1. Field of the Invention
The present invention relates in general to automatic transmissions, and more particularly to automatic transmissions of a type which comprises a clutch drum, a planetary gear unit and a friction mechanism which, when actuated, engages the clutch drum with an internal gear of the planetary gear unit, and a hydraulic actuator for actuating the friction mechanism. More specifically, the present invention is concerned with hydraulic actuators of a type which has a centrifugally operable drain valve in order to drain, when the hydraulic actuator assumes its inoperative condition, a remaining fluid in a fluid work chamber of the actuator thereby eliminating a centrifugal force caused by the remaining fluid.
2. Description of the Prior Art
In an automatic transmission, friction mechanisms are used which are selectively actuated by hydraulic actuators for obtaining desired speed gear ratios. That is, by hydraulically operating the friction mechanisms, automatic shift from one speed gear ratio to another speed gear ratio is achieved. This means that during operation of the transmission, at least one of the hydraulic actuators is forced to rotate even when it is in its inoperative condition wherein no hydraulic pressure is applied to its associated fluid work chamber. However, as is known, due to rotation of the hydraulic actuator, a non-negligible centrifugal force is produced, which is caused by a remaining fluid in the fluid work chamber. This force sometimes stimulates the actuator to operate, but loosely, the friction mechanism bringing about an undesirable draggling phenomenon of the friction mechanism (viz., incomplete loosed engagement thereof) thereby deteriorating a normal operation of the transmission.
In order to solve this undesirable phenomenon, a drain valve is usually employed, which functions to drain or discharge the centrifugal pressure. One of the transmissions of the type using the drain valve is shown in FIG. 14.
Referring to FIG. 14, there is shown a part of the transmission where a low clutch is arranged. Designated by numeral 1 is the low clutch which functions to selectively engage a clutch drum 2 with an internal gear 4. The clutch drum 2 is rotated together with a carrier of a front planetary gear unit (not shown), and the internal gear 4 is a part of a rear planetary gear unit 3. Upon application of fluid pressure to a fluid work chamber 5, a piston 6 is moved rightward in the drawing against a return spring 7 thereby to cause frictional engagement between clutch plates 8 and 9. With this, the clutch drum 2 is engaged with the rear planetary gear unit 3 to rotate therewith.
A drain valve 10 is associated with the piston 6, which comprises a valve ball 10a movably received in a restricted drain bore 10b formed in the piston 6. During rotation of the clutch drum 2 with the fluid work chamber 5 filled with a pressurized fluid, not only a certain fluid pressure but also a centrifugal force caused by the fluid in the chamber 5 is applied to the ball 10a.
Thus, under operation of the low speed clutch 1 wherein a certain fluid pressure is applied to the fluid work chamber 5, the ball 10a is forced to assume its close position by the force of the fluid, overcoming the centrifugal force inevitably applied thereto. Thus, under this condition, the operation of the low clutch 1 is not disturbed.
Under rest of the low clutch 1 wherein the clutch drum 2 is rotated with the fluid work chamber 5 supplied with "no" hydraulic pressure, the ball 10a is forced to assume its open position due to a centrifugal force produced by itself. Accordingly, even if any centrifugal force is generated in the fluid work chamber 5 due to presence of remaining fluid therein, the fluid is instantly drained through the drain bore 10b liminating or at least minimizing the centrifugal force caused by it. This prevents the undesired draggling phenomenon of the friction mechanism.
However, due to its inherent construction, the transmission of the above-mentioned type still has a following drawback which originates from the "Coriolis force".
That is, when the low clutch 1 (more specifically, the piston 6) is not rotated, the flow of the fluid is not applied with the Coriolis force, so that the flow is smoothly directed toward the drain bore 10b of the drain valve 10 in a manner as indicated by arrows in FIG. 15A. Thus, in this case, there is produced no vortex flows about a mouth of the drain bore 10b. Accordingly, under this condition, the pressures created at portions "a", "b", "c", "d" and "e" (see FIG. 16A) in the fluid work chamber 5 are substantially equal as is depicted by a solid line of a graph of FIG. 16B. This means that in the fluid work chamber 5 there is no portions where a great pressure drop is generated.
However, as is seen from FIG. 15B, when the low clutch 1 (viz., the piston 6) is rotated, the flow of the fluid is applied with the Coriolis force, so that the flow is directed toward the drain bore 10b in a manner as indicated by arrows in the drawing. Thus, in this case, there are produced vortex flows about the mouth of the drain bore 10b. (It is to be noted that the arrows designated by reference "D" is the direction in which the piston 6 is rotated.) The pressures at the portions "a", "b", "c", "d" and "e" in the fluid work chamber 5 are depicted in the graph of FIG. 16B by a broken line. As is seen from this graph, there is produced a great pressure drop at the mouth of the drain bore 10b, which is caused by the vortex flow.
Creation of such great pressure drop disturbs a smooth draining of the fluid through the drain bore 10b, so that elimination of the undesired centrifugal force is not effectively achieved. Furthermore, the creation of such great pressure drop causes increase in a critical pressure of fluid at which the drain valve 10 changes its open and close conditions, so that undesired function delay of the low clutch 1 may occur at the time when the fluid work chamber 5 is supplied with the pressurized fluid.
These phenomena bring about not only uncomfortable speed reduction feeling but also speed change lag.