How does HST "speed control pedal" control engine rpm?

[RangerGress]

Potentially the dumbest question of the week.

I recently bought a used L3800. I had expected the HST pedal to control engine rpm, but that is totally not happening. The HST is only controlling forward/reverse and surely that's not right. Hand throttle lever controls engine rpm fine. I followed thehand throttle lever linkage to the lower right front of the engine to a device that is likely to control fuel rate so that all made sense, but there seems to be no similar connection between HST pedal and that same little device. So I'm thinking:

1. There's some linkage bit missing that connects the HST pedal to the same fuel rate device, but dang I sure don't see where this mystery linkage bit would connect. I don't see anything disconnected and hanging free, nor do I see fastening points that would suggest a missing piece.
2. The HST is pedal is designed to solely control forward/reverse and the hand throttle controls rpm--which seems an unlikely design.
3. I am totally missing something.

I carefully went thru the both the owners manual and service manual and found no references to throttle linkage.

Looking over HST pedal linkages carefully, I see it changing state on a switch, connected to a small hydraulic cylinder that prob keeps it centered in neutral, and a connection to the tranny which would be for the F/R action. I see no connection to anything that looks throttle-ish.

Thoughts?

 

[Soopitup]

2. The HST is pedal is designed to solely control forward/reverse and the hand throttle controls rpm--which seems an unlikely design.

This

The throttle controls engine RPM. The HST pedal allows you to use engine RPM up to the limit you've selected.

 

[old and tired]

HST petal only regulates how much oil is used to go forward and reverse... Engine RPM's is a totally different controller, the hand throttle, and has nothing to do with what the transmission does.

If you go over pressure, it goes into bypass... so sometimes to get more power with the transmission, you actually back OFF the HST petal.

 

[kubotafreak]

You are referencing a kubota feature called auto throttle. Usually reserved for premium tier hst. Like the lxx40 or lxx60 series hst transmissions. You would see an adjustment for hst gain either knob, or digital display if you had the feature...

On your tractor you simulate this with higher idle rpm. Then when you use the hst pedal it is more sensitive, and has a wider range if you will...

Some of the kubota mowers have the auto-throttle feature, if that was something you had in the past.

 

[RangerGress]

This

The throttle controls engine RPM. The HST pedal allows you to use engine RPM up to the limit you've selected.

Wow! The hand throttle is the sole control of RPM. Dang, I didn't see that coming. I figured the very idea would have everyone laughing at me.

I noted that the owners manual referred to the HST pedal as "speed control pedal." That helped confuse me.

Thanks to everyone that posted answers in the thread. I'll quit trying to fix what isn't broke--advice from my father 50yrs ago.

 

[mcmxi]

Wow! The hand throttle is the sole control of RPM. Dang, I didn't see that coming. I figured the very idea would have everyone laughing at me.

I noted that the owners manual referred to the HST pedal as "speed control pedal." That helped confuse me.

Thanks to everyone that posted answers in the thread. I'll quit trying to fix what isn't broke--advice from my father 50yrs ago.

You can think of the accelerator on a regular HST as being like a gear selector. The more you depress it the higher the gear. Google Messicks on YouTube and you'll see that Neil has an episode about this very thing and it's his observation re the accelerator acting like a gear selector. A common mistake is to depress the accelerator too much when digging into a pile of dirt thinking that you'll have more power, but you have less since the tractor is now in a "higher gear". A CVT with pulleys and a belt is another analogy that can help conceptually with HST function and performance.

The engine spins the hydraulic pump at some rpm, and the higher the engine rpm the more work the pump can do because it's able to move more oil. So for every rpm setting you have a range of work that can be done based on oil flow rate. As the accelerator is depressed I'm thinking that it changes the angle of the swash plate and it increases the amount of oil flow. So varying the oil flow is like having an infinite number of gears between two extremes. This might be simplistic but it's close enough .... until someone says it isn't. In 3, 2, 1 ......

 

[Soopitup]

Wow! The hand throttle is the sole control of RPM. Dang, I didn't see that coming. I figured the very idea would have everyone laughing at me.

I noted that the owners manual referred to the HST pedal as "speed control pedal." That helped confuse me.

Thanks to everyone that posted answers in the thread. I'll quit trying to fix what isn't broke--advice from my father 50yrs ago.

It's hard to learn if you don't question things.

I think that's somewhat of a general setup for mobile hydraulic equipment.
You need a way to increase engine speed without moving the vehicle.
There are other ways to do it, this is how it's done on these.

 

[old and tired]

One other thing... feel free to throw out more questions, even if you think they are the dumbest ones... and that goes for anyone reading these replies. Easy way to learn about tractors!!!

 

[Trimley]

Speed is compared to water flow of a hose bib. Off (stopped) when valve closed. Open a little (move slow), get a little. Open more (go faster), get more.

 

[PaulL]

The other end of the understanding that took me a while to notice. The hand throttle is setting desired RPM, not a throttle opening. The governor actually controls throttle opening (well, I guess injectors), and it's working to keep at the exact RPMs that you set.

So actually neither of the controls is a throttle at all - the hand "throttle" is really an RPM set point, which the governor tries to then maintain by increasing or reducing the actual throttle. The HST pedal is a continuously variable gear stick, it's changing your gear ratio as you press it.