Planetary gear transmission systems or components – Differential planetary gearing – Bevel gear differential
Reexamination Certificate
2000-09-08
2001-07-17
Wright, Dirk (Department: 3681)
Planetary gear transmission systems or components
Differential planetary gearing
Bevel gear differential
Reexamination Certificate
active
06261201
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to transaxles including a hydrostatic transmission (“HST”) commonly used with riding lawn mowers and similar small tractors. Such tractors generally use an engine having a vertical output shaft which is connected to the transaxle via a conventional belt and pulley system. A standard HST for such a transaxle includes a hydraulic pump, which is driven by the engine output shaft, 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 of the pump pistons. The oil pressure created by this axial motion is channelled via porting to the hydraulic motor, where it is received by the motor pistons, and the axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft which drives the vehicle axles through differential gearing.
Among the advantages of transaxles with hydrostatic transmissions are the reduction of the number of parts and in the size of the unit, and, in some instances, the elimination of mechanical gears. As is known in the art, the use of a transaxle having a hydrostatic transmission enables the manufacturer to include all necessary elements in one unit, whereby the transaxle is easily incorporated into the tractor design, as it requires only the addition of a belt to connect it to the motor and a control lever for changing speed and direction. While the basic principles of transaxles with an HST are well known in the prior art, there are several disadvantages of present transaxles with HST designs. These disadvantages, and the present invention's means for overcoming them, are set forth herein.
A major problem with some prior transaxle designs is that the transmission is too large and too expensive to be used with the smaller tractors where it would be most effective. An attempt to solve this problem is shown in Okada, U.S. Pat. Nos. 4,914,907 and 4,932,209. The Okada '209 patent discloses a first mechanical deceleration means, namely the gear on the motor shaft and countershaft within the axle housing, and a second mechanical deceleration means in the differential. The gearing in the deceleration means eventually transmits power to the differential gears, which are then used to drive the output axle. However, these mechanical deceleration units add unnecessary weight and expense to the unit. An object of the present invention is to provide an transaxle design which does not require such additional mechanical deceleration means.
Another variation on the standard transaxle with HST design is shown in Thoma, U.S. Pat. No. 4,979,583. This patent teaches the segregation of the hydraulic units from the remaining portions of the transaxle through the use of separate segregated cavities to house each. In addition, the pump and motor in the Thoma design are mounted back-to-back, so that the input and output shafts have the same orientation. Thus additional gear units are required to re-orient the rotation of the output shaft so that it is parallel to the ultimate drive axle. Further gears then drive a differential which rotates the drive axle. This additional gearing adds weight to the unit and expense to the manufacturing process.
Thus, the Okada and Thoma designs present problems from the standpoint of manufacturing a small, economical transaxle including an HST which is easily adaptable to different size tractors or axle configuration. Okada requires multiple gearing and Thoma requires a housing having segregated cavities. The present invention is designed to overcome these and other problems in the prior art by providing a compact, economical transaxle with HST which substantially reduces the number of moving parts previously required.
SUMMARY OF THE INVENTION
The present invention, sometimes referred to generally as a “transaxle,” includes a split-axle housing which encases an HST. The HST includes a pump and a motor whose orientation to one another may be varied according to the space requirements dictated by the size and configuration of the vehicle. This transaxle also includes a novel hydraulic reduction means, an improved differential, a longer lasting, more effective means of preventing oil leakage from the axle shafts in the housing, a center section supporting the output drive shaft, an improved means for hydraulically bypassing the HST and a unique check valve arrangement. Each of the specific novel improvements are combined to provide a transaxle which is compact, reliable and economical to manufacture. These and other objects and improvements of this invention will be set forth in more detail herein.
One object of this invention is to provide an improved transaxle wherein the center section of the HST, on which the pump and motor are mounted, also serves as the bearing support of the output drive shaft. In the prior art, for example, Okada U.S. Pat. No. 4,932,209, one end of the gear drive arrangement is supported in the center section, but the other end is supported by the upper and lower axle housing casings.
The advantage of the present invention's arrangement is that it eliminates the need for an additional bearing support, thus reducing the costs and assembly time required. It also eliminates the tolerance concerns for aligning the bearing supports for the output drive shaft.
A further object of this invention is to provide a transaxle that may use multiple mechanical reduction units, but requires only a single such unit because a portion of the overall reduction is provided hydrostatically. The prior art generally requires dual or multiple mechanical reduction units in conjunction with the hydraulic unit. For example, as set forth above, U.S. Pat. No. 4,932,209 requires the use of two separate mechanical reduction units, including a separate counter-shaft between the hydraulic motor and the differential used to drive the output axle.
The present invention makes this same reduction through the hydraulics itself by the use of a motor which is larger in displacement than the pump. This eliminates the need for any secondary mechanical reduction units, thereby reducing sources of possible mechanical failure. The single reduction arrangement reduces the number of necessary components and the size of the transmission, and it eliminates the need for an additional support shaft or jack shafts, thus resulting in a smaller, simpler and less expensive transaxle. In a heavy duty application, the prior art often used two sets of mechanical reduction units to handle the necessary reduction. In such instances, the present invention's hydraulic reduction can eliminate the need for such multiple reduction units or could be used in conjunction with secondary units only.
A further object of this invention is to restrict the oil from having to extend to the outer axle support bearings, as is common in prior art models. The gearing and the hydrostatic transmission element of this invention are enclosed in a single chamber formed by an upper casing and a lower casing. The axle shafts extend through this chamber and are supported by separate bearing surfaces outside of the chamber.
In most of the prior art, the entire axle casing is filled with oil out to the outer axle bearings to provide lubrication to these bearings, in addition to the hydrostatic function of the oil in the pump and motor. However, after the outer axle bearings wear through use, the eccentricity or “play” in the shaft may distort the oil seal at said outer bearings, allowing the leakage of oil out of the main chamber. Maintenance of a leak-free joint is critical to the function and appearance of such a transaxle with HST unit. The entire internal hydraulic parts of an HST should be covered with oil, as an insufficient amount of oil in the main transmission cavity will cause foaming of the oil, damaging the hydraulic structures. Excessive oil leakage is a serious problem as it will hamper the ability of the HST to operate and cause damage to the internal workings of the HST. Oil
Hauser Ray M.
Johnson Alan W.
Hyatt John E.
Hydro-Gear Limited Partnership
Jarosik Gary R.
McDonough Thomas C.
Wright Dirk
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