BUILDING A WURLITZER 105 BAND ORGAN - Part 10
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: firstname.lastname@example.org