Help Untwist my HST Brain, please...

Henro · Feb 25, 2004

KevinJ,

I agree completely [almost]. Once a PRV valve pops, it holds the pressure of the system at it poping point, or at least somewhere above zero, so there would still be pressure to move a loader or other hydraulic cylinder. But the engine would not stall. IF for somereason the PRV valve did not operate in a single pump tractor, then the tractor would probably stall...

I think single pump tractors will not stall out for this reason, when their HST PRVs operate, but two pump tractors will.

This never registered in my mind...but now I see the light... /forums/images/graemlins/smile.gif

jrlichina · Feb 25, 2004

The number of pumps depends on the tractor. The HST unit has its own pump(variable displacement) and relief valve, there is separate implement pump(fixed displacement) and its own relief valve, and some have a separate PS pump(fixed displacement) with a priority valve and relief valve. If it has two pumps they are usually housed as a "single" double pump. So it looks like one but actually has two separate pump sections. FYI the BX only has one pump that powers both implements and the PS. The output of the pump first goes to the priority valve which gives fluid flow priority to the steering, then any remaining flow to the implements. They want you to be able to steer at all times which is a good thing.

Tractors4u · Feb 25, 2004

I run my gear tractor in High range, first gear and have yet to stall it working the loader. I am not just talking about scooping loose material either. I am breaking red clay.

slowzuki · Feb 25, 2004

The ps/implement pump (s) aren't on the same circuit as the Hst. Sometimes the ps/implements share a pump with a priority valve for ps. This is fun as steering will stop your loader or 3 pt in its tracks.

The point is curling a bucket or lifting the arms puts another load on the engine on top of the hst. This can be the proverbial straw that stalls the engine.

Ken

( KevinJ,
I agree completely [almost]. Once a PRV valve pops, it holds the pressure of the system at it poping point, or at least somewhere above zero, so there would still be pressure to move a loader or other hydraulic cylinder. But the engine would not stall. IF for somereason the PRV valve did not operate in a single pump tractor, then the tractor would probably stall...

I think single pump tractors will not stall out for this reason, when their HST PRVs operate, but two pump tractors will.

This never registered in my mind...but now I see the light... /forums/images/graemlins/smile.gif
)

Rick Jay · Feb 25, 2004

Henro,

( What am I missing?)

Obviously, an L series. Any self-respecting statistician/tractor owner will tell you that you need at least 3 tractors to make a valid experimental conclusion. You only have a BX and a B. You need the L now more than ever. And you might as well get it with a cab and lots of lights. /forums/images/graemlins/grin.gif

Seriously...I'm betting it has something to do with the mechanical advantage/disadvantage of the rear gearing but I'm not sure. Did anyone sleep at a Holiday Inn Express last night? Maybe they could answer. /forums/images/graemlins/grin.gif

~Rick

slowzuki · Feb 25, 2004

The rear gearing or range regulates what rim pull can be generated before relief. Obviously low range gives you more rim pull.

Your traction/weight will regulate the relief vs spin equation that also happens.

I tried to sketch some graphs to show what is going on but you end up with like 3 graphs with multiple curves.

It's a good old dynamics course with hydraulics rolled in to boot!
Ken

( Seriously...I'm betting it has something to do with the mechanical advantage/disadvantage of the rear gearing but I'm not sure. ~Rick

)

frank_f15 · Feb 25, 2004

MY HEAD HURTS! WAY TO MUCH TECHINICAL FOR ME /forums/images/graemlins/grin.gif

Henro · Feb 25, 2004

Seriously...I'm betting it has something to do with the mechanical advantage/disadvantage of the rear gearing but I'm not sure. Did anyone sleep at a Holiday Inn Express last night? Maybe they could answer /forums/images/graemlins/grin.gif

Well, 5030, you want to speak up??? [inside joke... /forums/images/graemlins/smile.gif ]

Rick, you have to be right...I certainly need that L series... /forums/images/graemlins/grin.gif

slowzuki,

I'm back at that tree now...for pretend...

This time my wheels are not only locked to the ground by the weight of the tractor, heaping loader bucket and backhoe, but I also have chained the front bucket to the tree so tightly that it will not budge.

Now I find that the my imaginary HST will whine when I try to move forward in any range I put it in, but does not stall.

If I leave off the directional pedal and try to lift the loader bucket, it will not lift but I hear a hiss of the PRV when the PRV activates...the engine does not stall.

I push the directional pedal, the HST whines, the engine does not die. But then I try to lift the loader...the backhoe weight prevents the back of the tractor from coming up...the engine starts to die and stalls if I don't lift my foot off the HST pedal or remove my hand from the loader control stick.

Both loads are felt by the engine, combined they are more than the torque of the engine can handle, so it bogs down and finally stalls...but for either one of them alone, the engine is happy to pump through the relief valve and back to the tank.

This is essentially what you are saying also, right?

kevinj · Feb 25, 2004

Henro,

You have way to much time to think !!!!!!!!!!!

JerryG · Feb 25, 2004

Henro,

You have way to much time to think !!!!!!!!!!! 

That is what I have been thinking for about a week now. If the weather doesn't get better to where Bill can get out, he is going to have all of us dead before long. /forums/images/graemlins/grin.gifROPS/forums/images/graemlins/grin.gif