Nasty Little Hydrostatic Transmission Secrets...

jinman · Feb 29, 2004

...your dealer "forgot" to tell you.

Okay, I admit being overly dramatic, but the recent number of posts regarding hydrostatic transmissions and "slippage" under load has caused me to do some detailed study. That study has led me to some discoveries that surprised me. Frankly, my study has been limited to New Holland and that's why I'm posting this here instead of in the general forums. Many times I have started to respond to Bill's (Henro's) posts and realized that I should not make comments about Kubota and/or Deere because I have no specific knowledge of their systems. With that said, here is the nasty little New Holland secret. 

New Holland has their tractors divided into classes, Class 0, Class I, Class II, Class III. Within a class, all tractors use the same hydrostatic transmission. There is no difference in that transmission for any range of engines within the class. As an example, let me use Class III. The TC35D, TC40D, and TC45D have the exact same transmission with the exact same parts, pressures, and specifications. The Repair Manuals and the Illustrated Parts Manuals show only one part number for these tractor's transmissions and sub-assembly parts. My discovered little "secret" is that the transmission relief valve pressure (which is fixed and unadjustable) is 5,000 psi on all three tractors.

What does this imply for operation, or why should any of us care? Well, this means that if you set up a circumstance where the TC35D will open the relief valve and stall, in the same situation, the TC40D and TC45D will do no better. The transmission physically cannot make use of the additional horsepower and torque of these engines. For pushing and pulling in Low Range-Turtle, the TC35D will have just as much power as its big brothers if the engine doesn't stall.

Did your dealer tell you this when you were shopping? /forums/images/graemlins/confused.gif I doubt it. /forums/images/graemlins/shocked.gif

So what is so different about these tractors? Well, the TC40 and TC45 both have higher hydraulic pressures for their 3PH and their loaders. I suspect the same is true in other classes of tractors since many of you have tested/adjusted your pressures with the gages Spencer so generously offered to everyone. The larger tractors also have all their horsepower available at the PTO because that is a direct mechanical connection.

This is probably almost a non-issue, but our gear-drive NH brothers certainly have a valid point that their tractors can outpull the same size hydrostatic transmission because all the power can get to the wheels. /forums/images/graemlins/blush.gif We hydro owners just have to realize that this is a trade-off for the convenience and operation we have with the hydro.

I in no way am ready to give up my hydrostatic transmission because I've discovered that the transmissions relief valves will not let me get all my power to the wheels. /forums/images/graemlins/frown.gif With pressures above 5,000 psi that would have to be available, the strength and seals internal to the transmission would have to be much stronger than they are now. For example, can you imagine what the transmission would look like that could handle 7,000 psi? If New Holland comes out with a hydro for the TC48DA and TC55DA, you can bet it will have to withstand tremendous pressures. /forums/images/graemlins/shocked.gif

Oh well, thanks for listenting to my "rant." It's sunny now and I think I'll go out and play instead of writing posts and raising controversy. /forums/images/graemlins/grin.gif

OkeeDon · Feb 29, 2004

What does this imply for operation, or why should any of us care? Well, this means that if you set up a circumstance where the TC35D will open the relief valve and stall, in the same situation, the TC40D and TC45D will do no better. The transmission physically cannot make use of the additional horsepower and torque of these engines. For pushing and pulling in Low Range-Turtle, the TC35D will have just as much power as its big brothers if the engine doesn't stall. 

Help me understand this. What if the relief valve is set up such that only the most powerful tractor in the class can make it stall? Wouldn't the lower powered tractors run out of "oomph" and quit pulling or pushing before popping the relief valve?

Say we hook up each of the tractors to a bungee cord of specific strength. A TC18 pulls it to 50 feet (the numbers are hypothetical) and can't pull it any more because it doesn't have enough power, but the relief valve never pops. A TC21 pulls it to 60 feet and gives up (I don't know whether the "giving up" involves stalling or just quitting). A TC24 pulls it to 70 feet, and the relief valve pops just as (or just before) it gives up. Am I seeing this correctly?

Let's set up a different scenario. In this case, the pressure relief valve is set so even the TC18 can pop it. Each tractor hooks up to a static weight that the TC18 can't pull, because the relief valve pops. In this case, the TC21 and TC24 can't pull it either, despite their greater power, because they will pop off at the same point as the TC18. If I understand correctly, the is the scenario you describe.

Now, let's reduce the static weight so that it is just under the relief valve threshold. The TC18 can pull it, but the TC21 can get it moving easier and faster, and the TC24 does even better. Does that sound right?

If I have a handle on this, then the only time the situation arises is (a) if the relief valve is set to the lowest specs, and (b) the job is impossible for the lowest powered tractor

It's an interesting question.

