CA1335422C

CA1335422C - Hydrostatic transmission bypass valve assembly

Info

Publication number: CA1335422C
Application number: CA000616712A
Authority: CA
Inventors: Denis James Del Ponte; Paul Anthony Meyer
Original assignee: Deere and Co
Current assignee: Deere and Co
Priority date: 1988-07-29
Filing date: 1993-09-09
Publication date: 1995-05-02
Anticipated expiration: 2009-07-06
Also published as: CA1335421C; CA1334816C; CA1336105C; CA1334368C; CA1334369C

Abstract

A vehicle hydrostatic transmission has a bypass valve incorporated therein which may be selectively manually moved to an open position establishing a fluid path between a pressure delivery port of the motor of the transmission and a sump whereby the vehicle with which the transmission is associated may be moved when the engine is stopped. The bypass valve is in fluid communication with the output of an engine-driven charge pump and is responsive to pressure delivered by the charge pump so as to be automatically returned to a closed position preventing flow from the motor to the sump when the engine is operating.

Description

HYDRO8TATIC TP~t-~MT~ION BYPAææ VALVB A88~MRLY
Related Application This is a divisional of Canadian application Serial Number 604,977 filed on 06 July 1989.
Background of the Invention The present invention relates to a vehicle hydrostatic transmission and more particularly relates to a structure for establishing a condition in the transmission for permitting the vehicle to be moved when the engine is not running.
Vehicles, such as lawn and garden tractors, for example, are commonly equipped with a hydrostatic transmission for powering the drive wheels of the vehicle. Typically, these transmissions include an engine driven pump unit coupled for delivering fluid under pressure for driving a motor unit having a driven output shaft coupled to the tractor drive wheels. In order for these tractors to be moved manually or to be towed in the event that the engine becomes inoperable, it is known to provide a bypass valve which is spring biased to its normal operating position and requires a manually operated lever, or the like, to be manipulated to open the bypass valve to establish a path permitting fluid to flow freely between inlet and discharge ports of the motor so that the drive wheels may rotate freely without causing the motor to act as a pump and cause damage to the hydrostatic drive system.
One problem with the prior art bypass valve actuator arrangements is that an operator may restart the engine while the manual control remains in its operated position where it holds the bypass valve open. This can result in the fluid pressure generated by the engine driven pump causing damage to the hydrostatic drive system or can cause inadvertent vehicle motion.
Summary of the Invention According to the present invention there is provided an improved bypass valve arrangement.
A broad object of the invention is to provide a bypass valve, that once moved to its open position, will automatically return to its closed position upon restarting the vehicle engine.
A more specific object of the invention is to provide a bypass valve arranged so that once moved to its open position starting of the vehicle engine will result in fluid pressure supplied by an engine-driven charge pump acting on the bypass valve so as to automatically return the valve to its closed position.
These and other objects of the invention will become apparent from a reading of the appended description together with the appended drawing.
Brief Description of the Drawing The sole figure is a sectional view taken through a vehicle hydrostatic transmission and showing a bypass valve for preventing pressure build-up and thus for permitting free wheeling when it is desired to move the vehicle while its engine is not running.
Description of the Preferred Embodiment Referring now to the drawing, there is shown a hydrostatic transmission 64 including a pump mounted to an input shaft 502 for being driven by the latter, and a motor mounted on an output shaft 504 for driving the latter.
Coupled to the pump input shaft 502 is a charge pump 510 which supplies fluid to a passage 506 when the vehicle engine is in operation. The passage 506 is coupled to one side of an annular groove 520 that permits fluid to flow about a bearing 516 supporting the motor output shaft 504. A bypass valve assembly 74 includes a valve bore 518 having an inner end intersecting the annular groove 520 and defining a shoulder 522 spaced and facing outwardly from the shaft 504. A passage 530 is connected for receiving pressure fluid delivered by the motor when the latter is driven by way of the connection of the output shaft 504 with the vehicle wheels when the vehicle is being moved with the engine not running. A bypass passage 532 spaced axially along the bore 518 in a direction outwardly from the passage 530 connects the valve bore 518 with a cavity 524 surrounding the pump and motor units. A valve spool 528 is axially shiftably received in the valve bore 518 and includes an outer end defined by an operating stem 526 that is reciprocably received in a throughbore provided in a cap 527 threaded into the outer end of the bore 518. The spool 528 further includes an inner end defined by an inner land 521 and an outer land 523 spaced axially outwardly from the land 521.
When the valve spool 528 is in an outward closed position, as shown, the inner land 521 is positioned in blocking relationship to the passage 530 and is spaced axially outwardly from the shoulder 522. At this time, the outer end of the outer land 523 will be seated against the inner end of the cap 527. When the valve spool 528 is shifted inwardly to its open position, the inner land 521 will be seated against the shoulder 522 and the passage 530 will be unblocked so that it is in fluid communication with the cavity 524 via an annular groove 534, defined between the lands 521 and 523, and the bypass passage 532.
Thus, it will be appreciated that anytime the vehicle engine is operating, the charge pump 510 will be pumping fluid and the pressure of this fluid will act on the inner end of the spool 528 so as to shift the latter to and/or hold the latter in its closed position so as to prevent working fluid from being bypassed into the cavity 524. However, with the engine stopped, the charge pump 510 will no longer be supplying pressure and if it is then desired to push or pull the tractor as may be necessary for service, this may be done without encountering back pressure that would be developed in the transmission due to the driving of the motor by merely pushing in on the stem 526 of the valve spool so as to place the valve spool in its open position.

Claims

1. In a hydrostatic transmission for use in a tractor and including a housing forming a sump and having a pump located therein and coupled to an input shaft adapted for connection to an engine of the tractor, a motor located in the housing and coupled for imparting rotation to an output shaft and coupled for receiving operating fluid from the pump, and a charge pump coupled for being driven by said input shaft and being coupled for delivering working fluid to the pump, and a bypass valve located in the housing and being fluidly coupled to said motor for bypassing fluid flow generated by the motor when the latter is being driven through said output shaft as when the tractor is being moved with the engine stopped, the improvement comprising:
said bypass valve including shiftable valve means having a surface connected so as to be in communication with fluid delivered by said charge pump and being biased in a first direction by the pressure of such fluid to a normal position wherein it blocks a fluid path extending between an output of said motor and the sump; said shiftable valve means being manually shiftable in a second direction opposite to said first direction to a bypass position wherein it opens a fluid path extending between the output of the motor and the sump, whereby an operator may place said valve means of the bypass valve in its bypass position so as to permit the tractor to be pushed or towed when the engine is stopped, the valve means thereafter automatically assuming its normal position once the engine is started and the charge pump is driven to deliver fluid to said surface of said valve means.

2. The hydrostatic transmission defined in claim 1 wherein said valve means forms a portion of a valve spool that is axially shiftably mounted in a valve bore and said surface is defined by one end of the valve spool.

3. The hydrostatic transmission defined by claim 1 wherein said valve means includes a first land of a valve spool and is located in a closed position blocking a fluid passage that carries fluid delivered by the motor when the latter is driven by rotation of the output shaft caused by the tractor being pushed or towed with the engine stopped; said first land being shiftable to an open position establishing fluid communication between said fluid passage and a bypass passage communicating with said sump to thereby permit fluid delivered by said motor to said passage to pass into said sump via said bypass passage.