Control device for continuously variable transmissions

Details Control device for continuously variable transmissions Control device for continuously variable transmissions

2000-05-15

2002-01-08

Fenstermacher, David (Department: 3682)

Endless belt power transmission systems or components

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

Fluid pressure actuator for adjustment of member

C474S018000

Reexamination Certificate

active

06336880

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a control device for a continuously variable transmission (CVT) that is based on the dual-piston principle.
The control of a continuously variable transmission based on the dual-piston principle is known, e.g., from the publication DE 195 46 293 A1. Continuously variable transmissions are cone-pulley transmissions that can be shifted within a continuous (step-free) range of transmission ratios. They have pairs of conical disks, i.e., one pair each on the input and output side of the transmission, with an endless chain-belt device making a torque-transmitting connection between the pairs of conical disks. More specifically, the continuously variable transmission according to
FIG. 1
essentially consists of a disk pair SSA that is rotationally locked to a torque-input shaft
20
and a disk pair SSB that is rotationally locked to an output shaft
21
of the transmission. Each of the two disk pairs comprises an axially displaceable disk and an axially fixed disk. An endless chain-belt device
22
transmits torque from the disk pair of one shaft to the disk pair of the other.
The disk pair SSA on the input shaft
20
can be axially tightened against the chain-belt device
22
by a first piston/cylinder unit
23
. In analogous manner, the disk pair SSB on the output shaft
21
can be axially tightened against the chain-belt device
22
by a second piston/cylinder unit
24
.
Third and fourth piston/cylinder units
25
and
26
, serving to shift the ratio of the transmission, are arranged to work in parallel, respectively, with the first piston/cylinder unit
23
on the input shaft
20
and the second piston/cylinder unit
24
on the output shaft
21
. The desired transmission ratio is set or changed by simultaneously adding pressure fluid to one and removing pressure fluid from the other of the pressure chambers of the third and fourth piston/cylinder units
25
and
26
, respectively. This is accomplished by connecting the pressure chambers either to a pressure-medium pump or to a drain conduit as needed in each case. In other words, a change in the transmission ratio is effected by adding pressure medium to one chamber and thereby expanding its volume while, at the same time, draining the other chamber at least partially of pressure medium and thereby reducing its volume. The respective pressurizing and draining of the pressure chambers occurs through a valve
1
as illustrated, e.g., also in FIG. 3 of the aforementioned publication DE 195 46 293 A1. The valve
1
has different ports, of which the port
2
is connected to a pressure-fluid pump (not shown). Port
3
of valve
1
is connected to the oil sump or reservoir tank. The third and fourth piston/cylinder units
25
,
26
of the disk pairs SSA, SSB are connected to the valve
1
through ports
4
and
5
(conduits L
1
and L
2
). Port
6
serves to control the valve
1
by means of a biasing pressure in pressure chamber
7
. The biasing pressure in pressure chamber
7
can be governed by a proportional valve (not shown).
The slide piston
8
of the valve
1
can be configured with a smaller cross-section in a portion
9
and a larger cross-section in a portion
10
. The corresponding bore widths inside the valve housing are dimensioned accordingly, i.e., differently for the respective portions
9
and
10
of the slide piston
8
. In addition, the slide piston
8
can have a portion with an axial channel
11
that has a radially directed opening at a location
12
. An internal piston
13
is arranged so that it can move inside the axial channel
11
.
A plurality of forces are acting on the slide piston
8
and, according to their sum total, can produce a resultant force that pushes the slide piston
8
either to the right or to the left. The individual forces are symbolized in
FIG. 1
by the arrows F
6
, F
4
, F
5
and F
14
.
Directed to the right and represented by arrow F
6
is a force that is proportionate to both the pressure at port
6
(thus also inside the pressure chamber
7
) and the cross-sectional area of the portion
9
of the slide piston
8
.
Also directed to the right and represented by arrow F
4
is a force that is proportionate to both the pressure at port
4
and the difference between the cross-sectional areas of the portions
9
and
10
.
Directed to the left and represented by arrow F
5
is a force that is nearly proportionate to the pressure at port
5
and the cross-sectional area of the axial channel
11
. The pressure at port
5
communicates with the axial channel
11
through the radial opening
12
. The radial opening
12
could also be designed as a hydraulic resistance element for damping the motion of the slide piston. The pressure acting in the axial channel
11
by way of the radial opening
12
is nearly the same as at port
5
. This pressure exerts a rightward push against the internal piston
13
which, in turn, bears against the plug
16
. The same pressure, acting on an effective area equal to the cross-section of the axial channel
11
, also exerts a leftward push on the slide piston
8
.
A further leftward-directed force, symbolized by arrow F
14
, is generated by spring
14
exerting a leftward push on the slide piston
8
and also bearing against the plug
16
.
FIG. 1
shows the valve
1
in a state where the slide piston is in its midway position. The force F
6
is about equal to the force F
14
. The portion
8
a
of the slide piston
8
closes off the port
2
leading to the pressure-medium pump.
Via the shutter edges
15
and
15
′, port
5
and port
4
are connected to port
3
. Given that port
3
has a connection to the oil sump, the respective pressures at ports
5
and
4
are nearly equal and very small. Consequently, the forces F
5
and F
4
, which have opposite directions and nearly cancel each other, are likewise very small. With the pressure being equal at ports
4
and
5
, no resultant displacing force is applied to the piston/cylinder units
25
,
26
through the conduits L
1
, L
2
.
If the force F
6
is greater than the force F
14
, slide piston
8
will move to the right. The connection between ports
5
and
3
is interrupted. Port
2
becomes connected to port
5
. An in-flow of pressure medium occurs, whereby the pressure at port
5
is increased. At the same time, the shutter edge
15
′ opens the connection from port
4
to port
3
and thus to the oil sump. This allows the pressure medium to escape to the sump. Consequently, the pressure at port
4
, and thus the force F
4
, is small, i.e., nearly zero. As the pressure rises at port
5
, the force F
5
will keep increasing up to the point where the force F
5
is equal to the difference between the forces F
6
and F
14
(F
6
minus F
14
). As soon as this is the case, the slide piston
8
will stop its rightward travel. If the pressure at port
5
and, consequently, the force F
5
continues to increase, the slide piston
8
will move to the left until the connection between ports
2
and
5
is interrupted and the further pressure rise is blocked. Ports
5
and
3
become connected, and the passage stays open until the pressure at port
5
has decreased to the point where the force F
5
is again equal to the difference between the forces F
6
and F
14
.
This process, which is appropriately termed pressure balancing, regulates the pressure at port
5
automatically to an amount of proportionate magnitude as the difference between the forces F
6
and F
14
.
If the pressure at port
5
is too high, fluid is drained off as the shutter edge
15
opens the connection between ports
5
and
3
, while the in-flow connection between ports
2
and
5
is blocked. If the pressure at port
5
is too low, the in-flow connection between ports
2
and
5
opens and the drain connection between ports
5
and
3
becomes closed off.
The pressure at port
5
acts on the piston/cylinder unit
26
by way of conduit L
2
. Conduit L
1
, along with the piston/cylinder unit
25
is nearly pressure-free. As a result, the loop radius at which the endless chain-belt devic

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