Dumb Question about HST

[Iplayfarmer]

Here's one that's more acedemic than anything.

I'm still brainstorming about hydraulic supply for my backhoe, and I was wondering why I couldn't rob the pressure and flow from the hydrostatic transmission to run the backhoe. The tractor won't be moving, so the power isn't needed to the wheels.

I'm assuming that this isn't a good idea or more people would be doing it. I'd like to understand why, though.

 

[dfkrug]

The HST hydraulic pump and the hydraulic motor it drives are usually
inside the same housing. How can you access the output flow of the
pump external to the housing? There are relief valves accessible from
the outside, but that's about it. They are also set at about 4500-5000psi.

 

[Iplayfarmer]

The HST hydraulic pump and the hydraulic motor it drives are usually
inside the same housing. How can you access the output flow of the
pump external to the housing? There are relief valves accessible from
the outside, but that's about it. They are also set at about 4500-5000psi.

Is that the only issue ...getting to the fluid? If there was a way to get the fluid out under pressure would that work?

 

[Wayne_Freeman]

I went with a PTO pump and it works great. No worries about contamination of fluids or strain on the tractor. Just my 2 cents!

 

[J_J]

I went with a PTO pump and it works great. No worries about contamination of fluids or strain on the tractor. Just my 2 cents!

If the HST transmission uses a piston pump, with squash plate to control pressure and volume, you would have to figure a way to control the transmission with out the tractor moving. It could also be a closed hydraulic system, with the fluid going from pump to motor, and back to pump. The hydraulic transmission requires more filtering and therefore you would not want to add fluid from you backhoe. Not a good idea. use external pump for best results.

 

[dfkrug]

If the HST transmission uses a piston pump, with squash plate to control pressure and volume, you would have to figure a way to control the transmission with out the tractor moving. It could also be a closed hydraulic system, with the fluid going from pump to motor, and back to pump. The hydraulic transmission requires more filtering and therefore you would not want to add fluid from you backhoe. Not a good idea. use external pump for best results.

A HST-equipped CUT uses one fluid reservoir for both the HST and implement
pump, so the fluid is shared by the tranny, PS, implements, 3-pt. It is true
that there may be a different suction filter on the inlet of the HST, vs. the
one for the implement pump. My Kioti and Kubota had (have) separate
filters, but none of my JDs had more than one hyd filter.

If the OP somehow found a way to tap into the HST housing and was able
to connect a manual flow diverter that would switch the piston pump output
to his implement, then he could use that flow. He would then have to
connect the OTHER output of his diverter back into the HST when he
wants to move the tractor. He would have to plug off the internal HST
passage, too. Theoretically possible, but completely impractical.

 

[Iplayfarmer]

...Theoretically possible, but completely impractical.

That was my original assumption. Thank you tons, though, for explaining it. I think what I'm after more than anything is a better understanding of the inner workings of a HST.

I've often wondered if there was some way to tap into the HST of one of these old broke down lawn mowers and use it as a pump.