Alternative Drive Construction

by Wallace Venable

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In building my "Senior 20" Organ I've generally followed John Smith's plans closely as far as dimensions go, but I've adopted a few alternatives in assembling the drive. I report them as methods which fit with my parts availability, skills, and tools, not as "improvements." Since I'm building in the United States of America, I'm working primarily in old-fashioned inch measurements and raw material sizes, and so I have used inches in my descriptions.

2-Piece Crankshaft
I decided to fabricate a 2-piece crankshaft. This is to facilitate parts installation. The main part of the shaft from the handcrank through the three bearings is brazed into one piece for rigidity. For easy alignment, the portion of the main shaft extending through the central throw was cut after brazing. I used ¾ inch by ¼ inch steel side plates, rather than the 1 inch by ¼ inch pieces described, in order to reduce the width of the slot required in the pressure box front panel. I could not find a specification for the rod diameter in the instruction package, so I chose 3/8 inch steel rod.

I made the front single-sided crank throw as a separately brazed part so that it could be removed for assembly. It is made from 1 inch by ¼ inch steel and 3/8 inch rod, and is drilled to clear a 6-32 machine screw through the main shaft and one side and tapped on the other.  Since the front throw can be removed I was able to use a solid wood drive wheel. A small hole drilled in the throw and a brass nail driven in the drive wheel to provide the driving force.  The drive train photo shows the removable throw, the wood drive wheel, and the pin assembled in place. It also shows a minor variation in the idler wheel construction.

I used a 3 inch hole saw to cut the disk, which resulted in a diameter of about 2 7/8 inches. I then grooved the outside of the disk and installed an O-ring with a 3 inch OD as tyre. This rubber ring came from a tractor parts supply house, and is sold as a repair part for hydraulic cylinders. I decided to use a ¼ inch steel shaft for the idler. I drilled the hole in the idler wheel slightly oversize and press fit a short bushing of 9/32 inch OD, ¼ inch ID brass tube in the hole. I used the same size tube to bush the holes in the pressure box sides for the take-up spool shaft.

The 2-piece crank also permitted the installation of a simple spacer cut from plastic pipe between the pressure box side and the center throw. The drive wheel and this spacer hold end motion of the crank to a small value.

Removable Hand Crank
I constructed the arm of the hand crank from 1/8 inch by ¾ inch steel. I drilled a 5/8 inch hole in the crankshaft end and brazed a short length of 5/8 inch OD, 3/8 inch ID tube in place. The arm was bent at two places before brazing in order to provide good clearance. I drilled the tube and crank to take a "hitch pin" so that the crank can be removed without tools for transportation and storage.

Take-Up Drive and Spool
I thought John Smith's drive clamp, disk, and "cam" pin on the take-up drive wheel was more complicated than necessary. I cut a plywood disk with a hole saw and used a classic "set screw and flat" drive. I used a milling machine to put the flats on my shaft, but they can easily be filed. My "set screws" are what we call "dry wall screws" in the US. I drilled a pilot hole in the disk and ground the point off the screw before installation. The brass pin sticking out of the wheel is to drive the arm of the animated conductor featured in the "Senior 20" design.

I could not find the 68 mm diameter drain tube recommended for the take-up spool, but I discovered that I had a considerable stock of 2 ½ inch OD, ¼ inch wall cardboard "mailing tube" already on hand. I soaked an appropriate length of tube in varnish and let it dry for several days before using it to construct the spool. I used a "set screw and flat" drive here as well, drilling a 3/8 inch diameter hole through the tube and grinding the screw point until the screw head was fully recessed. Instead of fastening the drive tongue to the tube with screws I cut some springy aluminum strip long enough to go about 2/3 of the way around the inside of the tube, curved the strip to approximately the outside diameter, and bent the hook so that spring action holds it in place.

Cutting Disks on a Drill Press
I used a variation of the "pivot and band saw" approach to cut several of the larger wooden disks needed. I drilled a hole in a length of scrap wood and inserted a short length of wood dowel in the hole. This jig was then clamped to the drill press table, and a router bit installed in the chuck. A pivot hole was drilled at the center of the disk to be formed and the plywood placed on the pivot. The drill press table is then rotated until the bit is accurately located and locked in place. After starting the drill press, the bit is lowered to plunge-cut an initial hole and then locked in position. The wood can then be rotated to cut the rest of the disk profile. For the 1/8 inch thick spool ends, the disks could be cut in a single rotation. For thicker material it is advisable to step down through the wood in cuts of 1/8 inch or so. I think this technique gives a smoother circle, and the plunge-cut means you don't have to work into the edge as you must with a band saw.

Crankshaft Bearings
I turned two solid crankshaft bearings from squares of ¾ inch thick oak. They are about 1 3/8 inch square, 7/8 inch OD, and drilled for the 3/8 inch shaft. They have oil holes in the top of the square. They are a light press fit in the panels and retained with small wood screws. I made a similar split bearing for the front by fastening two rectangular pieces of wood together into a square before turning and drilling. The purpose of these bearings is not to reduce friction, which will be minor in any case, but to increase bearing life by reducing contact pressure and increasing surface hardness.

Wallace Venable

2-piece crankshaft

Drive train

Crank spacer

Handle

Take-up drive

Take-up spool

Router

Bearings

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