HST pedal and torque relation

[ccsial]

I just purchased an L3400 HST last month. The only other HST I have had is my JD355D garden tractor, which hardly counts. I plan to use the L3400 for brush mowing and snow blowing only but I still was interested in pulling power. I have been reading on the forum that to get more torque you must NOT step on the HST pedal too hard since that is like high gear. That didn’t seem quite right to me since this is hydraulics, not gears. But, then, I’m an electronic engineer so I decided to experiment.

I put a chain (1/4 inch thick links) from my draw bar to a big dead tree. I selected LOW range, 4WD, and advanced the throttle to PTO speed. I stepped lightly on the HST pedal and it would not spin the wheels. I pressed the pedal harder and more noise, but no spin. I pressed it all the way down and the wheels spun, the chain snapped. My tractor seems to need to press the pedal further for more torque. This thing does have power. It impressed me. I under-estimated it and now I gotta fix my dang chain.

 

[dknarnd]

Just a thought of caution here. I am not sure if chain is same as cable but if a cable snaps while pulling with it the results can be deadly. With a cable it is always a good idea to drape something over the line like an old jacket or even a tree branch about midway the length. I imagine this would be a good idea with a chain also. /forums/images/graemlins/smile.gif

 

[ccsial]

I did think about snapping. I did not think the tractor would do it. I had the brush hog on and the chain ran under it. The tree was vey near the wheel of the brush hog so most of the chain was covered by armour.

 

[Dargo]

That would sound reasonable to me in low range. In higher ranges is where you will notice the "unexpected" more pulling with less pedal. Sometimes I think the less pedal just keeps the engine RPM in the desired range.

 

[ccsial]

In flying there are two statements I've heard. "Throttle controls altitude" and "Elevator controls altitude". Both are true under certain conditions.

I think ths same thing applies here. In a static condition, the more HST pedal the more torque. If your moving along and the engine is laboring, then you must back off the pedal or go to a lower range.

These little rules are great but there is no substitute for understanding the dynamcis of things.

 

[_RaT_]

Good analogy

 

[Muleskinner]

Think of the HST pedal like a clutch on a truck or car. The more you engage the clutch, the more direct hookup there is between the engine and the transmission. Better yet, think of how a motorcycle dirt racer rides his bike. If the engine lugs under hard pulls, he will pull in the clutch a small amount to let the clutch slip so the engine revs up a bit, putting the engine back up into it's torque range.

The same applies with the HST unit. The HST pedal encreases the hydraulic fluid flow to the transmission stator the more you depress it until it locks the engine to the transmission. If the engine begins to pull down and lug, you let up on the HST pedal decreasing the flow of hydralic fluid to the stator. This allows the engine to rev back up into its torque range creating more power. Just think of it as "letting the clutch slip."

Another way to look at it is using a lock up torque converter in a car or truck. When the converter locks up there is a lock up between the engine and the transmission. (IE no slipage) When the torque converter unlocks, the engine can now rev up a bit as there is no lock up only fluid engagenment.

Clear as mud?

 

[techman]

Theoretically the HST will produce full outrput torque at any speed (pedal position). The torque is set by the refief valve pressure in the HST circuit.

Now for a bit of reality. In any hydraulic system, and in HSTs as well, there is a certain amount of internal leakage flow. The leakage is a value that varies mostly with pressure, not as much with flow. At small pedal inputs (low pump flow) a sizeable percentage of the flow can go into leakage, preventing full pressure from being developed. As the pedal is pushed further, the greater flow now results in the leakage being a small percentage of the total, and full pressure can now be developed.

A good comparison. Take a garden hose and put a small hole in it to represent the HST internal leakage. With the shutoff only cracked open, the available water from the nozzle of the hose is low since part of the water is flowing out of the leak. If you open the shutoff valve fully, the leak is still flowing water, but the full available flow gives a large stream at the nozzle. This is not a perfect comparison since the shutoff is a throttle, not a fixed flow, but the idea is the same.

Bottom line is that at about 10% or less speed input to the pedal, you should have full torque. Some notice that as you push more you get more torque if stalled. This is due to the characteristic of the HST relief valve. As the flow through it goes up, the pressure rises somewhat due to internal flow drops. This increases pressure and torque, but do not hold it in that position for long since you are generating a lot of heat - the more the pedal is pushed, the more heat is generated when stalled. You can easily overheat and breakdown the hydraulic fluid if you "push" stalled for too long.

paul

 

[ccsial]

That makes sense, although, I think of it in terms of voltage and current. /forums/images/graemlins/smirk.gif