Hydrostatic transmission

An improved compact design for a hydrostatic transmission having a hydraulic pump and hydraulic motor mounted on a center section in a housing, wherein the pump and motor are mounted at generally right angles to one another, and the longitudinal axis of the input shaft is located between a first and second parallel planes located at respective ends of the motor shaft and the longitudinal axis of the motor shaft is located between a third and fourth parallel planes located at respective ends of the pump shaft.

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 leakage also presents an aesthetic problem for manufacturers of transaxles, as customers are usually quite disturbed by the presence of oil leaks and the accompanying oil stains. Thus, the reduction or elimination of oil leakage is critical for the continued success of transaxle sales.

In the prior art, maintenance of such a leak-free joint at the outer bearings requires the use of extra bolts and sealant, which add additional weight and cost to the unit. An additional problem with prior art designs is that such wear in the outer axle bearings can also cause contamination of the oil due to the presence of “shavings” and other detritus from the worn bearings.

Although such construction could be used with the other novel elements of the present invention, to solve these problems of leakage and potential oil contamination at minimum cost, the present invention also presents a unique means of restricting the oil to those portions of the transaxle where it is needed to lubricate the differential and to work the pump and motor of the HST. Thus, chambers separate from the main chamber enclosing the HST and differential surround the majority of each axle shaft. Therefore, the oil does not extend throughout the entire casing or to the outer axle bearings, removing the potential problem of oil leaking from the casing. Separate grease pockets are used to lubricate these outer axle bearings, resulting in a much more durable seal and allowing for the use of a higher viscosity grease lubricate these outer axle bearings.

This improvement also allows for a reduction in the amount of oil needed to fill the transmission case, and, due to the reduced sealant area at the outer axle bearings, a reduction in the amount of sealant required. Due to the fact that the maintenance of a leak-free joint at the outer axle bearings is not required, this invention also allows for reduced manufacturing tolerances, which reduces the manufacturing costs of the unit.

A further improvement is in the method used to place the transaxle into neutral gear to enable movement of the tractor without the motor running. A problem with the typical HST arrangement is that “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 rolled freely.

The prior art generally solves this 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 value through which the oil is diverted. Furthermore, machining such a valve requires precise tolerances, thus increasing the manufacturing costs of the unit.

In the present invention, this problem is solved by providing a mechanism whereby the motor block is mechanically lifted from its running surface, thereby allowing the oil to bypass the vacuum-pressure circuit and to exit the case completely. This operates to enable the vehicle to freewheel more easily than is possible with the prior art hydraulic valve method.

Another object of the present invention is to provide an improved design of the motor and motor thrust bearing in a hydrostatic transmission, whereby the motor shaft does not extend through the motor thrust bearing, and thus the bearing is fully supported and does not require an intermediate support plate, as is used on prior art models.

For example, U.S. Pat. No. 4,953,426 to Johnson teaches a thrust bearing having a motor shaft extending through its center section. As in the present invention, he thrust bearing in Patent '426 is supported by one section of the housing. However, because the '426 thrust bearing has the motor shaft extending through its center, it is not solely supported by the housing, but rather is supported by two “fingers” on either side of the thrust bearing. To support the thrust bearing against the hydraulic forces applied by the motor pistons, an additional structurally significant piece is required to support between these fingers.

In the present invention, the thrust bearing is fully supported by the housing part into which it is inserted, thus eliminating the need for an additional structural member. This results in an assembly that is simpler and less expensive to manufacture.

A further object of this invention is an improved differential gear assembly. In the prior art, differential assemblies generally require a cross-shaft to support the planet bevel gears. The arrangement of the present invention eliminates the need to use such a cross-shaft by providing a simple end cap axle support and bevel and planet gear entrapment.

A further novel feature of this invention is in the placement of the brake portion in the housing. Disk brakes are known in the art, and generally consist of a series of disks or plates, mounted on or about a rotating shaft, with at least some of the disks or plates rotating with the shaft. Such brakes generally have a brake arm or level which is moved to activate the braking feature by a means for transmitting the movement of the brake arm to the series of disks, causing the stationary disks to be pressed against the rotating disks, thus braking this rotating shaft through friction. This means for transmitting the movement of the brake arm to the disks generally consists of rods or shafts, and, in the prior art, these rods or shafts were mounted in a housing which is separate from the housing containing the HST. In the present invention, the brake rods are mounted directly into the HST housing through half-round sections formed into each of the mating housing sections, thus eliminating the need for this separate housing and reducing the manufacturing costs of the products.

