HST differences between brands

[getut]

This post if half question and half statement, but I see in thread after thread on here discussions of HST's and comparing them to automatics and people talking about the infinitely variable speed. Never once have I heard mention that there is actually a minimum speed at which full torque is available even with an HST (at least on my Kioti CK20HST). The question part is... is that behavior unique to Kioti? It is also possible that due to this, just like with a gear tractor, the combination of needed speed and needed torque could be impossible, even with an HST tractor that otherwise has enough oomph. It just may not have enough at a slow enough speed or high enough speed, albeit this is probably much less likely than with a geared tractor.

Using my CK20HST for example (and I have no idea if this same effect is reproducible on other brands or if it is specific to the Kioti CK20): If I slowly start pushing forward on the HST pedal, the torque and speed BOTH build up to about 20% of pedal depression (due to going from no hydraulic flow through the tranny up to the valve being 100% open at about 20% pedal depression). Then after the flow has reached maximum, then normal HST tranny physics are at work just as you guys talk about. See the graph that I made purely with fudged numbers, but just to explain the effect that I see, and that was also described to my by the dealer.

In effect, even HST, or at least MY HST does in fact have a minimum speed at which full torque can be applied... are all of them like this? I suspect so since I can't imagine any way around it.

 

[CTyler]

The available torque and HP is dependant on the engine rpm. Setting the engine at 2600 rpm creates max power in my case. The amount you apply to the ground is then dependant on how far you push the peddle.

For example pulling a box blade with the rippers down. I have on occasion stopped the tractor catching really hard with the tires just spinning. As long as the rpm is up the power is being applied. If you start pulling down the rpm then your applying all the power and then some.

 

[ericinmich]

I'll reply as not an expert on HSTs or tractors, but from my automotive engineering experience.

First, I'm assuming that in your post the term torque referrs to torque at the wheels, not PTOs. If not, please clearify.

Assuming that the engine is at PTO speed, then I would expect that your graph for torque be linear (negative slope) without any peaks. I haven't pulled much (yet) with mine, but at the slowest speed, just touching the pedal, the torque would be greatist.

I'll wait for the real experts...

- Eric

 

[getut]

ericinmich... that is exactly what it sounds like people are saying about HST that it is linear, but in fact it can't be unless I am missing something huge and Kioti also explained it this way to me.

When you aren't depressing the pedal, there is no flow through the transmission, no torque to the wheels and no speed. When you first start depressing the pedal, you are at a point of less than 100% hydraulic fluid flow, so even though the HST internals at that point would be configured for maximum torque and minimum speed, it can't actually achieve maximum torque because flow of fluid through it has not yet reached 100%.

The valve that opens and feeds the HST transmission does not fully open until approxitmately 10-20% pedal input. This is what causes the spike in torque at about that pedal input level, 100% flow has been achieved. From that point on through the rest of the pedal travel, the flow remains at 100% but the HST configuration changes to alter speed/torque.

 

[getut]

I guess another way to look at it like this. The HST pedal does not JUST reconfigure the internals of the HST to alter speed/torque. The first 10-20% of travel progressively opens the valve that provides flow to the HST in the first place. Until that valve is fully open, the HST is not recieving 100% flow (for the given RPM) on the input side.

Edit: This is the way that I understood it when it was explained to me.

 

[CTyler]

Why would you have to have 100% flow thru the HST(pump) to use all the available torque from the engine?

The HST is a pump, the more you press on the peddle the more you open the valve. You don't need the peddle pushed all the way to use all the engine torque.

If your tied to a tree and push the peddle down 20% and pull the engine down until it smokes and the tires are not turning you have applied 100% of the engines torque.

What is there to miss?

 

[getut]

Everywhere above I mention torque, I am talking about torque at the wheels, not engine torque. You will probably get what I am saying if you re-read it with that.

 

[CTyler]

There would be additional flow through the pump the faster the wheels turn. However I don't follow why the torque at the wheels would be less.

 

[getut]

No.. thats where the difference is.. flow through the pump (HST) is the same once the valve opens up (first 10-20% of pedal travel).

Same input flow (100%), but the geometry of that flow changes... large input reservoir/small output path = high torque slow speed, but push the pedal further and you get a small input reservoir/larger output path and you get higher speed but less torque. The amount of flow through the system is the same in both cases though.. speed doesn't change the AMOUNT of flow, just how fast it is flowing.

The case I am talking about is that 20% of pedal input BEFORE the amount of flow reaches 100%.

 

[Bird]

You're talking about the "flow" of hydraulic fluid when it seems to me that the pressure might be a more important factor. /forums/images/graemlins/confused.gif But I'm certainly no expert on the topic.