What is going on in an HST?

[gladehound]

I'm not debating hydro versus gear. Hydro should be superior in every way except efficiency of power transfer. What I am wondering is if this stalling in low range thing is the norm for these hydro Kioti's and hydro tractors in general. Is it a design flaw or a problem with a few machines? Some obviously dont' consider it a problem. I do.

And no, you can't stall my tractor out by pushing into a dirt pile when in the lower gears. And nor could you stall out my cousin's JD4300 hydro in low range. The tires would spin first. That's all I'm saying.

When I look at tractors, assuming they have the same tires, I always figured the one that could push or pull more was the heavier one. Now with these hydros that cannot spin their tires, that is no longer the case. Maybe my little LB1914 can pull and push more than your 30 hp hydros! And if that is the case, there is something very wrong because people by a certain size tractor to get a certain capability.

I hope I didn't rub anyone the wrong way with this. I probably did. I just want to understand what is going so I can consider it the next time I purchase a tractor.

 

[Tim M]

I think you understand it pretty well cuz you seem to have hit on all the factors. Particularly astute is the concept of leakage, which puts an unknown into the equation. Leakage is fairly slight, so it doesnt take much pumped fluid to crack the relief if the h motor doesnt turn. This relief pressure factored against h motor displacement governs the max torque that the motor will apply to the gearing. If this wont spin the wheels it amounts to a serious disadvantage over a gear tractor. Most geared tractors are able to spin wheels even carrying very heavy implements. In some cases the load is so heavy that the wheels only spin a little as the engine bogs. This is a tremendous driveline load. If the wheels lock hard with engine revs up you WILL shear something. The HST protects against this by preventing hard coupling of engine inertia to the wheels. When you are using a moderate pedal and the wheels stop, the relief is open and you can only get a couple % more torque by flooring it. The pressure goes up a little with the extra pump flow. Pressure and flow is HP. If the H system is sized correctly the engine should stall gracefully if the wheels dont turn. That situation needs a lower gear. Makes you mad if you dont have it.
larry

So as the flow increases through the open relief valve the pressure does go up enough to eventually stall the engine. Makes sense. It is very gracefull if I had to judge, since I really have to mash the pedal down to get it to stall.

Would bumping the relief valve setting up a bit help?

So the bottom line is how much power does the HST leakage consume? I have heard numbers as high as 15-17%. Seems like a lot. I suspect that this may vary from tractor to tractor. At what point do you say "hey mine is defective"?

Backhoe off, traction is the limiting factor (wheels easily spin).

Backhoe on, engine-(HST loss) is limiting factor (can make engine stall).

I definitely wish I had a lower gear.

 

[SPYDERLK]

[[[So as the flow increases through the open relief valve the pressure does go up enough to eventually stall the engine. Makes sense. It is very gracefull if I had to judge, since I really have to mash the pedal down to get it to stall.]]]

Would bumping the relief valve setting up a bit help?

So the bottom line is how much power does the HST leakage consume? I have heard numbers as high as 15-17%. Seems like a lot. I suspect that this may vary from tractor to tractor. At what point do you say "hey mine is defective"?

Backhoe off, traction is the limiting factor (wheels easily spin).

Backhoe on, engine-(HST loss) is limiting factor (can make engine stall).

I definitely wish I had a lower gear.

[[[Pressure goes up a little. That increases your wheel torque. If you cant quite make it up a hill holding moderate pedal you will do a little better by increasing pedal as you slow. As the relief flow rises it opens further and the pressure needed to keep it open thatmuch is a little more. Theres the extra wheel torque. But required engine power to drive the pump skyrockets. You are producing a little more pressure at a lot more flow. HP = PxF. You are wasting a whole lot of HP in that flow thru the relief. Thats why it stalls. Bumping up the relief would be great, but its a balancing act between there and abuse. Best to be sure its set only 100 PSI or less above cold spec. It will relieve slightly lower at hi temp.

I hear you about the extra gear. An HST at its limit is extemely frustrating.
I have a BX 1500 that wont stall its engine locked wheels at full pedal. Im looking for the relief valve.

BTW Power loss in a gear drive is about 10%. 15 to 17% Hydro sounds about right.
larry

 

[dynasim]

All hydros(except maybe computer controlled ones) will pop the relief valve before stalling the engine if the swash plate is moved away from zero in small enough increments, and the tires are fixed. If it didn't, things would break.

The HST acts as variable gearing device, with essentially an infinite gearing turndown. What that means is that, if the safety valve did not pop, the torque on the drive train would grow tremendously, surely breaking something important.

Creeper gear tractors have a warning about not pulling loads in that gear for the reason that there are no torque relief mechanisms. They will break things if they do. An HST will travel slower than any creeper gear machine.