An additional novel feature of this invention is the design of the check valve for the center section. Prior art check valve designs generally use hardened steel balls working against a steel or cast iron seat. To minimize the overall weight of the transaxle unit, however, the center section of the present invention is preferably made of cast aluminum, which is not strong enough to function as such a valve seat and to withstand the wear from such a check valve operation. This problem is solved by the use of a steel insert in the center section to support the steel balls.

To create a seal at such a location, it is known to use a machined surface on both the seat and the insert, so that a standard O-ring seal could be used. However, use of such a sealing means would require additional machining steps on the seat and insert, adding to the overall manufacturing costs of the unit.

To overcome these problems in the prior art, the present invention calls for the use of a powdered metal plate which acts as both the check valve seat and as the seal. The sealing functions of the plate are created through the use of a raised surface on the plate, which is pressed into the lower strength aluminum to form a seal. This design has the advantage of being simple and inexpensive to manufacture, while maintaining the advantage of a light overall weight.

It is a further object of this invention to provide an improved hydrostatic transmission wherein the pump and the motor of the HST need not be orientated at a 90-degree angle to one another as required by the prior art. In the present invention, the 90-degree orientation is the preferred embodiment. However, an orientation other than 90-degrees can be achieved by use of a helical gear between the output drive shaft and the differential.

Further explanation and details of the above objects of this invention, as well as other benefits and advantages of this invention, will be set forth in the following sections.

DETAILED DESCRIPTION OF THE DRAWINGS

All hydrostatic transmissions operate on the principle of an input shaft driving a pump, which, through the action of its pistons, pushes oil to a motor, which rotates a motor shaft. This rotation is eventually transferred through a differential gearing system to drive an axle shaft. With these general principles in mind, we turn to the drawings of the present invention showing the various improvements made by this invention on the prior art.

FIG. 1shows an overview of the entire transaxle of the present invention including an HST system. Referring also toFIGS. 2 and 3, the transaxle is encased in an upper housing1and a lower housing2which are secured by a plurality of bolts145and a liquid gasket seal82at the joining surface of housings1and2. Input shaft75, which has a longitudinal axis75A, extends through shaft opening116and is supported by bearing7and ring5, which are retained by seal4. Input shaft75is driven by a belt (not shown) which is powered by a vertical shaft engine (not shown). As shown most clearly in FIG.3, the rotation of input shaft75rotates the cylinder block14aof pump14at the speed of input shaft75. Pump14is of conventional construction, containing a series of piston receiving chambers146, each of which movably mounts a pump piston13and piston spring12in a direction axial to cylinder block14aof pump14.

Pump pistons13are powered by piston springs12against thrust bearing9, which, as is well known in the art, is rotatably supported in swashplate10by a standard bearing and bearing guide structure, including bearing112. Swashplate10is itself supported in upper housing1by bearing cradle8, as shown in FIG.4.

Thrust bearing9acts as a ramp against which pump pistons13are pressed. The rotation of pump14causes pump pistons13to travel up or down this ramp, thus creating an axial motion for pump pistons13. Swashplate10may be moved to a variety of positions on bearing cradle8to vary volume of oil pumped, which ultimately varies the speed of motor27, as described herein.

Movement of swashplate10is accomplished by the user's manipulation of trunnion shaft15, which in turn moves bearing guide18. As is known in the art, trunnion shaft15is supported by journal bearing17, which is retained by seal16. For example, if thrust bearing9is perpendicular to input shaft75and thus perpendicular to the axial plane of pump pistons13, there will be no point along thrust bearing9where pump pistons13are forced axially, thus resulting in no axial motion for pump pistons13and no oil flow between pump14and motor27. This position is effectively a “neutral” position for the HST, in that rotation of input shaft75will not ultimately result in movement of the vehicle.

