be controlled essentially independently from the rotational speed of a pulley assembly.
The known CVT functions satisfactorily, but possesses as a drawback that the second force exerted by centrifugal pressure compensating means only partly compensates the centrifugal pressure in the pressure chamber, especially at relatively high rotational speeds of a pulley assembly. A seemingly available solution to this insufficiency would be the increase of the dimensions of the compensation chamber. However, increasing the radial dimension of the piston/cylinder assembly does increase said second force, but also increases the centrifugal component of said first force. In the known construction the cylinder of the movable sheave is extended axially to locate the compensation chamber in line with the pressure chamber. This requires additional axial space to accommodate the movements of the movable sheave.
SUMMARY OF THE INVENTION
It is the object of the present invention to provide for a compact CVT with hydraulically operated adjustable pulley assemblies comprising centrifugal pressure compensating means providing adequate compensation within a confined space. According to the invention the CVT is provided with an adjustable pulley assembly with centrifugal pressure compensating means comprising a further cylinder, fixed to the axially movable sheave in radial direction beyond the piston/cylinder assembly, and a further piston fixed to the pulley shaft. Said further piston and said further cylinder interact in a sealing manner, forming a further piston/cylinder assembly. A CVT according to the invention possesses small dimensions with respect to a state of the art transmission as well as a number of additional advantages, that are elucidated below.
The centrifugal pressure is dependent of the radius to the second power, as is the surface area on which said pressure acts, so that the second force acting on the axially movable sheave is dependent of the radius to the fourth power. Since the further piston/cylinder assembly according to the invention defines a further hydraulic chamber partly located in radial direction beyond the pressure chamber, the surface area required for generating a desired compensating force is significantly reduced with respect to the known construction, thereby decreasing the dimensions of the CVt. By providing said further cylinder with an appropriate shape, a smaller or larger further hydraulic chamber may be effected. A CVT according to the invention possesses the advantage, that full compensation of the centrifugal pressure (or even over-compensation) is easily achieved, by inward relatively independently from the dimensions of the pressure chamber selecting a suitable surface area for the annular wall part of the further cylinder extending radially. A large second force can be effected by increasing the radial and axial dimension of the pulley assembly. Additionally, according to the invention a cheap and technically simple supply tube may be adopted to supply fluid to said further hydraulic chamber, because the centrifugal pressure compensating means may be located radially spaced from the pulley shaft. To this end the supply tube is provided with a supply hole substantially over an opening in said further hydraulic chamber. In the known construction expensive and complicated supply means are adopted, such as a bore in the pulley shaft or a calibrated supply opening in a piston.
According to a development of the invention, said further piston is fixed either directly to the pulley shaft or by means of an intermediate element such as the piston of the piston/cylinder assembly.
In a further development of the invention, said centrifugal pressure compensating means comprise a baffle plate for guiding fluid. Said baffle plate is fixed to said further piston in such a manner that fluid is allowed to flow between the baffle plate and said further piston to said further hydraulic chamber. In this manner centrifugal pressure is already build up outside the further hydraulic chamber, increasing the centrifugal pressure level inside said chamber. A large second force can thus be effected within an axially confined space.


REFERENCES:
patent: 4753627 (1988-06-01), Kawamoto
patent: 5145464 (1992-09-01), Mori
patent: 5676612 (1997-10-01), Schelle

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