Help Untwist my HST Brain, please...

[Henro]

I have seen reports about stalling and not stalling hst tractors in the past, and a thread in the Kubota forum has my mind twisted again... /forums/images/graemlins/confused.gif

What I don't understand is how/why I can stall my HST tractor when I am in low range and digging into a pile of dirt, while at the same time it is reported that it will not stall it when trying to go up a hill in high range. In the latter case, it just stops and whines, or so I understand.

Case 1:

I have done this several times. Grabbing a full bucket of dirt, the engine lugs, I am too slow getting off the HST forward directional pedal, the engine stops dead. I am unhappy with myself.

Case 2:

I have not done this yet, but it is reported as common. In high range, going up a steep hill. The tractor stops going forward. The engine does not stall, but continues running. To continuie moving up the hill, the tractor must be put into a lower range.

And I ask:

What’s wrong with this picture? The same pressure relief valves are in the circuit in both cases. Why would it/they function in case 2, but not in case 1??? Common sense (or lack of common sense) tells me that if the pressure relief valve(s) function in case 2, they should function even quicker in case 1. One would think that the sudden high pressure spike that would likely occur in case 1, in the two seconds or so that it take for the engine stall to happen, would cause the PRV(s) to pop. But that doesn’t seem to happen.

Now I have to admit I don’t know a lot about HSTs. But what I think I know is that the HST is basically a Bi-directional pump that is tied directly to a hydraulic motor. Engine RPM sets the RPM of the HST pump, and moving the directional pedal sets the amount of flow out of the HST pump, and the direction the fluid is sent to the motor. So the foot pedal tells the HST pump which way to push the fluid, and how much to send.

Now the coupling between the HST pump and the drive motor has to be fairly good or the efficiency would suffer badly. And being in low or high range is a mechanical gearbox change, on the output side of the hydraulic motor, so it seems like that gear range selection should be able to be set aside, when looking at what is happening within the HST portion of the equation.

So then does it come back to pressure within the HST loop somehow being a function of HST pedal position? Why would this be? I am having trouble seeing it. It seems to me the pressure in the loop between the HST pump and the Hydraulic drive motor, would be function of the load on the drive motor and the power being supplied by the tractors engine. Up to the pressure limit set by the circuit’s pressure relief valves.

So now let’s pretend I am sitting on my tractor, with the backhoe on the back and a full loader bucket on the front. I have the bucket against a tree. My engine is set to run at 1,800 RPM. My wheels will not slip. I am in low range, and push the directional pedal in the forward direction and the engine lugs and stalls.

I restart the engine and put the range lever in the high position. What will happen this time when I push the forward directional pedal?

Many reports here say the tractor will not stall, a pressure relief valve will pop, and I will hear the whine of the fluid flowing through that valve back to the tank.

My brain says the tractor should stall in both cases, just like a gear tractor would. But I believe the reports of others here too.

What am I missing? /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[trkbldr]

Henro, I believe the only thing you are missing are some well-placed holes drilled into your ROPS /forums/images/graemlins/shocked.gif .That should solve your HST problems /forums/images/graemlins/grin.gif /forums/images/graemlins/wink.gif .

 

[thcri]

Henro,

If it is any consolation, my New Holland TC29D does the same thing. Hit the pile and uuugggg. Go up a steep hill in higher range and the tractor just stops.

murph

 

[kevinj]

I think you kick out the relief in high range which will then lower pressure and stall. In low range you enough gearing to keep the wheels turning and stay below the relief setting. Maybe one of the experts has a better explanation.

 

[slowrev]

Yes, it must be the holes in the ROPS, or maybe the gas vs diesel hp thing, or you are not using $6.00 qt syn oil /forums/images/graemlins/smile.gif Maybe Henro has a Fram oil filter on it.

I guess it is just a quirk of HST's.
With gears it will stall out either way.

 

[OkeeDon]

I think there must be an override depending on which range you are using. In my usual manner, I have no idea what the technical terms are, but I think I can describe it like this:

You are in a higher range. You encounter a hill or obstacle that requires the pump to do more work to overcome it. The tractor says, "Phooey on this. I ain't working that hard. I'll just quit, and make him shift to a lower range that can do the work." So, you shift to low range, and repeat the exercise. Once again, the obstacle is encountered. The tractor says, "Wow, this is tough. But, he's already in the lowest range, so he must want to get the job done. I'll work as hard as I can, even if it stalls the engine!"

In other words, the range selector must decide whether the pressure relief is active or not...it's the only solution I can think of.

PS -- I ran ieSpell against the above, and it accepted "Phooey" and "ain't" -- I'm more impressed than ever!

 

[rockyridgefarm]

Duh! You forgot to turn on LoadMatch!

 

[slowzuki]

I'll try to answer:

I don't know if the variable displacment component is the pump or motor but here it goes.

I like your questioning attitude!

There are many loads on the motor we'll look at the three primary ones discussed here:
1. HST pump
2. PS pump
3. Implement pump

IF the combo of torque required to operate these 3 exceeds the torque output of the engine, it stalls. So if you are steering while raising the loader and pushing into a pile you will likely load up the engine.

The governor will try to keep up to the load as your accelerator is only one of the things influencing the fueling it is delivering to the engine.

The activation of the relief valve is limited to the HST system. The system can basically generate several different peak torques at the wheels given optimal hst pedal position. These would be right before the relief operates in each range the transmission is in.

In high range the peak wheel torque may be too small to propel the tractor up a hill or into a pile. So the system will hit max pressure and relieve.

in low range the peak wheel torque maybe high enough but the available engine torque too low, the system will not hit max pressure and the engine should stall if the operator doesn't back off the foot pedal enough.

So if you are careful with your foot, you should be able to make the HST relieve instead of stall in most situations. Provided you don't load the engine with steering or implement hydraulic tasks at teh same time.

 

[Henro]

<font color="blue"> in low range the peak wheel torque maybe high enough but the available engine torque too low, the system will not hit max pressure and the engine should stall if the operator doesn't back off the foot pedal enough. </font>

My thought would be if I have the engine is set to run at 1,800 rpm the torque would be the same regardless of what gear range I am in.

BUT I think I see the answer to my question now! Maybe...

What pump actually works the hydraulic cylinders?

When I stall out digging into a pile, I am also lifting/curling the bucket. The reality of the matter may be that it is NOT the HST system that is stalling the tractor at all! It is probably the second pump that operates the loader/steering/whatever...!!!

My let's pretend "For example #1" above might not be valid. If I try that without moving the loader, in low range or high range it may be exactly the same!

I ASSumed that pushing against the tree would have the same effect as driving into a dirt pile when working with the loader. Now my guess is that it is not!

Anybody know if it is the second pump on a two pump tractor that normally works the loader? I'll bet it is and that if I don't pop the PRV that is connected with the loader circuit, then the engines stalls.

Boy...at least now I have dreamed up a reason... /forums/images/graemlins/crazy.gif

Time to back to playin' with my drill now... /forums/images/graemlins/blush.gif

 

[kevinj]

Henro,

I think the BX models are single pump. I know the B7500 is a two pump and one runs the hydraulics and one the tractor hydro system. Our old B7100 was a single pump and it behaved much like the BX models. Slozuki mentioned the power of the drive motor, it may not have enough torque even if the pump is capable so you could create a stall on the drive and still have an operating FEL.