Hydrostatic transaxles

Details Hydrostatic transaxles Hydrostatic transaxles

2000-04-17

2002-02-12

Estremsky, Sherry (Department: 3681)

Planetary gear transmission systems or components

Variable speed or direction transmission combined with...

C475S083000, C074S60600R

Reexamination Certificate

active

06346059

ABSTRACT:

This invention relates to hydrostatic transaxles which are now used in increasing numbers for lawn care and other outdoor duties as the preferred choice for power transmission drive lines in products such as lawn and garden tractors, pedestrian walk-behind mowers and snow blowers.
Hydrostatic transaxles of the type currently sold in the marketplace require careful assembly and service practices in order to avoid certain problems occurring that may result in lower than expected operational life of the product. Hydrostatic transmissions operate most effectively and efficiently when they are constructed with exceedingly small clearances between their reciprocating and sliding elements. The transmission of power by such hydrostatic transmissions has now become widespread and the attendant small fluid leakage loss from the internal pressurised circuit during operation which is inherent with this type of speed changing device is generally considered insignificant as the resulting retardation in vehicle speed most often goes largely unnoticed by the end user. Hydrostatic transmissions work well and have a long and useful life so long as the level of contamination suspended in the power transmission fluid remains low. High levels of contamination carried by the power transmitting fluid can rapidly wear out the aforementioned fine clearances resulting in an increase in fluid leakage, especially during high pressure operation.
It is therefore of paramount importance during both the initial assembly process when the hydrostatic transaxle is built as well as at subsequent service repair intervals, that the possibility of contamination entering the hydrostatic transmission and its surrounding fluid chamber be minimised. In simple terms, the chance for the hydrostatic transmission components being contaminated during handling on the assembly lines in the factory recede as the number of components in the total assembly build is reduced. It follows therefore, that if the hydrostatic transmission could be fully assembled and sealed in the housing prior to the reduction gearing and differential shaft components being added, there would be an advantage.
In the past, because hydrostatic transmissions were of the self-contained bolts in order to separate the unit from the transaxle and replace it with another unit. If on the other hand, a bearing or shaft seal needed to be replaced in the transaxle drive train, such a repair could be easily effected just by disassembly of the transaxle and without disturbing the internal components of the hydrostatic transmission which would stay in-place in their own housing. A typical design of mating to a separate transaxle device is shown in Eaton Corporation U.S. Pat. No. 5,234,321 incorporated herein as reference.
Due to improvement in the art during the past decade or so, the vast majority of hydrostatic transmissions now in use are of the integrated type whereby a common housing is used to surround both the hydrostatic elements as well as the speed reducing gearing (and differential when required), typically as shown in
FIGS. 3 through 5
in Thoma et al. U.S. Pat. No. 4,979,583 incorporated herein as reference. Although the improvement of the “integrated” type over the earlier “bolt-on stand-alone” type of hydrostatic transmission and transaxle combination has provided significant economic benefits in terms of lowering manufacturing cost of the drive line such that hydrostatic transmissions are now better able to compete more effectively with mechanical-shift gear transmissions, inconveniences can arise when repairs are needed. The consequence of shipping units back to the factory for repair is both costly and inconvenient for the vehicle owner.
Furthermore, in the event of a service agent electing to make the repair himself, for example, a normally relatively simple repair involving the replacement of a worn bearing or seal, it is at present a fact that this would first necessitate the splitting open of the transaxle housing in order to gain access to those elements needing replacement. As such action results in the hydrostatic transmission components being exposed to what may well be a relatively unclean working environment, a distinct possibility exists that the hydrostatic transmission might have become contaminated such the repair is only short lived. Consequently, the service agent may elect to substitute the faulty unit with a brand new replacement but this has the disadvantage of much additional expense for the vehicle owner, especially if the existing hydrostatic transmission or conversely, the original gear train components were considered by the agent to be in good and still usable condition. There therefore is a need in the art for a new integrated hydrostatic transaxle that will allow simple repairs to be undertaken by the dealership on the non-hydrostatic components without exposing the internally disposed hydrostatic transmission components to contamination.
With all known integrated hydrostatic transaxles currently sold, factory testing can only take place once the transaxle is fully assembled as the hydrostatic portion as well as the geared portion are contained within a surrounding two-piece housing structure. In the event the factory test indicates that the hydrostatic transmission is not operating satisfactorily, repair and rectification can be both costly and time consuming as the complete housing must first be dismantled in order to be able to replace deficient hydrostatic componentry. What is therefore needed in the art a new form of integrated hydrostatic transaxle in which the two types of power transmitting componentry within the complete product package are separate from each other such that the hydrostatic transmission can be tested and approved before the remaining non-hydrostatic components are assembled in place. What is also needed is a new form of integrated hydrostatic transaxle allowing rectification work, when needed, to be speeded up and therefore more economic to perform. What is further needed is a new solution whereby the amount of handing required during assembly on the assembly lines is minimised before the hydrostatic transmission is fully sealed within the surrounding housing structure.
As integrated hydrostatic transaxles of the type currently available in the market require a large housing structure for containing both hydrostatic and non-hydrostatic components, the machine tools needed to perform finish machining operations on the housing are expensive due to their size. There would be a saving in machine tooling investment if the size of transaxle housing were smaller in size, and there would be further saving in terms of economies of scale if one part of the housing structure of the hydrostatic transaxle could be used for numerous other product types. What is therefore needed is a new form of integrated hydrostatic transaxle having a relatively small housing component requiring machining for the mounting of the hydrostatic transmission such that the remaining and larger housing members required for completion of the transaxle housing structure can be used in their as-received die-cast condition. What is further needed is a universal cover housing element for the mounting of the hydrostatic transmission such that the sub-assembly can be used in combination with any number of different case housing elements to satisfy a range of products types.
SUMMARY OF THE INVENTION
From one aspect the invention consists in a housing structure for a hydrostatic transaxle where the housing construction comprising three housing elements that inter-relate to form separate chambers for the hydrostatic transmission components and the geared components. An input shaft is supported in the housing and extends into the chamber containing the hydrostatic transmission to drive the hydraulic pump. An output shaft is also supported in the housing and extends into that chamber containing the geared components. In instances when a mechanical differential is also located within the chamber containing the geared components, the output sha

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