Hydraulically actuated piston with an air bleed

Details Hydraulically actuated piston with an air bleed Hydraulically actuated piston with an air bleed

2000-04-24

2001-10-23

Marmor, Charles A. (Department: 3681)

192 clutches and power-stop control

Clutches

Operators

C192S085060, C092S079000

Reexamination Certificate

active

06305521

ABSTRACT:

TECHNICAL FIELD
This invention relates to hydraulically actuated pistons for torque transmitting mechanisms and more particularly to such pistons having an air bleed structure.
BACKGROUND OF THE INVENTION
Torque transmitting mechanisms such as clutches and brakes employ a hydraulically operated piston to enforce frictional engagement between a plurality of friction plates that are alternately splined to a housing containing the piston and a hub member that is generally connected with a gear member. When the torque transmitting mechanisms are inactive during vehicle operation, a cavity or chamber formed between the piston and the housing remains filled with very low pressure oil. However, when the vehicle is stopped and the engine is not operated for a period of time, such as overnight, it is possible for the hydraulic fluid in the cavity to drain to the sump and the cavity becomes filled with air.
On a subsequent start-up of the vehicle, the air must be expelled from the cavity before consistent shift quality is attained. This may require five or more shift cycles which are objectionable. With today's electro-hydraulic controls, consistent hydraulic fluid fill times and volumes are necessary for shift quality consistency. The entrapped air in the cavity prevents the required consistency. The electronic controller of the electro-hydraulic control uses the data from the previous shift to calculate the optimum flow and pressurization rates to be employed during the current shift event. Since air is a compressible medium and hydraulic fluid is a relatively incompressible medium, if air is present in the cavity, it is difficult for the controller to determine the optimum flow and pressurization rates. During the first five or more shift events, the air is slowly bled from the cavity under the piston seals until the cavity is filled with hydraulic fluid only.
Some currently available power transmissions incorporating hydraulically operated torque transmitters utilize ball bleed valves and other such devices to provide a controlled passage through which the entrapped air can be evacuated. These devices often allow a significant amount of hydraulic fluid to leak from the cavity after the air bleed function is complete. The repeatability of these devices is inconsistent due to the variation in oil viscosity due to the operating temperature changes in the transmission.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved air bleed structure for the apply chamber of a torque transmitting mechanism.
In one aspect of the present invention, a hydrophobic material is secured in fluid communication between the apply chamber of a fluid operated torque transmitting mechanism and the interior of the transmission casing. In another aspect of the present invention, the hydrophobic material is disposed in either the piston or the housing of the torque transmitting mechanism. In yet another aspect of the present invention, a passage is provided between the apply chamber of a torque transmitter and the surrounding environment and a material permeable to vapor but substantially impermeable to liquid is secured in the passage.
Fluid operated disc type torque transmitting mechanisms can be either a rotating transmitter, such as a clutch, or a stationary transmitter, such as a brake. The entrapped air in a clutch is forced to the inner radius thereof as the clutch rotates since the heavier hydraulic fluid is centrifuged to the outer radius of the clutch. Therefore in a rotating torque transmitting mechanism, a vapor permeable-liquid impermeable material, such as a woven polytetrafluoroethylene (PTFE) is located in a passage at or near the inner radius of the clutch apply chamber. One such product found to be applicable to this environment is marketed by PALL Specialty Materials under the trademark Hydrolon®.
Generally, during the first engagement of the clutch after an idle period, entrapped air will have to be evicted from the clutch apply chamber. The air will be forced radially inwardly by the incoming hydraulic fluid and pass through the PTFE into the interior of the transmission from which it can be vented to atmosphere.
In a brake, the PTFE material is located in a passage formed at or near the top of the outer radius of the apply chamber. As the brake is applied, hydraulic fluid will force any entrapped air to the outer periphery of the brake apply chamber. The PTFE material will allow the air to escape to the interior of the transmission housing but prevent the leakage of any significant amount of hydraulic fluid. While the initial engagement of the clutch or brake may be slightly objectionable, after the entrapped air is evacuated, subsequent engagements of the brake or clutch will be within the desired design specifications for performance and pleasability.


REFERENCES:
patent: 3623700 (1971-11-01), Botsler
patent: 4560324 (1985-12-01), Durieux
patent: 5090528 (1992-02-01), Massel
patent: 6029447 (2000-02-01), Stojic et al.

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