How Does It All Work?

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BobCat

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Because of my limited knowledge of hydraulics. I was wondering if someone could talk a little bit about Axial Piston Pumps. Later Bobcat.
 

Tazza

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You are referring to the variable displacement piston pumps that are used in skid steers i suspect?
I didn't understand them either until i pulled a pump apart, they are so simple in design yet very effective.
This is what i know:
The main section consists if what is known as a rotating group, this is a circular piece of steel with holes machined into it with tight tolerances and have hollow metal “pistons” that run in them, the pistons have a “shoe” that swivels on the end. The whole rotating group is the part that spins around with the pistons facing in line with rotation. The piston shoes run on a yoke that when vertical the pistons do not move back and forth so there is no pressure generated. When the yoke is twisted forward or back wards the pistons generate pressure as they are moving forward in side the rotating group.
There is whats known as a swash plate the rotating group runs on, this is where the pistons force the oil on to. Forward and back wards is determined by which way the yoke is twisted, as the pistons will be forced forward at the top or the bottom of the swash plate.
Here is a picture of a rotating group fitted with pistons. The plate next to it you see is the swash plate.


group.jpg

I hope that is the information you were looking for, if you want anything specific just let me know, and if i know it I'll tell you.
They are very simple devices, but they do have very tight tolerances.
 

140mower

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Jun 4, 2006
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BobCat,
I think Tazza has pretty well nailed one of the best descriptions I've seen. One thing I'll add is that in order for this type of pump to work it needs a charge pump, in this case it is the pump that runs your loader controls. There is a small hole in the mounting plate that "feeds" the hydrostats, this oil cools and lubricates the bigger pumps. It is advised not to pull or tow non-running machines for any distance as there is no charge pressue and the pistons in the hydrostats will over heat and score, ruining a pair of very expensive pumps.
If you want to see what makes one tick inside, but don't want to gamble the ones in your skid, keep your eyes open for an old John Deere garden tractor with hydrostatic drive. An old 317 would be a good choice as they have the same pump in them only smaller. They're even the same make that are in my 763. (Old JD garden tractors are kind of a hobby for me, but I'm trying to get help kicking the buying habit. Wife says the six she can find are enough.)
emotion-4.gif

Don
 

Tazza

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BobCat,
I think Tazza has pretty well nailed one of the best descriptions I've seen. One thing I'll add is that in order for this type of pump to work it needs a charge pump, in this case it is the pump that runs your loader controls. There is a small hole in the mounting plate that "feeds" the hydrostats, this oil cools and lubricates the bigger pumps. It is advised not to pull or tow non-running machines for any distance as there is no charge pressue and the pistons in the hydrostats will over heat and score, ruining a pair of very expensive pumps.
If you want to see what makes one tick inside, but don't want to gamble the ones in your skid, keep your eyes open for an old John Deere garden tractor with hydrostatic drive. An old 317 would be a good choice as they have the same pump in them only smaller. They're even the same make that are in my 763. (Old JD garden tractors are kind of a hobby for me, but I'm trying to get help kicking the buying habit. Wife says the six she can find are enough.)
Don
I did fail to mention the charge pump, i think this is also needed to push the pistons back on to the yoke.
As for cooling, there is always internal leakage inside hydrostatic motors, this oil is fed back through the oil cooler and fresh cool oil enters the system through the charge port or super charge port as my manual calls it.
The best way to understand the system fully is to pull one apart like BobCat said. They are so simple in design but you need to see it to fully understand how it operates. I want to get hold of an old hydrostatic motor so i can pull it apart to see how they work. I know how they *should* work, but i just can't quite see it without pulling it apart.
 

BobCat

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Sep 30, 2006
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124
I did fail to mention the charge pump, i think this is also needed to push the pistons back on to the yoke.
As for cooling, there is always internal leakage inside hydrostatic motors, this oil is fed back through the oil cooler and fresh cool oil enters the system through the charge port or super charge port as my manual calls it.
The best way to understand the system fully is to pull one apart like BobCat said. They are so simple in design but you need to see it to fully understand how it operates. I want to get hold of an old hydrostatic motor so i can pull it apart to see how they work. I know how they *should* work, but i just can't quite see it without pulling it apart.
So the pistons aren't spring loaded? I need to tear one down like you suggested. I do have a 1973 John Deere 300 Hydrostatic, I'm mean the wife does. HUMMMMM??? Thanks guys. Bobcat.
 

Tazza

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So the pistons aren't spring loaded? I need to tear one down like you suggested. I do have a 1973 John Deere 300 Hydrostatic, I'm mean the wife does. HUMMMMM??? Thanks guys. Bobcat.
No, they are not spring loaded, i am pretty sure the charge pressure pushes them back.
 

Luthor

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Nov 15, 2005
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No, they are not spring loaded, i am pretty sure the charge pressure pushes them back.
The pump's purpose is actually to create flow in the hydraulic fluid, resistance to that flow is what causes pressure.
 
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