Power plants – Pressure fluid source and motor – Condition responsive control of motive fluid flow
Reexamination Certificate
2001-02-01
2001-11-20
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Condition responsive control of motive fluid flow
C060S487000
Reexamination Certificate
active
06318080
ABSTRACT:
BACKGROUND
This invention relates to an improved design of a hydrostatic transmission (“HST”) and includes several novel features. Hydrostatic transmissions are well known in the art, and are more fully described in, e.g., U.S. Pat. No. 5,314,387, which is incorporated herein in its entirety. Many of the inventions described herein can also be adapted for use in an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles within a single housing.
In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked through a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself.
The pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The pump pistons engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Other HST designs may use a radial piston or ball piston pump and motor design, but the general operation is similar, and this invention is not limited to use with a specific design. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing rotation thereof. The motor pistons are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an IHT, or external thereto.
The pump/motor system is fully reversible in a standard HST. As the swash plate against which the pump pistons move is moved, the rotational direction of the motor can be changed. In addition, there is a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid.
The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. Such check valves may be located so that they directly contact the porting or they may be located separate from the porting and connected through additional bores to the closed circuit.
There is a need to have a means to open, or bypass, this closed circuit in certain circumstances. For example, when the vehicle is stopped, the oil in the closed circuit provides hydraulic braking, making it impossible to manually move the vehicle. Mechanical bypass designs are known in the art and are described in, for example, U.S. Pat. No. 5,010,733. Such designs generally achieve bypass by opening the closed hydraulic circuit to the sump by, e.g., opening check valves in the circuit, or by opening a shunt between the high pressure and low pressure sides of the circuit. Such designs are generally complicated and add significantly to the cost of the unit.
SUMMARY OF THE INVENTION
This invention addresses the shortcomings in prior HST bypass designs, as an improved mechanical bypass system for a hydrostatic transmission is disclosed herein. One particular improvement of this design is in the tolerances allowed, as this design reduces or eliminates many of the tolerance issues which arise from known bypass designs. This invention uses a filter housing secured to the bottom of the center section indirectly by the check plugs, and a filter secured to the filter housing. The bypass actuator is mounted inside the filter housing and is actuated by means of a bypass rod which can extend outside the housing of the hydrostatic transmission to be operated by the user. Rotation of the rod causes the actuator to engage the check balls to unseat them from the check plug and allow the unit to enter the bypass mode. Other embodiments include use with an HST where the hydraulic porting is integrally formed with the transmission housing and the filter housing and filter are thus secured directly to the transmission housing.
The housing design disclosed herein is a significant improvement over current transaxle designs. Using a traditional transaxle design, it is very difficult to achieve rear discharge, as the input shaft is near the vehicle centerline. Some designs have attempted to overcome this problem by mounting the transaxle on the same deck as the engine, and using connecting chains to another axle on which the tires are mounted. Such a design adds significantly to the overall cost of the unit.
One aspect of this invention is the use of a housing formed of two pieces, generally divided along a vertical axis with respect to the orientation of the output axles. One section of the housing or casing is much narrower than the other housing to maintain clearance between the body of the transmission and the vehicle frame on one side, in order to accommodate a rear discharge chute. Many of the HST elements internal to the housing are contained in the larger of the two casing portions. In addition, the external housing elements are designed to conform as closely as possible to the shape of the internal IHT elements, so as to minimize the amount of material needed and the overall size of the unit. In essence, this design allows the main housing component to be offset to one side of the vehicle, while still maintaining the input shaft at or near the vehicle center line. Thus, the discharge chute parallels the vehicle frame, rising up slightly to clear the axle horn.
A further object of the invention is to provide an HST having an improved swash plate mounted on at least one trunnion which is secured to the transmission casing, to offer lower control moments for the swash plate. This design offers improved control of the swash plate, which is particularly important for use of a foot control mechanism.
A further object of this invention is to provide an improved and novel design of a center section for an HST, whereby the output shaft of the hydrostatic motor is secured at least partially by the center section and is positioned so that the axis of the output shaft is located below the plane of the surface on which the hydrostatic pump is mounted on the center section. The benefits of this arrangement include, among other things, a reduced height of the pump, motor and center section, which can reduce the overall height of the unit and/or provide more flexibility for mounting other HST elements. The horizontal mounting of the center section also allows for the use of the vertical split line as disclosed herein and the unique arrangement of the HST elements within the housing units.
A further object of this invention is to provide an improved and novel expansion chamber that can be bolted or otherwise secured to the HST and which prevents leakage or spillage of the hydraulic fluid therefrom. In a preferred embodiment this chamber is external to the housing and includes an internal tube extending from the top of the tank to the bottom, although variations on this design will be obvious to one of skill in the art. The use of an external tank allows for use of a smaller transmission housing, and reduces the possibilities of leakage due to gear splash and oil movement at various operating angles. The internal tube provides siphoning action which allows for, among other things, greater flexibility in the location of the tank.
A fur
Hauser Raymond
Houser Michael
Ooink Juan Pedro Roberto A.
Schreier John
Altheimer & Gray
Hydro-Gear Limited Partnership
Lazo Thomas E.
Look Edward K.
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