The operator may move swashplate10by adjusting trunnion shaft15, which varies bearing guide18, in one direction to create a “forward” ramp at thrust bearing9, so that axial motion of pump pistons13forces the oil flow in one direction. The operator may also reverse the flow by moving thrust bearing9to the opposite, or reverse, position. The details, of the resulting oil flow through the porting system of the HST are set forth herein.

FIGS. 5 and 6show center section74of the HST, which is securely mounted to upper housing1through bolt openings103. Pump14is rotatably mounted on pump running surface130with center opening138corresponding to shaft opening116to receive input shaft75.

Motor27is rotatably mounted on motor running surface61by conventional means and is supported by motor shaft22. When the HST is not in operation, motor27is sealed to motor running surface61through the force of motor piston springs25against motor pistons26, which press against thrust bearing23to create this seal. When the HST is in operation, there is an additional force resulting from the oil pressure. Specifically, the interior of motor piston chamber147is sufficiently large enough that the flow of oil through passage102creates a resultant net balance of oil pressure in cylinder block27ain the direction towards motor running surface61, creating a seal at this point. Pump14is retained on pump running surface130in a similar manner.

Center section74includes bearing structures74A and74B, which are integrally formed therewith and include bearing openings88and89. Motor shaft22, which has a longitudinal axis22A, is installed through and fully supported by openings88and89and running surface140. The means of supporting motor shaft22is a significant improvement over the prior art, which discloses the motor shaft supported at one end in the center section, and at the other end on some other external bearing housing. The present invention eliminates the need for such an additional bearing housing for motor shaft22, reducing manufacturing expense and weight, as well as reducing the overall size of the unit.

Proper alignment of motor shaft22is critical to the performance of the HST. The design of the present invention eliminates the necessity of aligning such an additional bearing support with the support on center section74, resulting in an overall savings in weight and expense, as well as increasing the ease of manufacture of the transaxle.

As shown inFIGS. 2 and 3, the longitudinal axis22A of motor shaft22is located between a first plane P1formed at one end of input shaft75and a second plane P2formed at the opposite end of input shaft75, where planes P1and P2are generally perpendicular to the longitudinal axis75A of input shaft75. Similarly, the longitudinal axis75A of input shaft75is located between a third plane P3and a fourth plane P4, wherein planes P3and P4are generally perpendicular to the longitudinal axis22A of motor shaft22, and plane P3is formed at one end of motor shaft22and plane P4is formed parallel to plane P3and at the opposite end of motor shaft22. It can also be seen inFIG. 2that the two ends of motor shaft22are on opposite sides of a plane formed by the longitudinal axis75A of input shaft75and perpendicular to longitudinal axis22A. Similarly, as shown inFIG. 3, the two ends of input shaft75are on opposite sides of a plane formed by longitudinal axis22A of motor shaft22and perpendicular to longitudinal axis75A.

As most clearly shown inFIGS. 2 and 9, motor27also contains a plurality of piston chambers147, each of which contains a motor piston26and piston springs25. Each motor piston chamber147has a passage102to receive oil flow from arcuate ports106and107on motor running surface61of center section74.

Each motor piston26is driven by the oil flow received through arcuate ports106or107in a direction axial to motor27and against the generally circular motor thrust bearing23. As shown inFIGS. 3 and 9, motor thrust bearing23is fixed in its position relative to motor pistons26at an angle such that the action of motor pistons26against thrust bearing23creates a rotational movement of cylinder block27aof motor27. Motor thrust bearing23is of standard construction and is composed of bearing plates23aand23band bearing race23c. Motor27is supported on and drives motor shaft22. Cylinder block27aof motor27has internal gear teeth (not shown) which mesh with gear teeth45on motor shaft22to rotate motor shaft22at a speed equal to the rotation of cylinder block27aof motor27.

A major improvement that this invention presents over the prior art is the elimination of the need for an intermediate support for motor thrust bearing23. As shown inFIGS. 1,2and9, motor shaft22does not extend through the center of thrust bearing23. Therefore, thrust bearing23is fully supported at its proper angle by upper housing1without the need for an additional structural member such as is used for pump thrust bearing9, which must be supported by swashplate10. This results in a less expensive and simpler unit to manufacture, and the absence of the additional member reduces the overall size and weight of the transaxle unit.

