Pump flywheel

Modeling in O scale and in the middle of making a small oil engine I needed a flywheel. The problem was that it needed to be 3/4″ dia (that’s 3ft in O scale) .. and for the life of me .. I couldn’t find anything. I ended up scratch-building the darn thing and this is how I did it.

Parts – HO Driver

I finally got a set of old HO locomotive drivers from my buddy Gary. They were a brass tire with a plastic center pressed in. I popped the rim off .. filed the flange off and I ended up with a ring that measured .750″x.110″. That scaled to 3ft x 5-1/4″ in O scale. Great. The only problem was that there was knurling on the inner surface of the ring. I fixed that by super-gluing a strip of .020″ styrene over the knurling. That accomplished two things .. 1) Hid the knurling and 2) supplied a place for the styrene arms to adhere to.

Sketchup drawing

First thing I did was draw the center of my flywheel in Sketchup. In the drawing below I drew a circle .640″ dia. That’s the inside measurement of the rim (with the styrene strip). I made the arms from .022″ x .066″ styrene strip so I drew these in. The hub I made from 1.8″ tubing .. drew that .. and finally indicated the 3/16″ dia reinforcing ring.


Here’s all the bits cut out. You can see the brass tire with the styrene strip cemented on the inside.

Evergreen styrene tubing telescopes – odd numbered sizes of round tubing telescope into next odd numbered size – even numbered sizes of round tubing telescope into the next even numbered size. That’s why I used the 3/16″ tubing. I cut these .020″ wide. They will reinforce where the arms cement to the 1/8″ hub. The 1/8″ hub is in two parts – one is .125″ long and one .145″ long. Finally, the .022″ x .066″ strips.

work surface

Moving the camera back a bit and you can see the set-up. I glued the Sketchup printout to my marble work surface (nice and flat). I used the glue stick to do that.

Note: My “marble work surface” is a single marble floor tile that I got from Lowes. A couple of dollars at best and you have a perfectly flat work surface.

gluing up

I first used the school glue-stick again and glued the 1/8″ x .145″ hub to the center of the drawing. I used this length simply because I wanted the hub to stick out 1/8″ on either side of the flywheel. The .140″ is simply that 1/8″ + the thickness of the arms. The strips of styrene that make up the arms are simply glued to the 1/8″ hub. A small round-file stroked across the ends makes them fit. I used the Tenax for cement. Let that set up a few minutes and dropped the 3/16″ ring over the hub and cemented everything well.

Above I went through a series of steps to build up the flywheel center. One of the most difficult parts of this was figuring out the steps so everything lined up correctly.

Finishing up

I used a razor-blade to snip off the arms even with that .640″ circle on the paper and then when it had dried, popped it all loose. Flipping the center over I added the other 1/8″ tubing (.125″ long) centered on the assembly and then dropped the other 3/16″ ring over the hub and snuggled up against the arms and cemented everything well.

A slight filing of the arms and the assembly fit into the ring. I again used the Tenax to cement everything. I got it close .. centered one arm by eye .. cemented that .. flipped to the opposite side .. centered THAT arm to the ring .. and continued until the center was cemented to the ring. With that dry I came back with my super-glue and my hand-dandy applicator and reinforced each joint. This makes a small fillet which is fine with me.


Finished flywheel .. mounted on my scratch built steam pump. I created this for my pump house. All that work and you can’t see it except by twisting yourself into a pretzel.

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