Planetary gear transmission systems or components – Fluid drive or control of planetary gearing – Pump and motor in series with planetary gearing
Reexamination Certificate
2002-02-26
2003-07-22
Wright, Dirk (Department: 3681)
Planetary gear transmission systems or components
Fluid drive or control of planetary gearing
Pump and motor in series with planetary gearing
Reexamination Certificate
active
06595887
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to hydrostatic power transmitting and speed reducing equipment having independent sumps which are useful in many diverse applications, one being for instance, a vehicle drive-line of the type generally known as a hydrostatic transaxle. This invention is particularly concerned with an improved transmission or transaxle having a housing with an interior space divided by a partitioning device into a first internal volume for the hydrostatic transmission mechanism and its associated operating fluid and a second internal volume accommodating a speed reduction mechanism in the form of a lubricated gear train.
2. Description of Related Art
Hydrostatic transaxles are increasingly being used in lawn care and other outdoor power equipment duties such as snow-blowing and have become the preferred choice for power transmission drive lines; for example, in lawn and garden tractors with most employing a single hydraulic pump fluidly connected to a single hydraulic motor. Although in most instances single motor hydrostatic transmissions are coupled by speed reduction gearing to a mechanical differential, applications also exist where two hydraulic motors are used and where each hydraulic motor is connected by a respective gear train to axle output shafts.
Furthermore, two hydraulic pumps can also be used with two such hydraulic motors to create a hydrostatic transmission for each drive wheel which can be useful for zero-turn radius vehicle applications. Occasionally, single motor hydrostatic transmissions are used without the addition of a mechanical differential, such that the hydraulic motor is coupled by speed reduction gearing to a single output shaft, and in these instances, the output shaft may be the axle driving one wheel of the vehicle or be arranged to drive the axle of the vehicle by an interconnecting chain drive.
All hydrostatic transmission require hydrostatic power transmission fluid in order to operate and the fluid acts as the medium to convey power between the pump and motor of the hydrostatic transmission. As the positive displacement fluid pumping mechanisms used by all hydrostatic transmissions and hydrostatic transaxles require careful and accurate manufacture to achieve the necessary close tolerance fits in order to minimize internal fluid leakage losses associated with high-pressure performance, a preferred practice is to prevent damaging contamination generated by general wear and tear in the power transmitting gear train from reaching the pressurized circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurized circuit, especially important when sintered powder-metal gears are used in the gear train, a long and useful working life for the hydrostatic transmission can be expected.
Although by no means essential, it can nevertheless be desirable to position the hydrostatic mechanism in a fluid compartment which is physically separate from any adjacent compartments in which the gear train is located such that no exchange of fluid can take place and whereby damaging contamination in the gear train compartment remains confined to that compartment. Contamination containment by way of separate compartments is shown in U.S. Pat. No. 5,090,949 titled Variable Speed Transaxle, expressly incorporated herein by reference. Here a bulkhead is provided in the housing which carries a shaft seal, the shaft seal operating on the interconnecting drive shaft which mechanically couples the motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. Further quantifiable benefits are gained as the compartment providing the sump for the gear train need only contain the bare minimum quantity of oil to satisfy lubrication considerations. Thus by relying on what in effect is “splash lubrication”, expense is saved as the quantity of fluid needed is less and the efficiency of power transmission is improved as the associated drag losses of the fluid contacting the rotating gears is much less then with a sump carrying a full capacity of oil.
On the other hand, with some hydrostatic transaxles, the hydrostatic transmission is arranged to operate within the same oil bath as the speed reduction gearing (and mechanical differential when included) and such designs are commonly referred to as “common sump” types. Typically, the gear train and the hydrostatic transmission lie adjacent one another at the same elevation and the oil level in the sump is kept near to the brim to ensure that the hydrostatic components remain properly submerged at all times and also to avoid any ingestion of air. With a gear train operating submerged in the oil bath, power losses are greater due to the increase in fluid friction associated with the wetted area in contact with the oil than would be the case with the “splash lubrication” types mentioned earlier. Such gear drag losses can be especially noticeable in winter time when the gears are required to revolve from rest in a sump where the oil can be in an extremely viscous initial state, and the resulting higher than normal operational loads imposed on the components in the drive train are unavoidable. As it is not possible to select oils with different properties in the common sump design, a problem is posed as the optimum fluid type which would normally be selected as the preferred lubricant for a gearbox will have completely different characteristics as compared to the type of power transmission fluid most suited for the efficient operation of a hydrostatic transmission. Typically, a gear oil tends to be thicker with a high viscosity range whereas an automatic transmission fluid (“ATF”) tends to be much thinner with a lower viscosity curve. As the hydrostatic transmission normally prevails when a conflict in design arises, it is accepted that the gear train may be operated in a generally adverse environment of low viscosity fluid such that accelerated wear and resulting higher contamination levels are more likely. The common sump design has the further limitation in that grease cannot be employed as the lubricant for the gear train. For certain applications, grease can be a more economic choice of lubricant.
Under normal atmospheric conditions, hydraulic fluids contain about 9% by volume of dissolved air which has virtually no effect on the physical properties of the fluid and therefore does not lead to any reduction in the performance of the system. However, should any appreciable quantity of undissolved air be present, the fluid will be prone to foaming problems, especially should the fluid experience excessive agitation, for instance, by any revolving elements such as gears being operated in only a partially submerged condition in the fluid sump. If such foaming occurs, it will rapidly lead to the destruction of the hydrostatic transmission.
It is also a physical characteristic of the fluid to expand and contract in volume in relation to changes in its temperature. In general terms, the volume of oil increases by about 0.7% for every increase in temperature of 10° C., and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100° C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation.
Quite often, an external expansion tank is fitted to the transaxle housing to allow for such expansion and contraction of the hydrostatic fluid. However, an external expansion tank can be troublesome as it is most often situated directly above the transaxle where little space exists. Frequently the space available under the frame of the vehicle is needed for rear-discharge ducts for the grass clippings. Therefore, there is often an advantage in casting the housing such th
Hydro-Thoma Limited
Wright Dirk
Young & Thompson
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