As described below, oil flow from pump14to motor27is the means by which rotational power is transmitted by the HST. Arcuate ports136and137on pump running surface130provide the means for transferring oil from passage101of pump piston chamber146through oil passages104or105and to motor27. Arcuate ports106and107, which are located on motor running surface61and which coact with passages102of motor piston chamber147, act to receive the oil from oil passages104or105and return it to pump14.

It is to be understood that there are a plurality of pump pistons13and motor pistons26and their related parts and chambers, and, therefore, the discussion herein of these parts in a singular sense is for convenience only, and should not be read to limit the invention in any way. In the preferred embodiment, there are five (5) pump pistons and seven (7) motor pistons.

As shown inFIG. 3, each pump piston chamber146has a passage101opening for coaction with arcuate ports136and137on pump running surface130of center section74. In the “forward” oil flow direction described above, the oil flow created by the movement of pump pistons13moves through passage101to arcuate port137, and then through oil passage105to arcuate port106on motor running surface61, and finally to passage102of motor piston chamber147. The oil then returns to pump piston chambers146through passage102, arcuate port107, oil passage104, arcuate port136and passage101.

In the “reverse” oil flow direction described above, the oil essentially travels in a reverse direction, being forced by pump piston13through passage101and arcuate port136to oil passage104and arcuate port107and passage102, and finally to motor piston chamber147. The oil is then returned to pump14through arcuate port106, oil passage105and arcuate port137. The rotational direction of motor27depends upon whether this oil flow is in the “forward” or “reverse” direction, as this rotation, and ultimately the movement of the vehicle, will also be “forward” or “reverse.”

As can be seen inFIG. 2, the transaxle design includes expansion chamber121formed by external wall3and internal wall124. Such expansion chambers are well-known in the prior art and are used to provide a space for the oil to expand into during operation of the transaxle. Expansion chamber121may be located at different areas along the upper and lower housings1and2, and, in the preferred embodiment, expansion chamber121is located along upper housing1or lower housing2outside differential gear63.

As shown inFIGS. 1 and 2, braking for the transaxle is accomplished through a braking mechanism109located on, and supported by motor shaft22and comprising brake stator57and brake rotor58, triggered by brake arm53and brake actuator55. Braking mechanism109is located within a cavity110which is separated from transmission cavity48by a standard seal31.

The novel brake feature of this HST is clearly shown in FIG.19. Specifically,FIG. 19is a cutaway portion of the top view of a portion of the transaxle generally shown inFIG. 1, but including the novel brake feature. The remaining elements of the transaxle shown inFIG. 19can be the same as those shown in FIG.1.

Motor shaft222, which can be identical to motor shaft22previously described, has, at one end, gear teeth223integrally formed thereon. Brake mechanism250includes brake rotors258, which are rotatably mounted on gear teeth223of motor shaft222such that brake rotors258rotate with motor shaft222, and brake stators257, which do not rotate.FIG. 19shows lower housing202of the transaxle, which can otherwise be identical to lower housing2previously described. Brake arm253is connected to lower housing202through bolt254, washer255and nut256. When the brake mechanism250is to be activated, the user moves brake arm253, which causes pins259to move in a lateral direction towards brake stator257. This movement of pins259moves stators257into contact with rotors258, causing contact and friction between stators257and rotors258and thus effectuating braking. Pins259are not mounted in a separate housing but are instead contained and held in mating half-round sections formed into both lower housing202and the upper housing (not shown) of transaxle. The advantage this design presents is the elimination of separate housing elements for the pins, reducing the weight and cost of the unit.

As is known in the prior art, the present invention uses a differential to transfer power from motor shaft22to the pair of oppositely-extending axle shafts62and62′ which are used to drive the vehicle. As shown inFIGS. 1-3, motor shaft22contains a center portion46which contains gear teeth126which mesh with teeth63bon differential gear63. Differential gear assemblies known in the art generally include an internal cross-shaft that serves as the actual driving mechanism for the output axles. A key improvement in this invention is the use of a novel structure which eliminates the need for such an internal cross shaft on differential gear63.

