more power

[dfkrug]

[[[Its easy to fall into that trap.]]] Actually, regardless of how slow it will go, you get no force multiplication beyond the actual gear ratio the HST is delivering torque to. And regardless of the small amt of fluid the HST is pumping at low pedal, the torque output is limited by the relief pressure and the displacement of the hyd motor that the pressure limited fluid is being fed to.

YES! Well put, Larry.

How a HST works is not intuitive for most people. This discussion is
important but controversial because it seems to fly in the face of
"conventional wisdom". Precise terminolgy is neccessary for a clear
understanding. A clutch can increase torque by allowing it to slip
and a torque converter (in a conventional auto trans) increases torque
by the fluid bypassing the impeller/rotor. In both cases, torque
multiplication occurs just like in gear reduction. HST pumps don't work
that way. There are many forums that argue constantly about the
differences between torque (force) and power (work).

It is important to differentiate the force (torque) delivered by the
HST, which is variable, and the MAXIMUM force, which is limited by
the relief valve.

The HST will allow an infinitesimally low output flow and therefore lower
speed, without having to slip the clutch to achieve the same lower speed
on a gear tractor. If I can paraphrase, the gear tractor still wins the
output contest, but loses with maintenance costs (worn clutch).

 

[SPYDERLK]

Are you trying to tell us that a hydro transmission will deliver the same output torque at a low pump swash plate angle as a high pump swashplate angle? [[[And in the case of the NH Class III tractors, HST output torque is not affected by the HST motor swashplate angle being low or high?]]
I am either completely misunderstanding you or I am in absolute and total disagreement, not sure which.

[[[Didnt see that part.]]] Changing the swashplate angle in the motor would change the stroke of its pistons, thereby changing the motor displacement and the amount of torque output per psi. So we do not disagree on the 2nd point. The HST with a variable Hyd motor would still be limited in the same way however, "by the relief pressure and the [max]displacement of the [variable]hyd motor that the pressure limited fluid is being fed to.
larry

 

[RobJ]

If I can paraphrase, the gear tractor still wins the
output contest, but loses with maintenance costs (worn clutch).

I can't agree with the costs comment, clutch for my L costs, $82, add the pressure place for $101. Break the tractor in half on a small one not that big of a deal. Plus the tractor is apart for 30 minutes replacing the 2 parts before you can reassemble. HST, more moving parts galor, pumps, plates. Got to be expensive to repair. Probably not something anyone can do without proper test equipment gauges, etc.

My Grandaddys JD 4020's, 4230's went 20 years without replacing a clutch with 10,000+ hours pulling plows all day in the cotten and bean fields. These are 90-100hp tractors.

 

[dfkrug]

ROBJ:
The point is that the clutch will wear out when you slip it, the HST will not
as there is nothing to slip/wear.

I have just completed a total rebuild of my B21 and I am amazed that
the HST part of the driveline was in spec when the gearboxes, rearend,
clutch, etc all needed major repair/replacement. It leaves one with an
appreciation for how simple and robust the HST is.

 

[RobJ]

I got ya, I only slip my when starting and stoping. Usually not any load on it.

 

[RickB]

ROBJ:
The point is that the clutch will wear out when you slip it, the HST will not
as there is nothing to slip/wear.

Unless they are allowed to cavitate, run hot, or get contaminated. Hydrostatic units cannot be successfully fixed by cutting corners, and a catastrophic hydro failure can make a double clutch replacement seem like a stop at the local coffee shop. Fortunately, that doesn't happen often. Partly because the units are engineered generously for the HP they actually transfer, and also because many owners get tired of looking at their CUT's before they actually wear them out.

 

[Soundguy]

Rick and Rob are on the money. A clutch, used properly, should last a VERY long time. I'v opened many an antique and found original or at least vintage clutches still working fine.

Also.. on the flip side... splitting a tractor and dropping out and stuffing a clutch back in might set me back 150-500$ depending on the machine.. but I can do it in under a day unless I find other damage. I'm afraid I'd have to throw the gloves in if face with rebuilding an HST...

Soundguy

 

[dfkrug]

Too bad you would give up on rebuilding an HST before even trying,
SOUNDGUY.

I have now opened up, analyzed, repaired and rebuilt my first one,
and it was remarkably easy. Doing so was one of the reasons I
recently bought my current Kubota. Now that I got everything working
again, I will post the whole chronicle on another thread.

Don't give up before trying one. Others who have rebuilt more HSTs
than I have will hopefully post here.

I agree that a cared-for clutch should last a very long time. A cared-for
HST should last much longer. For comparison purposes, the clutch
pressure plate is $98, the disk, $82, while a complete HST is $1734.
However, if either the HST pump or motor is worn out, then each
assembly is $393 and easy to replace.

 

[JC-jetro]

Rick and Rob are on the money. A clutch, used properly, should last a VERY long time. I'v opened many an antique and found original or at least vintage clutches still working fine.



Soundguy

I mega ditto that.. below is the picture of my clutch after 28 years of use. It's got many more years left if I give it just a bit of TLC. As you said, splitting and changing an $80 clutch disk is not a major thing. I'll be watching the clutch thickness every once in a while. It is a good think that I have a hand hole with a rubber cap on the tranny allowing me inspection with no disassembly in couple of minutes.

JC,

 

[Soundguy]

I agree that a cared-for clutch should last a very long time. A cared-for
HST should last much longer. .

In fairness.. i think the jurry is still out on that one... While a clutch disc can last 50ys.. seals usually give up the ghost... an HST is plum chok full of seals.. Im not so sure that with age.. that the HST's won't have some sort of 'mandatory' shelf life due to seal material breakdown.. whereas yuo could probably make a clutch last 100ys with no use... while a seal may get hard and crumble.. etc.. )

soundguy