Legacy builder Rob Logan posted a message on the LML about a problem he had with excess play in his rudder bellcrank. He found insufficient bearing area between the bottom end of the bushing #6338K412 and the top of the bellcrank bracket #4651 (see page 17-25 of the Legacy Assembly Manual).
Reading of his concerns, I examined the rudder bellcrank assembly on my Legacy #257 and decided to increase size of the bearing surface. I’m no engineer, so who knows what kind of havoc I created, but here’s how I solved it. Use it at your own risk.
I started with a small piece of 1/8” thick ultra high molecular weight polyethylene plastic (UHMW PE). It sounds like something that might come out the toxic waste end of a nuclear reactor, but it’s a very tough, slippery material commonly used for bearings on sliding surfaces. (Available from www.onlinemetals.com) I used a Forstner bit to drill a 5/8” diameter flat-bottomed hole 1/16” deep. This counterbore provides clearance for the bottom end of the bushing, which projects less than 1/16” below the bellcrank. I then used a 1.75” hole saw, centered on the counterbored hole, to drill all the way through the plastic sheet, creating a 1.5” diameter disk. I cleaned up the raw edges on a belt sander.
I used the same 1.75” hole saw to create a 1.5” disk from a piece of aluminum sheet that was .015” thick. Coincidentally, aluminum flashing from Home Depot/Lowes might be about the same thickness.
Lucky for me, the pilot drill on my hole saw is ¼”, which is just the right size for the AN4-13A bolt that goes through the center of the bellcrank assembly, so the clearance hole in the center of the two disks was already the right size.
I put the UHMW plastic washer between the bellcrank and the bellcrank bracket. This is the heart of the modification. The plastic washer creates a huge bearing area between the rotating bellcrank and the stationary bellcrank bracket, making for what I hope is a smoother, more durable fitting than the tiny surface area of the end of the bronze (?) bushing rubbing against the bellcrank bracket.
I added a thin (AN960-19L) washer between the bellcrank bracket shear support and the bellcrank bracket, one under each of the outer two bolts that holds the assembly to the fuselage support. The purpose of the thin washers is to adjust for the added thickness of the UHMW plastic washer.
The aluminum disk goes between the bellcrank bracket shear support and the bellcrank, centered on the center bolt that goes through the entire assembly. I needed the .015” aluminum washer on mine because that was just enough to take out all the excess play created by the net effect of adding the plastic washer and the thin AN washers. You might need a different combination of washers and shims to make it come out right.
I sanded down the thickness of the plastic washer a small amount to make it fit better. UHMW PE is tough stuff to sand, which is why it works so well in this type of application. I made a jig to hold the washer against a belt sander by screwing a ½” OD steel washer into a block of wood with a small flathead wood screw. The counterbore for the plastic washer fits on top of the metal washer. Hold the plastic washer down on the belt sander using the wood block and the metal washer is trapped inside the counterbore and prevents the plastic washer from slip-sliding away from the force of the moving sanding belt. Frequently stop the belt sander and rotate the plastic washer 90° or so to make sure you don’t bevel the washer. Measure with calipers as you go to make sure the two faces remain roughly parallel.
I used the same AN bolts to attach the assembly to the fuselage support, since they turned out to be long enough. You can look under the fuselage support to confirm that. I put a little bit of grease on all the moving parts and it works as smooth as silk!