As shown inFIG. 1, the various differential gears are contained in differential housing64, which includes two identical opposing endcaps108and108′. Endcaps108and108′ are shown in detail inFIGS. 10,11and12. Axle shaft opening152is integrally formed therein to receive axle shaft62or62′. Bolt openings154and154′ are also formed therein to receive and secure bolts68and68′.

As is shown most clearly inFIG. 3, planet gear66is mounted onto the inside of differential gear63through opening63cby means of a key or raised portion66awhich fits into keyway63aformed in differential gear63. Planet gear66′ is similarly located. Planet gears66and66′ are thus held in place by keyways63aand63a′ and endcaps108and108′. This arrangement replaces the cross-shaft of prior art designs, where the cross-shaft was used to support the planet gears.

Planet gears66and66′ include a plurality of teeth66band66b′, which are meshed with and cause the rotation of bevel gears65and65′. Bevel gears65and65′ are meshed with respective axle shaft gears47and47′ to cause rotation of axle shafts62and62′.

Thus, each bevel gear65and65′ is located and held in place by planet gears66and66′ on one side and by endcap108or108′ on the other side. Endcaps108and108′ function to center and hold bevel gears65and65′ and to allow the entire differential assembly to be held together by two bolts and nut assemblies68and68′. This is a much more compact and less complicated design than has been used in the prior art. In addition, the elimination of a cross shaft removes the need for a hollow center section, thereby making the differential design of the present invention stronger than prior art models.

Another embodiment of this differential is shown inFIGS. 13-18, wherein planet gears266are secured by and mounted on end caps208. Specifically, as shown inFIG. 17, each planet gear266has a tab267which may be integrally formed therewith, and tab267is mounted for rotation on curved mounting surface215on end cap208. When two end caps208and208′ are mounted together as shown inFIG. 14, their respective mounting surfaces215combine to secure planet gears266in place.

Each end cap208has a notch220which may be integrally formed therein and which fits into keyway268formed into ring gear263. As ring gear263rotates, force is transmitted from the sides of keyway268to notch220of end cap208, causing the entire differential unit200to rotate. Thus each end cap208receives the rotational force of ring gear263through notch220and transmits that force to planet gear266, causing planet gears266to move with the rotation of ring gear263.

As shown inFIG. 14, the differential unit200is secured together through the use of a pair of bolts275and275′ mounted through and securing end caps208and208′. Each planet gear266engages and drives bevel gears265to cause the rotation of bevel gears265about the same axis of rotation as ring gear263. At the same time, each bevel gear265engages and drives a rotatable output shaft262to power the vehicle in which the differential is used. Each bevel gear265has an opening (not shown) which corresponds to opening271on end cap208, and which has gear teeth (not shown) to engage and drive an output shaft262, which has gear teeth280formed thereon. Bevel gears265are engaged on the inside of differential unit200by planet gears266, and are engaged at their outside surface269by mounting surface270on end cap208. Each end cap200has a shaft opening271which corresponds to bevel gear opening302to receive output shaft302.

As discussed above, end caps208and208′ may be bolted to one another using bolts275and275′ through bolt holes301to form a single differential unit. It is also possible to use one larger end cap in place of the two separate end caps. In such embodiment the one large cap unit is bolted to an outside face of ring gear263and holds and rotatably mounts both planet gears266.

The embodiment shown inFIGS. 13-18shows the differential unit being mounted within the center, i.e., between the outside faces of ring gear263. However, it is also possible for the planet gears266, bevel gears265, and end caps208to be mounted off-center, such as on the outside face of ring gear263, with rotational force still being transferred from ring gear263to planet gears266through the single end cap unit secured to ring gear263or through a set of end caps similar to those described above.

As shown inFIGS. 3,7and8, center section74contains a check valve mechanism including check valve plate41, ball39and spring40. Plate41is formed of powdered metal which is significantly harder than the cast aluminum used to form center section74. Bottom face79of center section74is shown in FIG.6. Plate41is mounted on bottom face79by three bolts42through bolt openings127and received by openings128on bottom face79of center section74.

