Brakes – Wheel – Transversely movable
Reexamination Certificate
1999-03-03
2001-04-24
Graham, Matthew C. (Department: 3613)
Brakes
Wheel
Transversely movable
C188S07700R, C188S347000
Reexamination Certificate
active
06220402
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a hydraulic servo apparatus which seperates frictional engagement elements for an automatic transmission. More specifically, it relates to exhaust of air from a hydraulic chamber.
2. Description of the Related Art
A band brake is one type of a frictional engagement element provided for fixing a particular shift element, e.g. a sun gear, a carrier, or a ring gear, to a transmission case for achieving a certain speed ratio in an automotive transmission. In a conventional automatic transmission, the hydraulic servo apparatus, which operates to engage/disengage the brake band to/from the drum, is a piston/cylinder mechanism. The band is operated on by the movement of a rod interlocked to the piston. The rod is moved with the piston by a hydraulic pressure applied to the piston to engage or release the brake. Such a hydraulic servo is ordinarily arranged in an upper portion of the automatic transmission case.
One problem associated with the above-described type hydraulic servo is that fine bubbles tend to become entrapped in the automatic transmission fluid and are prone to accumulate in the hydraulic servo. Because of air accumulated in the hydraulic servo, a rise in pressure to the hydraulic servo is dampened and pressure responsiveness of the hydraulic servo apparatus is reduced. The timing of engagement of the band brake may become displaced from the proper timing. As a proposed solution to the foregoing problem, Japanese utility publication No. 60-99336 teaches provision of an air passage in the rod so that the air backlogged in the cylinder is exhausted by the piston stroke.
However, in the conventional automatic transmission, the piston and the rod move integrally. Therefore, upon a return stroke, air is sucked into the cylinder through the air passage, i.e. reintroduced. Therefore, a volume of air must be exhausted from the cylinder by each piston stroke, and the stroke time for exhausting the air is lengthy, i.e. the degree of opening of the air passage is determined by the movement of the rod which, in turn, depends on the stroke time. A long stroke time is required for sufficiently exhausting the air. In the case of a long stroke time, the rod of the hydraulic servo operating the brake has to be moved through a long stroke when the brake is fastened. Therefore, responsiveness in brake engagement is diminished.
SUMMARY OF THE INVENTION
In view of the above problems associated with the related art, a first object of the invention is to provide a hydraulic servo apparatus for an automatic transmission which exhausts air from a hydraulic servo, thereby preventing loss of responsiveness in the engagement of a frictional engagement element, and which prevents air from reentering the hydraulic chamber of the hydraulic servo in the return stroke of the piston.
A second object of the invention is to provide a proper stroke timing for exhausting air from the hydraulic servo, using a simple structure.
A third object of the invention is to provide a proper timing for exhausting air in a stroke of the piston, again by provision of a simple structure.
A fourth object of the invention is to utilize the rod to prevent air from reentering the hydraulic chamber of the hydraulic servo during the return stroke of the piston and rod.
A fifth object of the invention is to utilize the piston to prevent air from reentering the hydraulic chamber of the hydraulic servo during the return stroke of the piston and rod.
A sixth object of the invention is to prevent a loss of a hydraulic pressure after establishing engagement of the frictional engagement element, utilizing stroke of the rod for exhausting air from the hydraulic chamber.
A seventh object of the invention is to provide for exhaust of air from a hydraulic chamber without loss of responsiveness during the fastening operation (engagement) of a frictional engagement element by making it possible to exhaust the air within a short period of time in which the rod and the piston slide relative to each other.
A eighth object of the invention is to provide an absolutely reliable exhaust of volumes of air accumulating in a hydraulic chamber.
In order to achieve the aforementioned objects, the present invention provides a hydraulic servo apparatus for an automatic transmission which includes a cylinder having an interior space which, at one end, is in communication with the interior space of the automatic transmission. A piston is slidably mounted in the cylinder and divides the interior space of the cylinder into a closed hydraulic chamber and a back chamber open to the surrounding, interior space within the automatic transmission. A rod is slidably inserted in the piston so that the piston and the rod can move relative to each other over a predetermined distance. A passage is formed between the piston and the rod and connects/disconnects the closed hydraulic chamber to the back chamber by the relative movement between the piston and the rod. A closer closes the passage at the end of the return stroke of the piston toward the hydraulic chamber side. In this hydraulic servo apparatus, the piston is moved toward the back chamber side by applying a hydraulic pressure to the hydraulic chamber, whereby the rod is extended from the one end of the cylinder and pushes against a frictional engagement element by the movement of the piston.
The hydraulic servo apparatus further includes a return spring which is mounted in the back chamber and returns the piston and a regulating spring which is arranged between the piston and the rod and regulates the relative movement between the piston and rod. In the hydraulic servo, timing of the relative movement between the piston and the rod is determined in part by (depends on) the difference between the loads of the return spring and the regulating spring.
The timing of the opening/closing of the passage connecting the two chambers of the cylinder, by the relative movement between the piston and the rod, is determined in part by the shape of the passage.
In one embodiment, the rod provides a contact face which comes into sealing contact with a hydraulic chamber end face at the end of the return stroke of the piston toward the hydraulic chamber side. The passage connecting the two chambers of the cylinder is closed by the sealing contact between the contact face and the hydraulic chamber end face.
A seal member may also be provided on the hydraulic chamber end face for contact with the piston at the completion of the return stroke of the piston toward the hydraulic chamber side.
In one embodiment of the hydraulic servo apparatus of the present invention, the passage is formed in the rod, one end of the passage is opened at the end face of the rod, the other end of the passage is opened at a point on the periphery of the rod, with these two spaced openings being opened/closed by the relative movement between the rod and the piston.
Optionally, the passage formed in the rod may have two spaced peripheral openings which are open at same time. Alternatively, the passage which is formed in the rod has one end which is opened/closed by the relative movement between the piston and the rod, with the other end of the passage remaining open to the back chamber at all times.
According to the present invention, the air in the hydraulic chamber is exhausted through the passage formed between the piston and the rod at a proper piston/rod stroke timing, and return of the air into the hydraulic chamber during the return stroke of the piston is prevented by the closer which closes the passage at the end of the return stroke of the piston toward the hydraulic chamber side. Therefore, residual air accumulated in the hydraulic chamber, which would adversely affect response in engagement of the frictional engagement element, is minimized.
The timing of the relative movement between the piston and the rod is determined by load settings of the return spring and the regulating spring. Therefore, the short time needed for exhausting the air is obtained, without a
Isogai Akira
Iwata Akihito
Nishida Masaaki
Suzuki Akitomo
Tsukamoto Kazumasa
Aisin Aw Co. Ltd.
Graham Matthew C.
Lorusso & Loud
Sy Mariano
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