why Hollow vs full pistons in variable pumps and motors

[kebecano]

Hi all, my students asked me why some pisons in carbiable piston pump motors and pumps are hollow, and why others are full.???

I have a couple of ideas why, but I'like input on this as some people have mentioned its just the way it is, but I honestly think that manufacturers won't spend extra money on machining a hole in the piston or pay for extra metal if it does not serve a purpose.

I was think hollow type pistons might decrease the the rotating mass (inertia) of the rotating group (cylinder block and pistons) but......


Thanks a lot

 

[MikeHalloran]

In many of the VD pumps I've seen, the slipper shoes are hydrostatically pressurized. Flow through a hydrostatic bearing is controlled by an orifice. Drilling a long orifice is very difficult. Drilling a short orifice at the bottom of a fat hole is much easier.

I'm sure that the reduced mass helps keep the shoes on the swashplate at high speeds, too.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydromech]

I was once lucky enough to ask that question of lead design engineer at Parker Hannifin directly.

They are hollow to reduce material cost and to make them easier to manfacture...lube hole in the middle.

The trade off is in the higher inertia. The higher inertia provides a flywheel effect aiding pump stability.

Adrian

 

[budt]

Instructors at another company school said they were hollow so they could expand like any hollow tube at pressure. The expansion was to make the pump more efficient since it reduced bypass as pressure increased and expanded the piston and kept a tight fit in the bore walls.

I also heard that most were made with Powdered Metal and that type production and tempering process worked better with less mass.

Probably just how a Draftsman from one company drew the first one since it looked good on paper.


Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING