Easy-Build Double-Acting Bellows

by Melvyn Wright

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Simple Double-Acting Bellows
To make the organs easy to build, one compromise that John Smith had to make in his organ plans was to use single-acting bellows. Commercial organs almost always use double-acting bellows which supply air on both the down stroke and the up stroke. John got around this by using two bellows working in opposition, which has the same effect; but each bellows is only half the width, so the whole system only supplies half the wind of a conventional system.

Double-acting bellows are quite difficult to make for the home builder. There are complicated blocks of wood to machine, and internal wind channels to drill, and the geometry is more complex. But after much thought, I have come up with a double-acting bellows system which can be built just as easily as the John Smith single-acting one. My system will supply approximately twice as much wind as the John Smith system, and it only needs a single crank to operate it.

I have never seen this idea before, but I do not claim to have invented it. In the hundreds of years that organs have been built, I am quite sure that somebody else must have thought of this before me!  I woud be interested to learn if anybody has ever seen this idea before.

Like John's system, my system consists of two separate bellows and a reservoir, but instead of opposing each other, the bellows are connected together and go up and down together. The clever bit is that one of the bellows is mounted upside down and feeds air into the top of the reservoir from above! Both the bellows and the reservoir are full width. The two bellows are connected together by a connecting rod and, because one of the bellows is upside-down, when the bottom bellows is open the top one is closed, and vice versa.

The reservoir is sandwiched between the two bellows, in such a way that the reservoir boards also form the bellows boards. So only four boards are required, as opposed to five in the standard system: 1) Bellows 1 moveable board; 2) Reservoir top board; 3) Reservoir bottom board; 4) Bellows 2 moveable board. The hinge end of the reservoir is at the opposite end from the bellows hinges, so that as the reservoir opens and closes it does not affect the opening and closing of the bellows.

Here is a sketch of the idea:

The bellows and reservoir are all made using the same simple techniques as used in the plans.  Both bellows are identical, and they have simple geometry with both boards being of equal length and converging to a point. Each bellows still has access to the outside air, and feeds air into the reservoir by a flap valve, just like the standard system. The only significant difference is the way one of the bellows is mounted on top of the reservoir.

Only a single crank is required, which makes the crankshaft considerably easier to make, and this is connected to the bottom bellows, as standard. Because of the various fitments required on the reservoir (the output connection, the relief valve, and the pressure spring) it is necessary for the opening end of the reservoir to project slightly beyond the bellows, to provide space for these fitments. So the reservoir will need to be slightly bigger than the bellows (which is not a bad thing!).

In practice, I found that the best arrangement was to take the output connection from the reservoir top board and also mount the whole assembly by the reservoir top board so that the output conection doesn't move (as shown in the above diagram). This means that the reservoir bottom board is the one that moves up and down and the one which carries the relief valve.  There is no reason why the system can't be mounted by the reservoir bottom board and allow the top board to move up and down.

The photos below show a demonstration version that I knocked up quickly out of scrap materials to prove whether the idea worked or not.  It was cut out of a scrap chipboard shelf and some blackout cloth! The dimensions are random and based on the materials I had available.

These are the four boards required. The bellows boards on the left, the reservoir boards on the right. Ignore the very small holes, they don't go all the way through! The flap valves and relief valve are fitted.  Battens are fitted around the outside of the reservoir boards to act as the 'missing' bellows boards. The large hole is the outlet hole

This is how the boards will be arranged. The reservoir is in the middle, and the two bellows are on top and bottom

Covered with blackout cloth. 'The connecting rods have been fitted, and the two battens along the side are for mounting the whole thing into the organ. (See note below)

Another view. Note the outlet hole, and the cut-out in the battens to allow the connecting rods to pass through I knocked up a quick demonstration rig to test it. Note the make-shift pressure spring underneath the reservoir!

 Watch the demonstration video

In this video, the relief valve is not operational. The outlet hole is half open in order to demonstrate the rise and fall of the reservoir within safe limits.  In fact, the pressure relief valve turned out to be much too small to cope with all the surplus wind, even when fully open.  I wasn't expecting so much wind to be produced!

Some Notes
The two bellows could be made as completely separate units, each with two boards, and then each bellows glued onto the reservoir when finished. This would make construction even more similar to the John Smith plans, but would be quite wasteful of wood, and needlessly heavy.

I used solid boards for the construction, but you could just as easily use thin boards with battens around the edges, as in the plans.

When covering, cover the reservoir first and leave to dry.  Then cover one of the bellows, then the other one.

The connecting rods must be at the corners, otherwise when the reservoir opens it will restrict the movement of the bellows.

Instead of the mounting battens at the side, a neater way of mounting it would be to extend the top reservoir board at both the crank end and the hinge end, and mount it by these extensions. A cut-out would need to be made for the connecting rod to pass through to the crankshaft, just like in the original plans. Having already made the reservoir, I couldn't use this method, but I will do it like this in the future.

In my demo, the pressure spring is at the opening end of the reservoir, which is not the best position.  It should really be about half-way along. This can easily be achieved by making the top bellows slightly narrower than the reservoir, and mounting one pressure spring on each side of the reservoir. The springs can then be moved backwards or forwards as required for adjustment.

Note that only a single crank is required, which is a great benefit for those without engineering facilities, as the crank can be a simple one on the end of the crankshaft.

This bellows assembly is slightly taller than the John Smith bellows, but they provide almost double the wind because both bellows are full width. Therefore they are not direct replacements for those in John Smith's plans.  To use these bellows you would need to increase the height of the case slightly for them to fit.  They could possibly be made to fit in place of the standard bellows by reducing the opening angle and the stroke of the crank, and even then they would still provide more wind.  It's up to you to experiment!

I would not recommend anybody use my design when making their first organ. Stick to the plans to get the experience necessary. Then perhaps experiment with my design on subsequent organs.  They would work well in the Universal, as that is rather short of wind when all the stops are pulled!

Note that these bellows are not quite as neat as conventional double-acting bellows because they take up a bit more room. But they are much easier to make and they do supply roughly the same amount of wind.  Other advantages are that the outlet port is stationary so the air connection can be fixed; and the relief valve is mounted upside down so that it is held closed by the air pressure and by gravity. No spring required.

Major update coming soon!

Have fun!

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