What is going on in an HST?

[Tim M]

I've been trying to get my brain around this whole HST, high pressure relief valve and how it relates to engine stalling/power. I am going to ramble a bit. Please feel free to comment or add to anything I say.

The engine drives a swash plate pump whose angle determines flow output and is set by the HST pedal. More pedal more flow. The hydraulic output of this pump drives a hydraulic motor which in turn drives the gearbox whose gear ratio is set by the range selector. Given an equal amount of hydraulic pressure/flow, the Low range will apply more turning force (albeit slower) to the wheels than the medium range. In between the hyd pump and hyd motor is a high pressure relief valve.

Assume you are at max throttle, in the low gear range and pushing the bucket into a pile of whatever.


scenario #1
If the HST pedal is depressed slightly, yet the tractor is not moving, the tires are not spinning and the engine is still running then

is the high pressure relief valve open?

(i.e. with a heavy load at this low flow/swash angle the engine and pump combo can produce enough pressure to open the relief valve?)

Or is there just not enough flow yet to overcome the "leakage" inherent in the hydraulic pump/motor system?


scenario #2
Now the pedal is depressed a little more still no wheel movement, but the engine begins to lug.

Hydraulic flow is increasing and the engine is working harder to maintain the relief pressure at the new higher flow?

Or the flow is now getting very close to the amount "relieved" by the leakage in the hyd pump/motor causing the pressure to finally build enough to create more load on the hyd pump/engine.


scenario #3
Finally you push the pedal far enough to stall the engine.

Was the relief valve ever activated?

It would seem not since the engine did stall.

And if not then more engine power would have been able to apply more turning force to the wheels.


My service manual covers both the CK25 and the CK30 and it has no distinction between the 25 and 30 in terms of high pressure relief valve setting. This leads one to believe that engine power and not relief valve setting is the limiting factor.

Thanks,

 

[gladehound]

Hmmm... what I don't understand is how you can stall the engine in low range period.

My lb1914 has 19 hp. It's mechanical not hydro. But I can't stall it in the bottom 4 gears. All 4 wheels will spin first. And I have filled tires and normally have 800 pounds on the 3pt. when doing heavy loader work.

My cousins JD4300 was hydro and you couldn't stall it in low rang. Would just spin the tires.

I've heard so may people talk about not being able to spin the tires with a hydro from Kioti that I would be leary of buying one. Are there people out there with Kioti hydros that have no problems spinning all 4 tires? Please someone tell me yes. Otherwise I can only think that Kioti is not doing the hydro thing right.

Is low range just way too high of a gear?

 

[Tim M]

Sorry. Forgot to mention both rears are filled and I have a woods BH80x(1000 lbs) on.

With the backhoe off I can easily spin the wheels, even in 4WD with the diff lock on.

 

[gladehound]

the JD4300 did not have filled tires but it did have a 1500 pound backhoe on it. Do you have ag tires or industrial?

What is the maximum tavel speed in low range? That will give me an idea of the gearing.

 

[Tim M]

Industrials.

Not sure of the top speed in low, but with the backhoe on I rarely use medium and forget about high.

My setup w/backhoe weighs very close to 6000 lbs.

How much does the JD weigh?

 

[gladehound]

The JD was 5200 with the back hoe and loader. I think it was around 33 Hp at the engine.

 

[Steve_Miller]

Hmmm...

I've heard so may people talk about not being able to spin the tires with a hydro from Kioti that I would be leary of buying one. Are there people out there with Kioti hydros that have no problems spinning all 4 tires? Please someone tell me yes. Otherwise I can only think that Kioti is not doing the hydro thing right.

Is low range just way too high of a gear?

