A recent article concerning converting Duo-Art cross valves to the
modern round valve replacement has sparked an old interest of mine.
Modern day rebuilders keep reinventing the wheel, or the valve, as it
were. I am sure that the designers (in the early years) of pneumatic
devices had a "desk" reference manual defining certain ratios in valve
design. Those ratios would have specified the working area of the
valve in respect to the working area of the pouch.
The working area of the valve is the opening of the valve seat. The
working area of the pouch is the contact area of the pouch with the
valve. The contact area is usually the pouch disk or the valve lifter
button. In the case of the old Duo-Art cross valve, the valve seat is
.1468 square inches, while the contact area of the lifter button is
.3066 square inches. This gives a ratio of about .478. Making the
ratio a smaller number will give faster valve response. Conversely,
as the ratio approaches the number "1.00", the valve will at some point
ceases to function.
When one replaces the cross valve seat with the round valve seat,
the area of opening increases by 25 %. The round valve seat has a
working area of .1962 square inches. This drastically changes the
original ratio. The new ratio is now .639. If nothing else is done to
bring the ratio back to original design specifications, the new round
valve will be much slower to activate. The pouch will simply be too
weak to give fast response. The results are poor repetition and poor
soft note reliability.
I gave a technical session on this topic at the 1983 Chicago AMICA
convention. The conclusions there still remain valid. One cannot
change one part of the valve design formula without adjusting the other
parameters of the valve equation and have no change in valve
performance. Those other parameters are: 1. overall mass of the
movable valve, 2. the working area of the pouch, 3. the gravitational
forces on the valve.
Cross valves can be terrible because they tend to leak. They do have
incredibly fast response. Merely replacing cross valves with round
valve seats will dramatically slow down the valve response time, if all
other parameters remain the same. The original ratio of .4787 can be
restored by placing a lifter disk on the pouch. The lifter disk must
have an area of .4098 square inches. This can be achieved with a 3/4
inch disk, which is very slightly over the size needed.
Perhaps each rebuilder could submit ratios for the work they have in
progress. The ratios could be the beginning of a new "desk" reference
showing ratios of various specific designs. Only valves with bleeds
need be considered. The ratio is the open area of the inside valve
seat divided by the area of the pouch lifter disk or pouch button. The
area of the pouch itself is not important.