Plate41has top surface148, which is flush with bottom face79of center section74when mounted, and bottom surface149. As shown inFIG. 8, bottom plate surface149has generally circular opening133formed therein, while top plate surface148has a slightly larger opening131formed therein. Openings133and131coact with each other and with valve openings156on bottom face79to form check valve139. Check valve139includes ball support surface135to support ball39when check valve139is in the closed position, as shown in FIG.3. When the check valve139is opened, ball39lifts off of ball support surface135to allow oil from sump155to flow through check valve139. Oil filters43are used to prevent contaminants from entering sump155from transmission cavity48.

A raised annular surface or ring129is formed around opening133on top surface148of plate41, and is pressed into the lower strength bottom face79of center section74to form a seal between plate41and center section74. The minimal leakage which may occur due to deflection in the metal does not affect operation of the transaxle because center section74is within main transmission cavity48, which is filled with oil. Thus, the present invention provides a simple, low cost sealing mechanism which allows for the use of a lighter cast aluminum center section without the need for the use of additional machining to use an O-ring, as is done in the prior art.

Prior art HST designs have the pump and motor mounted either at a 90-degree angle or in a parallel arrangement, whereby the pump and motor are set “back-to-back.” In the present invention, the preferred embodiment calls for these elements to be positioned on center section74at the standard 90-degree angle to one another, as shown in the drawings. However, if necessary, center section74could provide for motor running surface61to be inclined upwardly or downwardly in the vertical plane of FIG.2. Such an orientation, which may be required by the configuration of the vehicle, would also require motor shaft22to remain parallel to motor27. In this position, motor shaft22is no longer perpendicular to axle shafts62and62′ and differential gear63, as is required to have gear teeth63band gear teeth126of motor shaft22to mesh using standard gearing.

To allow such an arrangement, the present invention would require the use of a helical gear at motor shaft center portion46or on differential gear63to allow these gears to properly mesh. Such helical gears are well-known in the art, but have not previously been used in HST designs to allow the pump and motor to be oriented at angles other than the standard 90-degrees. The angle of the helix on such a gear is determined by the angle between the motor shaft22and the axle shafts62and62′.

With a transaxle, it is necessary to reduce the rotational speed of the input shaft as it is transmitted to the final drive axles. One of the disadvantages of prior art transaxle designs is the need to provide a reduction of angular shaft speed through mechanical gearing. Such mechanical reduction requires the use of extra gears, shafts, supports and various other related parts, as shown in prior art patents. This results in additional expense in manufacturing as well as additional weight in the transaxle. Furthermore, mechanical gears are subject to failure if stressed sufficiently or repeatedly.

In the present invention, at least a portion of this shaft speed reduction is accomplished through the hydraulics. In a preferred embodiment, this is accomplished by internally sizing motor27at a larger capacity than pump14. As an example of the preferred embodiment, it has been discovered that if the capacity of motor27is 21 cubic centimeters (cc), while the capacity of pump14is 10 cc, a significant reduction in the speed of motor shaft22is achieved. With such sizing it has been found that the angular speed of motor shaft22is generally reduced to about one-half of the angular speed of input shaft75.

In light duty applications where the prior art would require a double mechanical reduction, the present invention can eliminate this secondary mechanical reduction altogether. In heavy-duty applications which would require two or three mechanical reduction units, the present invention may only require a single secondary mechanical reduction unit. In either event, the present invention results in a significant savings in size, weight and expense over prior art designs. This also results in an improvement in reliability, as a hydraulic reduction is less susceptible to breakdown due to the fewer number of moving parts required. Furthermore, a hydraulic reduction is less likely to break from being overstressed than is a mechanical gear reduction.

As seen inFIG. 1, axle shafts62and62′ extend from differential housing64through transmission cavity48and axle cavity49to outer axle bearings72. The wheels (not shown) of the vehicle are then attached at axle ends150and150′. In prior art models, oil extends throughout axle cavity49along the length of the axle shafts to lubricate outer axle bearings72and is sealed in cavity49at seal120.

However, inherent in the manufacture of any such axle shaft is a slight deviation from the main axis at either end150or150′ of axle shafts62and62′. Such minor deviations occur through imperfections in the manufacturing process and do not affect performance of the axle shaft or the transaxle. Further deflection occurs due to axle loading at ends150and150′. The sum of these deflections together with any wear at the outer axle bearings72can create minor gaps at seal120, which can cause leakage of oil from axle cavity49. Such a gap at seal120, and subsequent oil leakage, can also occur through normal wear and tear. Wear of seal120and outer axle bearings72can cause detritus from the seal, bearing and surrounding structures to contaminate the oil.

