Hydrostatic transmission with internal fluid expansion chamber

Details Hydrostatic transmission with internal fluid expansion chamber Hydrostatic transmission with internal fluid expansion chamber

2001-05-15

2003-09-30

Marmor, Charles A (Department: 3681)

Machine element or mechanism

Gearing

With fluid drive

C074S60600R, C475S072000, C475S083000, C060S487000

Reexamination Certificate

active

06626065

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to stand-alone hydrostatic transmissions as well as combined hydrostatic and gear transmissions having housing structures provided with either independent or common sumps, such transmissions being usefully employed for many diverse applications such as vehicle drive lines of the type commonly referred to as hydrostatic transaxles.
This invention is particularly concerned with an improved hydrostatic transmission or transaxle drive line disposed within a surrounding housing structure and where the interior space inside the housing can be said to be divided by structural walls or bulkheads into two distinct internal volumes. The first internal volume containing the hydrostatic transmission submerged in its operating fluid whereas the second internal volume, being either in the form of a spill over chamber or alternatively, a chamber containing a gear train, are arranged to be fluidly linked together at all times by a communication duct in the form of a siphon.
2. Description of the Related Art
Hydrostatic transmissions and transaxles are increasingly being used in the lawn care industry and for other outdoor power equipment duties such as snow-blowing. They 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 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 pressurised circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurised 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 hydraulic motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. As such, 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 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 very 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 in which 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 a 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 deg. C. and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100 deg. C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion to occur without restriction over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation.
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