Power plants – Pressure fluid source and motor – Having a mechanical clutch or brake device in the power train
Reexamination Certificate
2002-02-25
2003-11-25
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Having a mechanical clutch or brake device in the power train
C060S494000, C475S074000
Reexamination Certificate
active
06651427
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to hydrostatic transmissions (“HST”) and transaxles including hydrostatic transmissions, and in particular to a bypass system for such transmissions. Specific exemplary embodiments discussed herein relate to riding lawn mowers and similar small tractors.
BACKGROUND OF THE INVENTION
The description of art in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such.
Riding lawn mowers and small tractors
10
of the type discussed herein and depicted in
FIGS. 1A and 1B
generally use an engine having a vertical output shaft
12
that is connected to the transaxle
14
via a conventional belt and pulley system
16
. A standard HST for such a transaxle includes a hydraulic pump that is driven by the engine output shaft
12
, and a hydraulic motor, both of which are usually mounted on a center section. Rotation of the pump by an input shaft creates an axial motion for the pump pistons. The oil pressure created by this axial motion is channeled via porting to the hydraulic motor, where it is received by the motor pistons. The axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft that drives the vehicle axles through differential gearing. For additional background, the reader is referred to U.S. Pat. No. 5,201,692, (“the '692 patent”) issued to Johnson et al., and incorporated herein by reference in its entirety.
A problem with the typical HST arrangement is that a “neutral gear” does not exist, as it is merely a point where the hydraulic pressure in the pump goes to zero. However, at this point the oil remains in the transmission, preventing the vehicle from being freely rolled. The present invention is an improvement over prior art methods used to place a transaxle into neutral gear and enable movement of the tractor without the motor running or, more generally, without the transmission being engaged.
Prior art has solved the problem by diverting the oil through a hydraulic valve from the pressure side to the vacuum side of the HST center section. The problem with such a design is that the hydraulic valve allows for the movement of only a limited amount of oil due to inherent design limitations, such as the diameter of the hydraulic valve through which the oil is diverted. Machining such a valve also requires precise tolerances, thus increasing the manufacturing costs of the unit.
The '692 patent solves this neutral gear problem by providing a mechanism whereby the motor block is mechanically lifted from its running surface. This mechanism allows the oil to bypass the vacuum-pressure (hydraulic) circuit and to exit the case completely. This mechanism operates to enable the vehicle to free-wheel more easily than is possible with prior art hydraulic valve methods.
FIG. 1C
(which is similar to FIG. 2 of the '692 patent) is a section view through the transaxle
14
. To activate the bypass feature disclosed in the '692 patent, a bypass arm
18
is manipulated by the user to rotate a bypass actuator
20
. The bypass actuator
20
includes a rod
22
which is shaped at its base
24
so that rotation of the rod
22
forces a bypass plate
26
to press against the base of the motor
29
, thereby breaking its seal to the motor running surface (See FIGS. 2 and 5 of the '692 patent). It will be apparent to those of skill in the art that a pin or multiple pins, or other mechanical means may be used to lift the motor block. These mechanical lifting mechanisms allow the oil to flow between the motor and the transmission cavity.
A bypass rod
28
is connected to the bypass arm
18
to facilitate manipulation of the bypass arm
18
.
FIG. 1D
shows the bypass rod
28
in an unlocked position. The prior art means for activating the bypass mechanism includes pulling the bypass rod
28
through the tractor hitch plate
30
and securing it in place with a weldment (or cross pin)
32
. To secure the bypass rod
28
in the hitch plate
30
, the hitch plate
30
must include a relatively complex stamp out, e.g., a keyhole
34
. See
FIG. 1F
which shows the bypass rod
28
extending through the keyhole
34
of the hitch plate
30
. Perhaps more troublesome is the requirement to have tight tolerances for setting the cross pin
32
relative to the hitch plate
30
and the keyhole
34
. This tolerance requirement is especially frustrating because manufacturing tolerances involving the hitch plate
30
and the vehicle frame are much looser.
While the present invention relates to hydrostatic transaxles and transmissions generally, it will be better understood within the discussion of exemplary embodiments directed toward riding lawnmowers and similar small tractors.
SUMMARY OF THE INVENTION
A primary object of the present invention is directed toward a bypass system, and in particular, a bypass latch, for a hydrostatic transaxle or transmission. A hydrostatic transaxle according to one embodiment of the present invention comprises a casing including a hydrostatic transmission located in the casing. The transmission comprises a motor on a motor running surface and a pump connected to the motor via a hydraulic circuit. A bypass actuator, in a preferred embodiment, is rotatably positioned to lift the motor from the motor running surface when the actuator is rotated such that hydraulic fluid flows out of the motor (a part of the hydraulic circuit). Alternatively, other components can be shifted to allow fluid to flow out of the hydraulic circuit. A bypass arm is preferably positioned external to the casing (though need not be) and affixed to the bypass actuator and rotatable therewith. A latch arm that is rotatable about an axis parallel to an axis about which the bypass actuator rotates is adapted to releasably engage the bypass arm. The latch arm is coupled with a brake rod such that operation of the brake rod disengages the latch arm from the bypass arm, whereby the bypass actuator is disengaged and the transmission is thereby engaged.
Accordingly, an embodiment of the invention is directed toward a bypass assembly comprising a latch arm having a latching end for releasably engaging the bypass arm and a brake end coupled with a brake rod. A returning spring, preferably an extension spring, is connected to the latch arm to apply a returning force to bias the latch arm toward an at-rest position. In a preferred embodiment the at-rest position is the position where the brake end of the latch arm abuts, or is stopped against, a brake arm. A second spring, preferably a compression spring, cooperating with the brake rod is used to apply a braking force to the brake arm when the brake rod is actuated. In a preferred embodiment, the compression spring transmits a force to the braking end of the latch arm prior to transmitting a braking force to the wheels or axles of the tractor. A slidable member adapted to control the timing of the force may be positioned on the brake rod.
One advantage of the design is that it is integral with the transmission. One method of integrating a bypass latch with a hydrostatic transmission according to the present invention comprises placing the hydrostatic transmission in a casing and connecting a bypass actuator to the transmission. This allows the transmission to roll more freely when the bypass actuator is engaged than when the bypass actuator is disengaged. A bypass arm positioned external to the casing is connected to operate the bypass actuator. A latch arm adapted to releasably engage the bypass arm is positioned external to the casing as well. The method of integrating further includes linking the latching arm to a brake rod such that operation of the brake rod causes the latching arm to release the bypass arm, whereby the bypass actuator is disengaged.
By integrating the design, the manufacturer could provide a means to actuate the bypass that is less expensive tha
Hauser Raymond
Poplawski Herb M.
Schreier John D.
Hydro-Gear Limited Partnership
Kershteyn Igor
Look Edward K.
Neal, Gerber & Eisenberg
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