Mine (CK30HST) will in the dirt, depending on how compacted the dirt is. I can fully load my bucket at a dirt pile without spinning the tires now. I look back at tire spinning and realize that once the tire spins you've reached your tractors limit anyhow, yes there are times where the tires won't spin, but the times when they do spin you build confidence that the tractor is putting most of the power available to the wheels and even if they did break traction how much more is this going to do for you. You've got to remember that a hydro has that relief (or safety), a geared tractor only has clutch slippage and if the clutch doesn't slip and the tires won't give something else is going to. A hydro 's relief is what it says, if it doesn't relieve, something else will, whether its a gear that breaks or tire slipping in a rim or you blow the hydro motor out, it has it's pluses and minuses and the relief is one of them albeit good or bad in your view. I was hesitant for years on getting a hydro, but I finally converted and won't look back. The only thing I can see that a hydro can't do is when your stuck in a swamp/bog/mud hole is pop the clutch and dig yourself deeper, so I just avoid trying to test my limits anymore and you know what, I don't miss those tests as I've had a few of them over the years.

Steve

P.S. I have Ag tires on mine.

 

[gladehound]

"The only thing I can see that a hydro can't do is when your stuck in a swamp/bog/mud hole is pop the clutch and dig yourself deeper"

That and get full power to the ground when it is useful

I'm just messing with you. Hyros are productive but I wont buy one if it is the limiting factor to getting power to the ground. If I'm in a foot of mud and trying to pull something out of the woods, I don't want the wheels to stop turning until I decide it is time for them to stop turning.

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

Just my oppinion. I'd love to have a hydro. But after hearing all these stories I'll be sure to test it thoroughly before buying to make sure it can perform to my expectations.

 

[HODAK]

Hydro - Gear.....Hydro - Gear.............Hydro - Gear. The only MAIN issue I see between the two is operator preference. Power to the ground when used correctly should be basically the same. Stall a hydro tractor into a pile of dirt, stall a gear tractor into a pile of dirt. Both can be done. Seems to me that most users who switch from gear to hydro say they would never look back. Not everyone, but most. I've owned both, (presently hydro), and sometimes I wish I had the gear, but for the majority of my use I prefer hydro. Now a DK35SE HST with cruise that actually works might take care of those times when I wish I had gears........hmmmmmmmmmm.... where's that cab model by the way? My opinion.

 

[SPYDERLK]

I've been trying to get my brain around this whole HST, high pressure relief valve and how it relates to engine stalling/power. I am going to ramble a bit. Please feel free to comment or add to anything I say.

The engine drives a swash plate pump whose angle determines flow output and is set by the HST pedal. More pedal more flow. The hydraulic output of this pump drives a hydraulic motor which in turn drives the gearbox whose gear ratio is set by the range selector. Given an equal amount of hydraulic pressure/flow, the Low range will apply more turning force (albeit slower) to the wheels than the medium range. In between the hyd pump and hyd motor is a high pressure relief valve.

Assume you are at max throttle, in the low gear range and pushing the bucket into a pile of whatever.


scenario #1
If the HST pedal is depressed slightly, yet the tractor is not moving, the tires are not spinning and the engine is still running then

is the high pressure relief valve open?

(i.e. with a heavy load at this low flow/swash angle the engine and pump combo can produce enough pressure to open the relief valve?)

Or is there just not enough flow yet to overcome the "leakage" inherent in the hydraulic pump/motor system?


scenario #2
Now the pedal is depressed a little more still no wheel movement, but the engine begins to lug.

Hydraulic flow is increasing and the engine is working harder to maintain the relief pressure at the new higher flow?

Or the flow is now getting very close to the amount "relieved" by the leakage in the hyd pump/motor causing the pressure to finally build enough to create more load on the hyd pump/engine.


scenario #3
Finally you push the pedal far enough to stall the engine.

Was the relief valve ever activated?

It would seem not since the engine did stall.

And if not then more engine power would have been able to apply more turning force to the wheels.


My service manual covers both the CK25 and the CK30 and it has no distinction between the 25 and 30 in terms of high pressure relief valve setting. This leads one to believe that engine power and not relief valve setting is the limiting factor.

Thanks,

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