November 15, 2019

Rear Elevator Torque Tube Modification

I decided to modify how the rear/short elevator torque tube is held in place on the floor ribs. After spending hours in previous weeks fiddling with the floor ribs, I just couldn’t find a fit that allowed the tube to turn smoothly. The floor ribs that support the torque tube have to be exactly parallel to each other, and basically I found they are not. On top of that, the channels cut into the ribs for the bushings are not exactly lined up either, so the bushings were not co-linear, which also causes friction. It’s pretty difficult to adjust that alignment without removing too much material from the rib; once you take away too much, then the bushing starts turning in place, which it isn’t supposed to.

My design change replaces the bushings with ball bearings. I found a thin section ball bearing that was small enough to fit in the existing rib channel without having to cut away additional material. To hold the bearings in place, I fabricated new bearing plates from 0.040” sheet of 6061-T6 (same as the close-out brackets that went over the bushings). I quickly designed the plate in Fusion 360, and then used its manufacture capability to develop the CNC program to mill the shape out of aluminum sheet stock. After exporting the NC program from Fusion, I use a tool used Candle to send the program to a small desktop mill that runs a firmware image called GRBL.

I’m very pleased with the result – the torque tube moves freely now. At one point I was going to make this same modification to the forward torque tube, but I already have that all closed out and it’s moving pretty smoothly. I don’t want to mess with it since it seems to be working. Here’s a video I uploaded a video to YouTube to talk about the changes:

I made the initial bearing plate on my desktop CNC router. This is the first time I’ve tried cutting aluminum sheet with it. This CNC machine is not quite strong enough for the task, but it got the job done.
After making the first plate, I decided it would be quicker to cut the blanks by hand, and then use the CNC to cut just the hole, which needs to be a very precise diameter since I’m press fitting the bearings into the plate
Cutting out the hole for the bearing on the subsequent plates.

One other key difference in my design change is how I aligned the bearing plates. Rather than just assume everything would line up perfectly and pre-drilling the rivet holes, I first temporarily held the plates in place on the ribs, and then marked onto the plates where I needed to drill. This results in the ball bearings all being co-axially aligned – something I couldn’t achieve with the original bushings except by trial and error.

Temporarily holding the bearing plate in place to mark off where to drill the holes
Finishing bearing plate with the ball bearing press-fit into the hole

I also changed up how the torque tube is held in place. I purchased 1″ aluminum shaft collars from McMaster. Between the shaft collar and the ball bearing, I used a small plastic spacer to ensure that the shaft collar only contacts the face of the inner ring of the bearing. I can achieve a more precise fit with these shaft collars compared to the original kit design, which uses a short length of slightly-larger OD tube riveted onto the torque tube.

A view of the stack-up of the shaft collar, spacer, and ball bearing plate
After tightening the shaft collar down. I applied a few drops of Loctite onto the set screw to ensure it won’t back out
View of the finished assembly

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