Center section for hydraulic transmission

A two piece center section assembly for mounting inside the housing of a hydrostatic drive apparatus is disclosed. The assembly includes separate pump and motor support portions where the check valve is formed between the two pieces. Fasteners extend through the two pieces and into the housing for the drive apparatus to secure the assembly thereto without the need for additional fasteners. The need for additional sealing material between the pump and motor portions can also be reduced by the use of raised portions on the mating surfaces between these two portions.

BACKGROUND OF THE INVENTION

This invention relates to a center section for use in a hydrostatic transmission (“HST”) or an integrated hydrostatic transaxle (“IHT”).

The invention disclosed herein is an improved center section for use in an IHT or an HST, where the center section is of a two-piece design. Center sections containing hydraulic porting for connecting a rotating hydraulic pump and motor are known in the art and are disclosed in, e.g., commonly owned U.S. Pat. Nos. 5,314,387 and 6,122,996, the terms of which are incorporated herein by reference.

Two-piece center sections for use in an IHT are also known. For example, different two-piece center section designs are disclosed in the file history for U.S. Pat. No. 5,505,279 and other patents claiming priority from U.S. Pat. No. 4,903,545. These designs included two separate pieces, one for the motor running surface and one for the pump running surface, and the two pieces were bolted together at a right angle to one another for insertion into an integrated hydrostatic transaxle housing. Two of these known prior art designs are shown inFIGS. 12–15.

The first prior art design inFIGS. 12 and 13shows a two piece center section120having a pump portion130and a motor portion132bolted together by two bolts134. Separate openings180are shown for bolting center section120into an integrated hydrostatic transaxle such as that shown in U.S. Pat. No. 4,903,545. Pump kidneys143and motor kidneys142are hydraulically connected via porting160to connect the hydraulic pump cylinder block and motor cylinder block (not shown) of the integrated hydrostatic transaxle.

FIGS. 14 and 15show a similar but slightly different two piece center section220where pump portion230is bolted to motor portion232by bolts234. Again, hydrostatic porting260is formed therein to connect pump kidneys243and motor kidneys242. Separate fasteners (not shown) are needed to secure two piece center section230to the transaxle housing via openings280. It is understood that the pump portions and motor portions in both of these prior art designs can be reversed.

SUMMARY OF THE INVENTION

The subject invention is an improvement over these prior art two-piece center section designs. In particular the disclosed invention discloses an improved method for assembling and mounting a two-piece center section in a transmission housing. It also discloses an improved check valve design.

A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and is indicative of the various ways in which the principles of the invention may be employed.

DETAILED DESCRIPTION OF THE DRAWINGS

An exemplary vehicle10is shown inFIG. 1where hydrostatic transmission15comprises pump12and motor14mounted on center section20. Sump25is formed inside transmission15to hold the hydraulic fluid for the operation of pump12and motor14.

As shown, e.g., inFIG. 2, hydraulic pump12comprises a rotatable cylinder block16having a plurality of axially movable pistons17mounted therein. A spline engagement21is formed to drive rotatable cylinder block16through pump input shaft22. Similarly, motor14comprises a rotatable cylinder block18having a plurality of axially moving pistons19mounted therein. A spline portion23is engaged to drive motor output shaft24.

Prime mover13is also mounted on vehicle10and drives pump input shaft22through a pulley arrangement26. Motor output shaft24drives gear train28which is engaged to differential40which, in turn, drives output axles44and wheels47. The specific details of exemplary vehicle10and various details of the hydrostatic transmission15are not critical to this invention and will not be shown or discussed in detail.

Center section20is comprised of two portions, namely a pump portion30secured to motor portion32by means of fasteners34, which are depicted herein as bolts. Fasteners34extend through openings38in pump section30and openings39in motor section32. Fasteners34are also used to secure center section20to transmission housing27.

It will be understood that housing27depicted inFIG. 11is not a complete transmission housing; this is a vertical split design, where the housing pieces are joined at a plane perpendicular to the axis of the output axles (not shown), and another housing element (not shown) will be joined to housing27along the split line76. Other housing designs can be used in accordance with the spirit of this invention.

A benefit of this design is that center section20may be easily assembled into housing27. As shown inFIG. 11, motor14may be inserted into housing27, followed by motor portion32of center section20. Check balls48then may be inserted in check ball chamber56of motor portion32, then pump portion30of center section20is secured to motor portion32using fasteners34, which are also used to secure the entire assembly to housing27. This eliminates a major concern with two piece center sections, which required multiple fasteners to both secure the two pieces together and additional fasteners to secure the assembled center section to the housing.

Hydraulic porting is integrally formed in center section20to hydraulically connect pump cylinder block16to motor cylinder block18. For example, motor kidney ports42formed on motor running surface33are connected to ports36formed in motor section32. These are connected to ports45formed in pump section30, which in turn are connected to pump kidney ports43which are in communication with pump cylinder block16.

As shown most clearly inFIG. 1, hydraulic porting60comprises a first pressure side61and a second pressure side62, each of which connect pump12to motor14, and during operation one of pressure sides61or62will be under high pressure while the opposite side will be under low pressure, often referred to as vacuum.

Porting60is open to sump25through a pair of check valves58, shown schematically inFIG. 1, each of which is connected to one of the hydraulic pressure sides61or62. In the preferred embodiment, depicted inFIGS. 3 to 10, check valves58comprise a pair of check openings50formed in bottom surface37of pump portion30. The plane that is formed by bottom surface37and includes openings50intersects motor shaft24. Plug in filters52comprising a mesh filter element54are located in check opening50to prevent contaminants in the hydraulic fluid from reaching porting60.

Check balls48are mounted in respective check ports49as shown inFIG. 5to control the intake of fluid through check openings50. While the use of check plugs in general is known in the art and will not be described in detail, the present invention simplifies the assembly of the overall unit. A benefit of the simplified design shown herein is that gravity acts to locate check ball48in the area of seat55. When center section20is assembled, check balls48are sandwiched between pump section30and motor section32as shown most clearly inFIGS. 8and9. InFIG. 8, check ball48is against seat55to close check port49from the rest of hydraulic circuit60. This would correspond to this respective side of hydraulic circuit60being under pressure. InFIG. 9, on the other hand, check ball48is lifted off seat55to permit hydraulic fluid to flow through check opening50to passage49and thus to hydraulic circuit60. This is accomplished when that side of hydraulic circuit60is under low pressure or vacuum.

A ramp68is formed in motor port36to guide check ball48to the area of seat55, which permits gravity to assist in seating the balls. This design eliminates the need for a separate check plug assembly which must be inserted into the center section. It also permits the check plugs to be formed between pump portion30and motor portion32of center section20, as opposed to being entirely formed within a single component. A rib66is formed in port36to locate check ball48in check ball chamber56and to keep it from interfering with motor kidney42. Seat55is preferably located as that it is below pump running surface35and above the center axis of motor shaft24.

Motor shaft24extends through opening64in motor section32and may be at least partially supported by the bottom surface37of pump section30or by a separate bearing on the transmission housing. A recess72is formed on bottom surface37to provide clearance for motor shaft24.

A benefit of the present invention is that it eliminates the need for sealant or a gasket between pump section30and motor section32. This is preferably accomplished by providing a raised area41that is preferably machined, and surrounds both motor porting36as well as bolt openings39. When the unit is assembled, raised surface41contacts engagement surface46on pump section30, providing sufficient contact to minimize leakage. It will be understood that a certain level of leakage is permissible due to the location of center section20in sump25, but excessive leakage can diminish performance of the unit.