Making Double-Acting Bellows for the John Smith Organs

by Melvyn Wright

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Making Double-Acting Bellows for the John Smith Organs
The main purpose of this article is to demonstrate how easy it is to make double-acting bellows to my new design (shown on this page). But rather than build a useless demonstration version, I decided to build a version that could be put to use afterwards.  So I made these bellows with dimensions compatible with the Busker plans.  They are 11.5" long and 7" wide.  A larger version could just as easily be made for the Universal organ.  In each case they will supply more than double the amount of wind.  They are unashamedly made from 1/2" thick MDF.  MDF is perfect for making bellows as it is strong, doesn't warp or split, and accepts holes with perfect edges.  It does need sealing well though.  1/2" thick MDF is used to provide a large gluing area for the leather around the edges, but thinner could be used if extra thickness is provided by battens arouind the edges.

I deliberately made these bellows as three separate units, to show how easy they are for a beginner to make. However, some economies can be made if the units are combined together, and then covered as a whole unit. For instance two of the boards could be replaced by glueing battens around one of the other boards.  You can decide for yourself exactly how you want to make it.  In case of difficulty, refer to the Busker plans, as the construction techniques are exactly the same as the ones in the plans. In fact these are easier to make than the ones in the plans, where the reservoir floor is formed from the top of the two bellows and a sealing strip.

See also my separate article on how to make and cover bellows


Diagram of the finished double-acting pump

Making the Bellows

Left: Cut out the six boards from ½" thick MDF: Bellows1 top and bottom and Reservoir top and bottom are all 11.5" x 7". Bellows2 top and bottom are 9.5" x 7". Label the boards at the top left to prevent them getting mixed up, or disorientated.  This is essential otherwise you will get hopelessly confused!.

Right: Cut out the centre of two of the Bellows boards (Bellows 1 top and Bellows 2 bottom) leaving a square framework. A jigsaw or scroll saw is suitable for this job.  Alternatively, these two boards can be replaced by battens forming a rectangle, but I found it easier, quicker, and stronger to make the framework from solid MDF. (MDF is cheap!) The centre cut-outs can always be used to make other parts.  The purpose of these is to clear the flap valves in the adjoining boards.


Left: Drill six holes (not four as shown) in each of the solid boards with a 20mm flat bit. The position of these holes is not important, I put them in the centre.  Drill from each side of the MDF to avoid splintering and breakout of the surface. Sand flat.

Right: Fit the four leather flap valves in the four solid boards. See the diagram at the top for where they fit. Notice that two of them go above the board, and two go below the board.  I used leather strips 1.5" wide.  Lay the strip over the holes and glue it at both ends.


Left: Take the top reservoir board and drill a large hole with a flat bit, (suggest 25mm) to take the air supply tube to the pressure box. Alongside this hole, cut out a square hole 25 x 40mm for the spill valve. Both of these holes must be close to the front edge of the board, outside the area of the top bellows and opposite the hinge.  I put them on a 1" centre line from the edge.

Make the spill valve (see my article for making a suitable spill valve) but do not fix it on permanently yet.  The bellows will be much easier to cover if the spill valve is not fitted. Because of the movement of the reservoir, the spill valve is operated by a length of cord rather than a fixed stop, so make sure that the operating lever is long enough to extend over the side of the reservoir.

Right: Make and fit the connecting rod to the front of the bellows. Screwing into the edge of MDF may not provide a strong enough connection for the connecting rod, so take a piece of hardwood and glue it into position inside the front edge of Bellows 1 top.  It is important that this piece of wood does not protrude above the surface of the MDF as that would cause leaks when the board is later glued to the reservoir.  When dry, drill a pilot hole and fix the pivot to the bellows with a No.8 screw. When satisfied with the fit, remove it again as it will make the bellows easier to cover.  Depending upon your crankshaft, you may wish to move the pivot along to coincide with the horizontal position of the crank. Realise that you can still get full access to the back of this connection after the bellows have been covered.


The three spacers added at the hinge ends

The leather hinges glued on

Set your bandsaw or table saw to ½" in preparation for cutting lots of ½" strips from 3mm MDF. You will need around 15 feet of this to go around the periphery of all the bellows, but they do not need to be particularly accurate or even. You will need 8 x 11.5", 4 x 9.5", and 9 x 7" strips.

Left: Take three of the 7" strips and glue them at the hinge ends of each of the bellows (Bellows 1 bottom, Bellows 2 bottom, Reservoir bottom). These act as spacers between the two boards. Make sure they do not protrude over the sides.

That finishes the basic construction. Now the internal surfaces of each of the bellows and reservoir need to be sealed with sealer. These are the same four surfaces that bear the flap valves. Brush the sealer over the entire surface of the boards, but take care not to allow any sealer to get underneath the flap valves, or into the air holes. The sealer may harden to a rough surface and prevent the valves from working properly.  Apply more coats as necessary, the more the better!

Right: Cut out three 7" strips of thick leather and glue them on to form the external hinges.  This operation is made much easier if you first align the pairs of boards and clamp them together.  When dry, roughen up the smooth surface of the leather with sandpaper, as the bellows covering will be glued onto here, and the glue won't stick so well onto the shiny surface.


The three bellows units ready to be covered.  Loosely placed as they will be arranged.

Covering the Bellows
See my other article on making and covering bellows. (Click here).  These bellows are 11.5" and 9.5" long, and are all 7" wide, but you can make them any size you like.  I made the bellows and reservoir openings all 2.5". Two of the bellows have one of the boards cut away, which makes it particularly easy to cover them, as you have full access to the interior while they are being covered.

