Hydrostatic tranny?

davesisk · Oct 14, 2002

Hi folks:

I'm currently a Power-trac owner, and I'm curious about the hydrostatic transmissions in Kubota's (and other more traditional CUT's). The Power-trac has a hydro-static drive pump that sends hydraulic fluid (actually, 10W30 motor oil!) to a hydrualic wheel motor at each wheel (very much like a skid steer, I believe).

On hydrostatic drive Kubota's, how does that drive system work? Is it the same type of deal where there's an HSD pump and 4 hydraulic wheel motors? Or is it an HSD pump and one big hydraulic motor that drives regular drive-train with differential and gearbox? Or is it somewhere in between? Any explanations appreciated!

Also, I'm curious about if other CUT brands (like JD, New Holland) do HSD differently. I don't want to start any sort of brand war, I'm just curious about how they work, how they are put together, and what the differences are.

Regards,
Dave

Anonymous Poster · Oct 14, 2002

I am /w3tcompact/icons/blush.gif to say I don't know the whole answer...

But I do know there are no independent motors on my Kubota B2910, unlike on your PowerTrac.

The setup on my Kubota B2910 is that there is a drive shaft that goes from the engine to the rear part of the tractor. The rear wheels are driven right from the rear section of the tractor. Another drive shaft comes out of the rear end area, and transfers power to the front differential. Much like a conventional 4x4 pickup truck.

I will let the experts add more details...

Bill in Pgh, PA

davesisk · Oct 15, 2002

Thanks for the reply. So, even though it's HST, you have a shifter for several forward/reverse speeds and a clutch?

Thx,
Dave

Jeff396 · Oct 15, 2002

I can't say that I have looked real closely at the Kubota manual for my B7500 yet. However, this summer I got to work on my parents Case garden tractor that we have had since the mid 70's. It has a hydrualic pump on the rear of the engine plumbed to a control valve. From there the fluid goes to the hydraulic motor which is mounted to the gearbox in the rear of the tractor. With the gear box you can select either high, low or neutral. I assume the the CUT's work pretty much the same way except they also will have a transfer case somewhere to put it in 4wd./w3tcompact/icons/smile.gif

Jeff

techman · Oct 15, 2002

Dave:

Since I am very familiar with the technology (I work for a major supplier of hydraulic transmissions, components and systems). All hydrostatic transmissions work basically the same. There is a hydraulic pump that can pump oil in a volume that is set by the operator input (stroke control). The pump has 2 ports. One is the outlet and one is the inlet side. The pump can go "overcenter", which means that the outlet becomes the inlet and vica versa. This allows the oil to be delivered in either direction to the motor. The motor is driven by the pumped oil, and the oil leaving the motor returns to the inlet of the pump. By adjusting the pump stroke, you can determine the oil flow (motor speed) and direction.

The pump doesn't care if the motor is in the wheels or driving the input to a gearbox. The most common design in small tractors is a module that contains the pump and the motor in one housing. There is a shaft in to the pump and a shaft out to the trans. Most are equal displacement, which means at full speed, the output shaft is equal to the input shaft, less some small losses. This is what is use on most Kubotas.

The other option is to have external hydraulic motor(s). These are typically wheel motors, which are either low speed motors directly connected to the wheel in the hub, or high speed motors with integral gearboxes that connect to the wheels. You can have 2 or 4 wheel motors. The advantage of wheel motors is the lack of mechanical drive linkage to the wheels. The disadvantage is that the basic configuration is like an open differential - one can spin free. To have a lock, or limited slip, involves some fancy valves that add complexity. Most skid-steer use 2 transmission pumps driving separate sides, so you can control the speed and direction of each side separately.

No matter what brand, all HSTs work the same. As I said the modular-gearbox uses lower cost HST and a costlier gearbox. You can have multiple gear ranges (advantage) and the differential lock features. The wheel motor version simplifies the mechanical design, and can eliminate BIG gearboxes in higher horsepower construction equipment.

One last comment. I would be surprised to see 10W30 motor oil used as a hydraulic fluid. Motor oil does not have the high pressure characteristics nor the proper anti-wear additives that hydraulic fluid has. In less demanding applications it may be OKay, but for modern high efficiency products, service life would be comprimised using motor oil.

paul

davesisk · Oct 15, 2002

Cool. Thanks for the post. This clears up alot of my personal lack of understanding the mechanical differences...

Thx,
Dave

art · Oct 15, 2002

The different ranges allow them to probably use a smaller hydro than yours for power lowering the cost to enable them to use a range transmission to give more or stronger power in the lower ranges and still give them a road speed. On the note about longevity with the use of 10w30 oil. On several skid steers we sell they use the 10w30 oil for there drives but there is an additive that is added.

duane · Oct 15, 2002

Hi Techman,

Interesting post. I am curious regarding your statement about the 10w-30 motor oil. I have a power trac which calls for a 10w-40 motor oil for the hydro pump. My other lawn tractors that have hydrostatic transmissions use 10W-30 oil. Do the tractor manuf. do this for cost or convience. What would you use for a fluid? Also, I have been trying to find a link to what the viscosity of a 10W-40 motor oil is at various temperatures, say 130 F in SUS values. Do you have info. on that.

Love to hear from the insiders
Duane

techman · Oct 15, 2002

Duane:

I am not a fluid expert, but I can give a little insight on oils that I am familier with. Motor oil and hydraulic fluids are optimized for different conditions. Motor oil has viscosity additives to maintain properties at higher temperature. Also motor oil is formulated to prevent breakdown from fuel and exhaust blow-by.

Hydraulic oil is optimized for characteristics under high compression pressures (high bulk modulus and high shear strength). Also there are no detergent additives in hyd. oil, and there are minimal viscosity additives since the oil operates over a narrower range of temperatures (running temperatures). In our industry, one of the greatest concerns with "other" fluids is the compatability with materials in the hydraulic components. A variety of materials are often used in the pumps and motors. These are sometimes composites, and various bronze like alloys. Some oils can become acidic, especially in the presence of dissolved water. This slight acidity can start to dissolve certain materials (slowly) and result in premature wear out.

My gut feeling is that the lubricity of motor oil under high shear force conditions (in a pump or motor) may not be high enough to prevent metal-to-metal contact in extreme conditions (high pressure + high temperature). This can lead to accelerated wear and perhaps internal damage. If a particular manufacturer permits the use of motor oil, it would be at the manufacturer's risk as a recommended use. I definitely would not use motor oil in a device that specifies hydraulic fluid. No system or use that I have seen in 25+ years in the industry used motor oil as a fluid. If it is used in a product, my guess is that it is for convenience (only one product to maintain at the manufacturer), and the product is sufficiently lightly loaded to not risk damage due to fluid properties.

One quick note is that I had built a log splitter some years ago. I used an old GM power steering pump, belt driven off of the engine. Due to circumstances, I had tested it at work (a source for hyd. oil), and drained the oil to fix something. I ended up hauling it home w/o oil. Went to use it on the weekend and discovered I had no oil. Well buying hyd. oil retail was a price shock to me, so I used cheap 30 weight motor oil. It was easily available any time and any place. It ran for years like that, till I sold it. I can only guess that it is still running. This was not a real demanding application, however, as hydraulics go.

paul