In the prior art, oil leakage has been dealt with through the use of extra bolts and sealant at the location of seal120as well as at additional locations along sealing surface125. This results in additional parts, expense and weight for the unit.

Since the present invention does not fill axle cavity49with oil, this problem is eliminated without the need for such extra bolts or sealant. As shown inFIG. 1, seals71are used to prevent oil from flowing from transmission cavity48to axle cavity49. Seal71thus operates as the primary oil seal for transmission cavity48and, in a preferred embodiment, seal71is a seal made of nitryl.

In the present invention, a conventional higher viscosity grease within axle cavity49provides the necessary lubrication to outer axle bearings72. Use of this higher viscosity grease provides better lubrication to the outer axle bearings72than is available through the use of oil. Seals120serve to maintain this higher viscosity grease within axle cavity49and thus do not serve as the primary oil seal. Moving the primary oil seal from outer axle bearing72to seal71eliminates or minimizes oil leaks, extends the life of the product and reduces the quantity of oil needed in the casing. Seals120further act to minimize the amount of outside contaminants which reach outer axle bearings72.

Another important and novel feature of this invention is the hydraulic bypass shown inFIGS. 2 and 9. The effect of this bypass system is to enable the vehicle user to roll or “freewheel” the vehicle without resistance from the oil in the HST. When an HST does not have any power being applied to it through the tractor motor, pump14and motor27are not being rotated. Therefore, any attempt to roll the vehicle would transmit the rotational energy through axle shafts62and62′, and through differential gear63to motor shaft22. This in turn will rotate motor27, and the action of motor pistons26against motor thrust bearing23causes axial motion of motor pistons26, causing oil flow through the porting of center section74. However, with pump14at neutral there is no place for the oil to go, and high pressure results. This high pressure causes resistance to further motion of motor shaft22and axles62and62′ and prevents the user from pushing the tractor.

Prior art solutions to this problem generally involve placing a valve between arcuate ports106and107to allow the oil to flow between these two ports, i.e., between the pressure side and vacuum side of HST center section74. However, such a hydraulic valve allows only a limited amount of oil to pass between the ports due to inherent design limitations, such as the diameter of the hydraulic valve through which the oil is diverted. Such a valve also requires accurate machining to maintain minimum clearances to reduce leakage during normal operation of the unit.

The present invention solves this problem by use of a mechanism to lift motor27off of motor running surface61of center section74, thus breaking the seal at that point and allowing oil to flow out of arcuate oil port106and into transmission cavity48. Thus, the oil is not ported from the pressure side to the vacuum side, but rather bypasses this entire circuit within center section74.

To activate this feature, bypass arm50is manipulated by the user to rotate bypass actuator29. Seal28is used to retain oil within the main transmission cavity48at this point. Bypass actuator29includes rod115, which is shaped at its base so that rotation of rod115forces bypass plate30to press against the base of motor27, breaking its seal to motor running surface61. This allows the oil to flow from arcuate port106to transmission cavity48. The oil is then returned to motor27through arcuate port107. This design enables the vehicle to readily “free wheel” with less resistance from the oil.

Further manipulation of bypass arm50and rod115causes bypass plate30to withdraw off of motor27, allowing motor27to return to its normal position on motor running surface61, reestablishing the seal at that point. The design of the present invention could also be used in a different embodiment to lift pump14off of pump running surface130, as this would have the same effect.

An advantage of this design is that it is very simple and inexpensive to manufacture and install because it does not require precise tolerances. Prior art hydraulic bypasses using valves to move the oil between its porting sections require very precise machining of the valves to prevent unwanted leakage, and are therefore more expensive to manufacture. In addition, this mechanism dissipates the oil into the cavity rapidly to allow immediate movement of the vehicle.

The above descriptions are intended to illustrate the various features of this invention and are not intended to limit it in any way. Further advantages will be obvious to one of ordinary skill in the art. This invention should be read as limited only by the following claims.