MikePA · Feb 29, 2004

New Holland has their tractors divided into classes, Class 0, Class I, Class II, Class III. Within a class, all tractors use the same hydrostatic transmission. 
Class II Tractors - TC25, TC29 and TC33 are slightly different.

According to the Operator's Manual the TC25 has a 7.1 gpm pump capacity while the 29 and 33 have a 7.6 gpm pump capacity. All three have the same System Relief Valve Setting at 2,133 psi.

DK35vince · Feb 29, 2004

Mike,
I believe you are refering to the main hydraulic pump mounted on the engine that supplies fluid to the 3 point and loader Ect.
As I understand it hydro transmissions have there own seperate pump.

tc35dforme · Feb 29, 2004

Hmmmm.....I'm thinking !!! /forums/images/graemlins/smirk.gif

jinman · Feb 29, 2004

( Each tractor hooks up to a static weight that the TC18 can't pull, because the relief valve pops. In this case, the TC21 and TC24 can't pull it either, despite their greater power, because they will pop off at the same point as the TC18. )

That's exactly right, Don. If the TC18 pops it's relief valve without spinning its wheels, the TC21 and TC24 can do no better. They have the same relief pressure and their valve will release at the same point. Technically, I guess someone could make the argument that since the TC21 and TC24 are slightly heavier than the TC18 a few milli-pounds of pressure are lost and they would actually not be able to pull as hard as the TC18. /forums/images/graemlins/blush.gif I don't want to split hairs quite that thin, but I believe I am correct in my other assertion.

jinman · Feb 29, 2004

( According to the Operator's Manual the TC25 has a 7.1 gpm pump capacity while the 29 and 33 have a 7.6 gpm pump capacity. All three have the same System Relief Valve Setting at 2,133 psi.
)

Mike, as Vince pointed out, that 2,133 psi pressure and the different flow rates sounds like the engine-driven hydraulic pump for the 3PH, FEL, and remotes. The hydrostatic transmisson has two pumps. One pump takes fluid from the return side of the engine-driven pump and supplies it to the main hydrostatic pump under high volume and low pressure. The main pump on your tractor is set to relieve at about 3,700 psi I think (The relief valves are located between the pump and the hydraulic motor in the transmission.). I'm not sure if that is a Class I pressure or a Class II pressure, but someone recently quoted that number as having been checked by their dealer.

The fact that this "issue" relates only to the hydro tranny's ability to pull/push is something I realized about my tractor. I can pop the relief valve in low range with the engine hardly dropping 500 rpm. It's nowhere near to stall. I think now I know why. /forums/images/graemlins/crazy.gif

techinNY · Feb 29, 2004

Even as a tech, I never looked at it or thought about it like that. Yes, the pto is a direct connect to the engine, but even sitting still, no tractor motion, you are still turning the mass in the hydro, so you are not really getting all the engine hp at the pto. Good research any how! /forums/images/graemlins/grin.gif

SacandagaBrad · Feb 29, 2004

Jim,

You knew you would hear from me!

( We hydro owners just have to realize that this is a trade-off for the convenience and operation we have with the hydro. )

I realize it now and am learning to live with it!

( I in no way am ready to give up my hydrostatic transmission because I've discovered that the transmissions relief valves will not let me get all my power to the wheels. )

Me either!!! And neither is my left knee. It has been tuned up a couple of times and hopes in never sees another clutch.

( I can pop the relief valve in low range with the engine hardly dropping 500 rpm. It's nowhere near to stall. I think now I know why. )

I'm in the same boat, but didn't know why until you did my homework for me.

But it still seems to me that the tractor should be able to back itself out of a hole, carry its own weight so to speak. Twice I have been buried where I couldn't get the wheels to spin. The loader pushed me out no problem, so I guess with some coordination, the motor did deliver enough power to get the tractor out. Just not all to the wheels.

I am also finding that if I engage it more abrubtly, it seems to have more power to the wheels before popping the relief valve. Who knows? The traction can vary a lot depending on what you are doing with the loader, with a lot of pressure on the front wheels and the diffy lock engaged, that is a lot of traction!

Thanks for looking into it. This is the best explanation I've had. It still has plenty of power, I just expected it to lock up tight rather than slip, but I realize something has to give. The other benefit/trade off to keep in mind is smooth continuous power rather than popping a clutch and breaking traction. Back in my 4 wheeling days in the mud, the automatics alway had the advantage for the same reason.

I still love the tractor and have no regrets in buying it. Thinking back to the fall, I peeled 3 to 4 inches a rip off the compacted gravel road with the rear blade, and never "slipped", it either spun or kicked sideways very little. I guess I can't ask much more out of a 2800 pound 24 hp tractor.

Thanks again for the research.

Brad