HST - How Does It Work?

[Harv]

Ya know, Mark, I was around for that discussion you mentioned, but I guess I was so enamored of my brand new (used) manual shift 'Bota at the time, I just didn't stash it in my gray matter storage. /w3tcompact/icons/tongue.gif

I see that TresCrows has added some actual hardware diagrams to the pot and, by golly, I think I'm getting it this time. /w3tcompact/icons/clever.gif

 

[DFB]

Harv, maybe this can help too. I bookmarked this link Gordon had posted a while back on hydraulics. <A target="_blank" HREF=http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/5-499/toc.htm>http://www.adtdl.army.mil/cgi-bin/atdl.dll/fm/5-499/toc.htm</A>

Chapter 3 covers pumps including fixed and variable displacement pumps along with axial piston pumps.
Chapter 4 covers hydraulic motors.
A very informative site indeed. Color diagrams too./w3tcompact/icons/grin.gif

DFB

 

[DFB]

Mark, thanks for locating that previous thread and posting the link back to it. An interesting discussion for sure!

DFB

 

[Harv]

Great illustrations, DFB!

Thanks.

 

[MossRoad]

So referring to diagram A, the driveshaft passes through the swash plate and turns the pistons, correct?

The only thing the swash plate can do is change it's angle, not rotate. This would explain everything nicely. When I step on the forward or reverse pedals, there is linkage that actuates this arm on the top of my variable volume pump(see attached). It moves in both directions and centers when the pedals are centered. It must be attached to the swash plate.

 

[MChalkley]

MossRoad - It depends on whether you're talking about the pump or the motor. In the illustration, it's pump that's being discussed, so it's the swashplate that moves. With the pump, you're trying to convert the rotational force of the crankshaft into hydraulic pressure and flow, so the crankshaft turns the swashplate, which pushes the pistons in and out, which generates flow.

With the other end of a typical HST transmission, the process is exactly the reverse. The pressure enters the motor, pushing the pistons against the fixed swashplate, causing the motor to turn the hydraulic flow back into rotational force by spinning the block the pistons are mounted in.

Incidentally, if your PowerTrac is designed like the 2465 I looked at, you don't have a single piston-type motor, you have an individual gerotor-type wheel motor at each wheel.

 

[DFB]

For clarification this is the text that should accompany the diagram. The picture certainly doesn't make it clear if the swash plate is non rotational.

In an in-line piston pump (Figure 3-19, diagram A), a drive shaft and cylinder block are on the same centerline. Reciprocation of the pistons is caused by a swash plate that the pistons run against as a cylinder block rotates. A drive shaft turns a cylinder block, which carries the pistons around a shaft. The piston shoes slide against a swash plate and are held against it by a shoe plate. A swash plate's angle causes the cylinders to reciprocate in their bores. At the point where a piston begins to retract, an opening in the end of a bore slides over an inlet slot in a valve plate, and oil is drawn into a bore through somewhat less than half a revolution. There is a solid area in a valve plate as a piston becomes fully retracted. As a piston begins to extend, an opening in a cylinder barrel moves over an outlet slot, and oil is forced out a pressure port.

I'll also scan and post a diagram from my parts manual showing the specific components for my tractor too.

DFB

 

[MChalkley]

DFB - Thanks for the text. It means I sit corrected, according to that. I'll have to go back through my hydraulics books to see if they all work that way, and my memory is faulty (likely), or if some work the way I described.

 

[MossRoad]

If the swash plate rotated with the shaft you couldn't get reverse out of the pump, could you?

 

[DFB]

Mark, I apologize for creating any confusion with the diagram.

From the the same web site, is this the type of pump you described?

Wobble-Plate In-Line Pump. This is a variation of an in-line piston pump. In this design, a cylinder barrel does not turn; a plate wobbles as it turns, and the wobbling pushes the pistons in and out of the pumping chambers in a stationary cylinder barrel. In a wobble-plate pump, separate inlet and outlet check valves are required for each piston, since the pistons do not move past a port.

DFB