Theory Question - Piston 1st Stage Balancing

[guruboy]

Re-reading Regulator Savvy by Wolfinger. Having trouble understanding some of the piston balancing details as they are described. This is on page 31 in the book.

The balancing occurs because the force on the piston stem at the o-ring is the same as the force on the piston at the hard seat. However, Wolfinger says that since the diameter of the hard seat is physically smaller due to the presence of an edge, the forces are actually slightly imbalanced. I understand this concept but I'm having trouble understanding this diameter thing.

Is he talking about outer diameter or inner diameter? Is the hard seat edge a single or dual bevel design?

If the hard seat is a dual bevel, I could see how the outer diameter would be slightly reduced and cause a minor imbalance.

However, if the hard seat is a single edge on the inside, the outside of the piston should be able to remain the full diameter. This would not cause the slight imbalance Wolfinger mentions. Assuming pistons are machine made, it would make the most sense to me to have the hard seat made with a single edge bevel on the inside.

 

[redacted]

Yes, it is a single beveled edge. I also had some trouble with that explanation. I suspect the imbalance that is seen is related to the pressure on the piston stem o-ring and friction between that o-ring and the piston stem.

 

[halocline]

Even with a single bevel and a very sharp edge, there is some difference between the inside and outside edge, so there's a tiny area for HP air to "push" on the piston. But probably not nearly enough to produce the 7-10 PSI difference in the MK10 over the supply range. The only explanation I can think of is what awap said, increased friction at higher supply pressure due to pressure on the piston stem o-ring. Later SP balanced piston 1st stages use bushings to minimize extrusion of this o-ring and they also have less IP swing between full and empty tanks.