This writeup was originally done for an article in Live Steam magazine based on my Mann steam wagon model. The authors agreement with the magazine states that I retain ownership and can redistribute it in its original form, as long as I don't use any of the changes, artwork, etc that the magazine did, so its okay for me to post it here. The topic of making taper locks came up in another build thread, and I am posting this to help that builder (and any others who may want to use this) out. Since this was written I've used the same arrangement for several other models, including my current Kearsarge winch build to hold the crank discs in place.
Chris
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In this writeup we’ll be looking at options for mounting wheels onto shafts, whether they be flywheels, road wheels, gears, sprockets, etc. Most kit engines will give you a flywheel casting, and show mounting it to the crankshaft by drilling/tapping an angled hole for a set screw. While this works, without very tight tolerances on the center hole there is usually a bit of wobble on the flywheel. Also, every time the flywheel is put onto the shaft during the build the set screw will tend to mar the surface of the crankshaft, sometimes so badly that the flywheel is difficult to remove. A simple remedy is to use a taper lock rather than the set screw. It will give a very secure, repeatable, and removeable connection that will not damage the surface of the shaft. This is an old technique, and I would like to credit the late Chuck Fellows for teaching it to me in one of his online build logs.
The basic type of taper lock is shown in Picture 1.
At the top in yellow is the lock piece with three screws. Below that in blue is the hub with its center hole tapered to match the lock, and two tapped holes. This hub is shown as a separate piece, but it could be the center of a larger cast or machined wheel. At the bottom is the assembled hub and lock, with the round shaft in red. The center hole in the lock piece is sized to be a close fit on the shaft, and its outside is tapered at 5 degrees. That angle is a good compromise between holding power and strength of the lock – too shallow and the lock gets flimsy, too steep and it takes a lot of pressure to run the lock in. One side of the lock has a slot sawn through it to allow it to compress around the shaft, in the same way that a collet does.
On either side of the slot is a clearance hole for a screw used to draw the lock in tight to the hub, compressing the tapered length down around the shaft. The third screw opposite the slot is tapped into the lock, and is used to push against the hub to remove the lock. You can omit this third screw, but removing the lock will then require prying against the hub. The hub itself is bored at the same angle as the lock, and has two screw holes drilled/tapped.
The next set of pictures shows the sequence used to make a lock and hub set – these were used on a truck model to hold the rear wheels onto the axle shafts. To start, a length of steel rod was drilled to be a close fit on the shaft, then the outside was turned down at a 5 degree angle using a compound rest on the lathe.
The same thing could be accomplished by swiveling the lathe headstock over 5 degrees. Pick a piece of material that will give a lock with a flange with enough width to take the screws. The taper is cut in until the thin edge at the outside almost meets the center hole – you want the material here to be thin so that it can flex enough to grip the shaft.
Next the hub is also drilled to be a close fit on the shaft.
If this is a one-piece flywheel, then that entire part would be chucked up on the lathe – in this example the hub was a separate piece later attached to the spokes, but the process is the same. Bore the hole out using the same angle setting as was used on the outside of the lock. If possible, leave the compound rest at that angle throughout the machining of both pieces so the tapers are a perfect match. Use the lock to test fit the bore – you want about 1/16” to 1/8” of gap to the wide part of the lock.
Once you have a good fit, move over to the mill or drill press and drill/tap the holes. The two holes either side are clearance-sized to the screws through the lock, and tapped in the hub. For the clearance holes, use a drill that gives a loose fit, so the lock can flex in without binding on the threads of the screws. The third hole is tapped in the lock, but not drilled into the hub.
The last step is to saw cut a slot through the lock opposite the third hole.
That’s it – ready to assemble onto the shaft.
To assemble, leave out the third screw, and use the first two to draw the lock into the hub while its on the shaft. Tighten them evenly, back and forth between them to get the lock snugged down on the shaft. The grip is remarkably good, I’ve used it to hold flywheels for machining the spokes with no problems.
There are a number of variations on the theme that will work out just fine – one is to put slots either side of the lock, but not through the end flange.
For this to work, turn a flat onto the taper up against the flange (shown by the arrow). This setup is closer to the collets used in small pin vises.Another version does away with the separate lock, and puts the taper directly onto the shaft, with a flange for the screws.
Or, the same thing without the flange but with a screw on the end of the shaft to push the hub into place.
This last one gives a look like the old racing wheels on cars, with a single large lug nut in the center. These final two will not grip quite as well as the others since they do not have a piece that can flex to grip, so they act more like a Morse taper. The one with the screws will put some shear force onto the screws but it is fine for small flywheels on a slower engine.
Hope those designs are useful on your next model. For now, I am back into the shop for some more lessons from the elves…
