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Function of the gap inside the displacement chamber of a constant speed drive?
4

Function of the gap inside the displacement chamber of a constant speed drive?

Function of the gap inside the displacement chamber of a constant speed drive?

(OP)

Is this for thermal expansion of the fluid? The piston has a constant size, so when it reaches the bottom, there would be a tiny gap around the end of the piston. Below is an Axial Piston Pump that also has an expansion in the displacement chamber, but it's located in the middle of the chamber. not sure if they share a similar function.

RE: Function of the gap inside the displacement chamber of a constant speed drive?

2
I think the best term for what you are describing is "unswept cylinder volume. In your top picture the clearance allows for easier manufacturing as you cannot hone a cylinder all the way to a closed blind end. In the bottom picture, energy is saved by not having a longer than needed tight tolerance seal where there is intense shear in the hydraulic oil.

RE: Function of the gap inside the displacement chamber of a constant speed drive?

2
Supporting previous comment. It is an undercut help with the manufacturing. The bore that the piston sits in might be sleeved and the undercut avoids having to clean the bottom face up to make sure that the sleeve sits flush or under the face of the piston block at the slipper end.

If there is no sleeve, it still helps to make sure that there are no burrs or non clean up at the bottom of the galleries.

There are no fluid dynamic or thermodynamics reasons for it, just manufacturing.

RE: Function of the gap inside the displacement chamber of a constant speed drive?

I would agree, it's for manufacturing purpose.

Ted

RE: Function of the gap inside the displacement chamber of a constant speed drive?

(OP)
Thanks, guys. By "Unswept cylinder volume", are you suggesting that the piston does not go all the way to the bottom (highlighted area)? It is sleeved. Before the sleeve is installed, I guess the bore is a constant size all the way down. However, I could be wrong—they might use another method to finish the highlighted area. But why does the sleeve have to sit flush?

Is honing the main finishing method for this type of application? The tolerance is incredibly tight, something like .00005''

RE: Function of the gap inside the displacement chamber of a constant speed drive?

Unswept volume is oil that is not displaced by the movement of the piston.

Each of the 9 sleeves and pistons have to be the same in terms of position and interaction with the slipper plate and the response to the forces applied when the pump is running. They all need to be the same length and in same axial position, otherwise the pump will be unbalanced. Flush is the easiest way to make them all the same and there needs to be enough room at the bottom of the pocket and machining it out would be expensive and time consuming.

The piston and sleeve are often lapped to maintain the radial gap. It’s higher accuracy than honing.

RE: Function of the gap inside the displacement chamber of a constant speed drive?

(OP)
I did find a note pointing to the bottom of the sleeve and it allows for material to loss/degrade from the bottom of the bore, .120 max. I think this note is there due to the same reason you initially stated.

I just realized that the sleeve is a bi-metal cast to the housing, I initially thought it was thermal press fit. The sleeve and the base plate are initially one single piece of casting (see figure below).


So the initial highlighted area was machined away. In this case, it would be easier for the sleeves to go all the way to the bottom of the bore, correct? Like the figure below. You won't need to worry about the bottom being flushed or not since its casted.


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