Planetary gear transmission systems or components – Fluid drive or control of planetary gearing – Fluid controlled mechanical clutch or brake
Patent
1995-01-25
1996-04-23
Marmor, Charles A.
Planetary gear transmission systems or components
Fluid drive or control of planetary gearing
Fluid controlled mechanical clutch or brake
475140, 475141, F16H 354, F16H 6106
Patent
active
055098606
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a powershift transmission, in particular to a two-speed transmission that is preferably designed as an epicyclic transmission. The gear steps of said transmission are engaged by hydraulically actuated friction clutches. The friction elements (inner and outer disks) of the friction clutches are engaged by spring force and disengaged by oil pressure.
Two gears can be engaged in a powershift transmission of the above kind. The friction clutches (one of said friction clutches acting as a brake) are alternatively disengaged and engaged when shifting the gears. The friction clutches are engaged by plate springs. When both friction clutches are pressureless, that is, engaged, the input and output are blocked (parking brake).
Due to the relatively strong force of the plate springs, the pressure medium (hydraulic fluid) is discharged from the respective pressure chamber toward the tank within a very short time. The time needed for disengagement of the friction clutches depends on the delivery rate of a source (pump) of hydraulic fluid.
By installing check valves, with throttles connected in parallel, in the pipes leading to the pressure chambers, it can be ensured that the alternative disengagement and engagement of the friction clutches are coordinated in a manner such that the friction clutch to be engaged does not engage until the friction clutch to be disengaged is disengage.
The check valves, complemented by the throttles, are arranged so that, during disengagement, the oil stream can flow unobstructed into the respective pressure chamber. During engagement, the oil stream exiting the pressure chamber is drawn into the tank.
The moment of engagement can be controlled with said valves. The engagement operation itself, that is, the torque transmission of the friction clutch over a period of time, cannot be controlled thereby. As soon as the piston acting upon the friction elements in an engagement direction has overcome the release play (sum of the spacings of the inner and outer disks of a friction clutch in a disengage state), the torque transmitted by the friction elements suddenly rises to the maximum value. Since the engagement operation takes place within a very short time a hard shift is noticeably felt, this becomes particularly disagreeable in a downshift from second to first gear.
This invention is based on the problem of improving the shifting quality, especially in a downshift.
The above problem is solved by means in at least one of the friction clutches which prevents the whole spring force from acting upon the friction elements when the release play of the friction elements is reduced to zero or near zero. Said means is applied in at least the friction clutch which is engaged in a downshift from a higher to a lower gear. With said means it is possible to control the torque transmission of the friction clutch to be engaged when no more release play exists. Hereby, the shifting operation can be made smooth when downshifting to a lower gear.
In a preferred embodiment, check valves with throttles connected in parallel are inserted in leading to the pressure chambers of the friction clutches. An accumulator, which is likewise filled with hydraulic fluid during disengagement of a corresponding friction clutch, is attached to at least one of the pipes. The volume of hydraulic fluid temporarily stored in said accumulator is emptied via the throttle point during the engagement of the friction clutch. The emptying begins when the friction elements of the friction clutch come into contact, that is, when the release play has become zero. The engagement operation can be additionally influenced by the type and size (accumulator volume) of the accumulator.
An especially simple design is obtained when the accumulator is formed by a cylinder and a spring-loaded piston. The engagement operation can be coordinated in a simple manner by the characteristic line of the spring used.
It is also advantageous if the piston reaches a final position when the friction clutch is entirely engaged. The fi
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Jensen Nathan O.
Marmor Charles A.
ZF Friedrichshafen AG
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