Finally, if gear guy will not admit to stalling the tractor while trying to ease the clutch out while pulling a stump, I wouldn't buy a car from em. The maximum wheel torque is likely(not certainly) greater in the HST, because few people will pop the clutch at full engine speed(necessary for maximim torque in a gear machine). The HST will reach maximum wheel torque at almost any engine speed, because the maximum wheeel torque is wholey dependent upon the relief valve setting.

An HST sucks for pulling implements because the HST sucks away a bunch of power to never never land when it is operating at full pressure.

Chris

 

[gladehound]

"Finally, if gear guy will not admit to stalling the tractor while trying to ease the clutch out while pulling a stump, I wouldn't buy a car from em. "

I have a sweet deal on barely used car... I guess you wouldn't be interested

If what you're pulling doesn't move and the wheels don't spin, in low gear, most gear tractors will lift the front until you push in the clutch or flip over backwards. Once up-side-down, the wheels have much less traction and will spin again.

 

[dfkrug]

Hydros don't have to be the limiting factor. If they are it's a design issue.

That's it in a nutshell. An HST tractor is usually designed to be able to
spin the tires in lowest gear range under good traction conditions. That
said, the engineer should design for AG tires and a weight in the upper
range of normal expectations. The CK30H is the first HST tractor I have
owned or driven that will go to relief in some low gear use conditions. So,
some engine power is wasted if indeed the CK30 has the same RV setting,
gearing, and HST as the CK25 (TIM's post). So far, I have not seen that
with the CK20H, and it has AGs, while my CK30 has INDs. I have not
checked pressures with a guage on either tractor.

My JD955s had new AG tires and backhoes mounted and they could always
spin tires. My B21 will bog the engine instead of going to relief or spinning
tires. All 3 HST scenarios covered.

 

[SPYDERLK]

All hydros(except maybe computer controlled ones) will pop the relief valve before stalling the engine if the swash plate is moved away from zero in small enough increments, and the tires are fixed. If it didn't, things would break.

The HST acts as variable gearing device, with essentially an infinite gearing turndown. What that means is that, if the safety valve did not pop, the torque on the drive train would grow tremendously, surely breaking something important.

Creeper gear tractors have a warning about not pulling loads in that gear for the reason that there are no torque relief mechanisms. They will break things if they do. An HST will travel slower than any creeper gear machine.

Finally, if gear guy will not admit to stalling the tractor while trying to ease the clutch out while pulling a stump, I wouldn't buy a car from em.

[[[The maximum wheel torque is likely(not certainly) greater in the HST, because few people will pop the clutch at full engine speed(necessary for maximim torque in a gear machine).]]]

The HST will reach maximum wheel torque at almost any engine speed, because the maximum wheeel torque is wholey dependent upon the relief valve setting.

An HST sucks for pulling implements because the HST sucks away a bunch of power to never never land when it is operating at full pressure.
Chris

Thats a good description. Perfect all the way thru except the part I bracketed. There are several issues involved here. The most important is the final drive ratio counting the full pedal ratio. That is pump ful output to H motor displacement and the low range hard gearing. Most [all that I know] have a final ratio capable of more speed than a geared tractors low gear [1 to 2mph]. With the pump and motor sizes you would have to go to very high pressures with the HST to deliver the torque of a 2mph gear. Also with a gear the engine inertia during bog down can be hard coupled to the wheels. The HST can never do this w/o the relief valve set to NO relief.
larry

 

[Tim M]

So what I am hearing here is that both the pump/motor leakage and relief valve setting are the limiting factors until the engine stalls. And along the way both the flow and relief pressure increase until the pump loads the engine enough to stall it.

So in theory a CK35(34 HP) with the same HST system and relief setting may be able to apply a little more wheel torque before stalling, but not the full 13% HP more since more HP will be consumed along the way by the leakage and relief flow. In fact it seems like the increase in wheel torque will be more function of the relief valve's characteristics (i.e. how much the extra HP can raise the pressure).

My guess is this would be barely noticeable. Now what if that 35 engine was tuned a bit perhaps giving 38 HP? Anybody venture to guess at what point you would notice a difference? This seems like a dead end.

So without a redesigned HST (or higher relief setting) the CK35 offers little wheel torque advantage. I am thinking it was brought about solely to increase PTO power and not wheel torque.

 

[Racer71]

I have the opposite problem. I can spin the ag tires anytime. My tires are filled and I use a 1,300 pound block when doing heavy digging. The engine will bogg when I lift the bucket while the tires are spinning though. I think I am getting the max out of the tractor.

 

[fcamillieri]

My CK20HST seemed very under powered when new. It would only run on level ground in high range. After 50 hours it was much better and now I use high range for most work as long as I'm on hard ground.

Frank