The three covered bellows 

Now you can pin the 1/2" wide strips that you prepared earlier around the sides of the bellows. These hold the cloth/leather in place and also prevent the stiffeners from forcing their way out.  It is important to ensure that these strips do not project above the surface of the bellows boards, as that would prevent them from being glued together when it comes to assembling the complete unit.   When dry, trim off any excess leather/cloth.

The edging strips are now pinned on, and the spill valve and conecting rod can now be re-fitted

Glueing them all together
The three separate bellows can now be glued together.  Make sure that the mating surfaces are completely flat and sanded smooth, to avoid any air leaks. I cut out two cardboard gaskets to go between the bellows, to absorb any unevenness, but this is probably not necessary.  However, the gaskets do provide a weak link to enable the units to be separated in the future if necessary (I'm not sure how easy this would be!)  Assemble the units from the bottom up. Apply plenty of glue to the surfaces to make sure there are no leaks. Glue the reservoir onto the bottom bellows, then the top bellows onto the reservoir. Make sure the reservoir hinge is at the opposite end!  Clamp everything up and then leave the glue for 24 hours to dry completely.  Do not operate any of the bellows or reservoir until the glue is completely dry as that could force air through the joints, causing a leak that would be impossible to fix.

Cardboard gaskets (optional, but recommended)

The whole unit clamped up.  Notice how it is like a completely flat solid block at this stage, which makes clamping easy.


Side Stays
Now all that remains is to fit the two side stays. These should go as near as possible to the front corners. If you have made the bellows boards from thick plywood you can screw the stays straight into this, otherwise fit four wooden blocks to the corners of the bellows and drill pilot holes to fit the screws you will be using. Preferably screw these into the side grain rather than the end grain.  I used No.6 x 1" screws. A neater method would be to recess four wooden dowel plugs into the edges of the MDF and screw the stays into these, but the block method is quite satisfactory although doesn't look as neat.  The blocks should overhang the sides of the boards slightly so that the moving boards do not rub against the stays. The length of the stays should be such that one bellows is completely closed and the other one is completely open, so fix the stay at one end and mark it off against the opposite block.  Drill the screw holes and fit, with spacing washers as necessary. In my case the distance between the top and bottom bellows corners turned out to be 6". See the photo below for another method of making the stays.

The three bellows now glued  together

The two side stays fitted

Finish off by attaching the cord which operates the spill valve.  It is attached to a screw eye in the bottom reservoir board.  This should start to open the valve when the reservoir is about 3/4 full, but by all means experiment to find the best position.

This alternative mounting for the connecting rod is similar to the one in the JS plans, and much more compact. Just a simple angle bracket

Alternative side stays. These are made from 3mm studding and locknuts.  This enables them to be easily adjusted as necessary


The completed unit
Note the operating cord, the pressure spring locating strip on the top, and the mounting strips underneath


Fitting into the Organ
The fitting is slightly different from the original bellows, as the mounting is by the base, rather than the centre boards. So you will probably need to glue two strips underneath the bottom bellows to mount it into the organ.

If you have already built the organ and find that the height of the complete unit is too tall to fit in the original space, the stays can be reduced in length.  This will reduce the wind suppply slightly, but remember that this unit will supply double the amount of wind compared to the original bellows, so this shouldn't be a problem.

The complicated double or triple crankshaft is no longer needed.  All that is needed is a single crank on the end of the rod. You could still keep the original crankshaft, and only use one of the cranks.  Make sure that the crank throw is not too large for the bellows movement. If you are making a new crank measure the possible bellows movement and make the crank to fit.

The stop block that operates the spill valve is no longer required.  I used the same pressure spring, but instead of mounting it underneath the pressure box where the movement is a bit limited, I mounted it on the front of the pressure box.  There's plenty of room and height there for the spring to move freely.  The spring tension will also be more constant if the other end of the spring is mounted higher up.


Comparison between the new double-action pump and the old original one removed from my 11-year old Busker organ! You can see that the overall size is roughly the same.  The new reservoir is slightly higher than the old one due to the stay blocks glued onto the top, but that could be reduced by shortening the stays by an inch.  Seeing the two units side by side makes it clearer how much greater the capacity of the double-action one is. Each of the bellows is larger than the pair of original bellows, due to the gap in between them; and you can see that the new reservoir is full size compared to the original one which is much narrower.  It wouldn't surprise me if the new unit could be reduced to about half the size and it would still provide more wind than the original.  It could certainly be cut down quite a lot, if necessary.

Small Confession!
When I tried to fit the above bellows into my Busker organ they fitted exactly - a bit too exactly actually! When measuring, I hadn't left enough room for the connecting rod to swing backwards and forwards, and when I came to re-install the bass pipes I found they would no longer fit in the space!

No problem, the bellows are easy to make, so I just made another set, but a bit smaller this time.  The photos above are of the larger version, but don't worry, all the measurements given above are for the smaller version which fitted!

As you can see there is plenty of space for these bellows, and they fit very neatly in the space of the original.

Bellows number 2 fitted into my 11-year old Busker Organ

Click to see a demonstration video of the bellows in action

One of the bass pipes is not fitted in this video, as it would obscure the view of the bellows, that's why you can hear some strange puffs of air throughout the tune, with air blowing into the non-existent pipe!  Note that the reservoir is constantly full, (watch the spill valve behind the outlet tube). By the way, this organ hasn't been tuned since it was built over 11 years ago!

As suspected, the new bellows do supply far more wind than necessary.  The consequence of this is that leaks no longer matter! The organ could possibly be made quite a bit smaller.  Or more usefully, the drive ratio could be changed so that you only have to crank at half the speed. That would be a real advantage.  It would mean increasing the diameter of the drive wheel on the crankshaft to make the roll move faster.  I think that's worth trying when I get an hour to spare!

Click here to see my general article on the design of these bellows.

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