by Howard Wyman
THE PRESSURE BELLOWS
Now that the pipes and windchest have been installed in the case, this
seemed to be a good time to build the pressure bellows. Some builders
have opted to use a blower to provide the wind supply for the pipes but I
preferred to use the traditional method. It would definitely be easier
to use a blower but in any case, a reservoir is needed whatever wind
supply is used. The reservoir with its spill valve provides a constant
pressure which is required for the pipes.
There are two nearly identical bellows sections which sit next to each
other with the reservoir mounted on top. Each of the sections has a
stationary top board and bottom board and a movable center board. The
center board is moved up and down by means of a wooden rod connected to
one of the lobes on the crankshaft. As the board moves it causes air to
be drawn into one of the bellows chambers while at the same time air is
being pushed out of the chamber on the other side of the center board.
This air is pushed into the reservoir. Leather check valves keep the air
from going back into the bellows when the center board starts moving in
the other direction. As I said earlier, there is another almost
identical unit sitting next to this one and its center board is connected
to another lobe on the crankshaft which is 90 degrees rotated from the
first one. This means that as the crankshaft rotates there is an almost
constant air flow going into the reservoir. I say almost constant
because there are still fluctuations in the flow. It is the job of the
reservoir to even out these fluctuations and also set the pressure at the
desired level. In PHOTO A one can see the pressure bellows at the
bottom of the picture. The bottom board cannot be seen because it is
below the white board which is part of the back of the cabinet. The
movable center board is just above this white board and one can see the
slot in the edge of it which is the air intake port. The flexible
leather sides of the bellows are black cowhide. In this picture the
crankshaft is not turning and so the reservoir is closed. The top of the
reservoir is being held down by three large leaf springs. In PHOTO B
one can see the reservoir in the full open position because the
crankshaft is turning and air is being pumped into the reservoir. The
top of the reservoir pushes against the three springs and the level of
pressure is set by adjusting the amount of tension in the springs. In
Photo A the spill valve can be seen sitting between the springs. When
the top board of the reservoir reaches its maximum height the arm of the
spill valve strikes the wooden bar causing the valve to uncover a hole in
the top board and exhaust the excess air. PHOTO C shows another view of
the bellows and reservoir. This photo shows more clearly the crankshaft
and the wooden rods going to the bellows. The crankshaft is supported by
the two large wooden bearings which can be seen in this picture. The
rear of the cabinet itself serves as the third bearing for the
crankshaft. In this picture that part of the cabinet has not yet been
installed
Most of my experience with player pianos, etc. has been with vacuum
bellows. Since those bellows are operating as a suction device the
flexible covering just naturally is pulled inward as the bellows operate.
However, in the case of pressure bellows such as the ones we are
building here, as the bellows closes it causes the leather to be blown
outward. Because of this it is necessary to glue stiffeners to the
inside of the leather covering. These can be thin wood or stiff
cardboard. I have been told that Formica works quite well for this
purpose also.
Next we will tackle the vacuum bellows and reservoir.
Editors note: Howard is a retired electrical engineer and lives in Florida. Most of his career was at the Army Night Vision and Electro-Optics Laboratory. He became involved in mechanical music with the purchase of a non-working player piano. As you will see in his articles, Howard is a highly skilled craftsman. Building your own band organ is a real accomplishment and Howard does beautiful work. Howard can be contacted at: hwyman@tampabay.rr.com