SonexBuilders.net

What's up everyone, I spent some time today building my aileron bell cranks, but I'm not sure how loose/tight they're supposed to fit. I have two questions:

1) How freely should they rotate? Like should they swing around under their own weight or have at least some resistance? (adjustable by adding shims between the brackets and the spacer block)
2) How much radial slop is allowed between the ID of the bronze bushing, and the OD of the steel bushing that rotates inside it? Is there supposed to be none at all, or is some okay? (adjustable by drilling out the bronze bushings more or less)

Here's what I have so far, and I have a video below to illustrate. For point #1, my RH bell crank swings around pretty freely even with no shims between the brackets. If that's too loose I guess I could sand down the spacer block a bit. My LH bell crank turns with some slight resistance, with two layers of aluminum foil as shims. If that's too tight I'll add a couple more layers.

I'm a little more concerned about point #2. I tried reaming my bronze bushings to 5/16", but the inner bushings still wouldn't fit at all. So I drilled up to the next size I could find, which was 21/64". My bit actually measured 0.325". The OD of the inner bushing is 0.314", so that's 0.011" of diametrical clearance. They fit pretty nice now, but there's definitely some perceptible slob, which I'm not sure is OK or not.

Take a look at this video and let me know what you think! If that slop is too much, I'll have to find a drill bit somewhere in between and redo my brass bushings! Thanks guys!

https://www.youtube.com/watch?v=NmOFUj8kjtw

Edit: alright I guess I can't figure out how to embed the video, but there's the link!

Hey, those are some good looking parts... Based on what I saw, the free-swinging rotation on the first bellcrank is fine, the second one seems a little stiff and may require a tiny bit more space between the flanged bushings (more on how to create that space in a minute).

However, if it were me I would not accept the slop you demonstrate in the video, with regard to the inner and outer bushings. I don't think that the slop you demonstrate in the video is necessarily unsafe; but its not desirable and will only get worse over time as the parts wear down.

I would get 2 new oiled bronze bushings to replace the ones that you drilled out to a larger size. Put the new bushings in the part and line-ream them to 5/16" again (or whatever size is slightly too small to fit the inner bushing). Also take the inner bushings and use a scotchbrite wheel or other sanding device to put a very small chamfer or "bull-nose" on the ends, to make it easier to get them into the bushing. Check the fit. If they still won't slide in smoothly, take a small dowel that's just a bit smaller than the hole size through the bushings. Get some fine-grit sandpaper or emery cloth (somewhere between 400 grit and 1000 grit) and wrap it tightly around the dowel a few times, until you can just get it into the hole. Spin it a few times while moving the dowel through the hole - if its not a tight fit don't let the weight of the parts (or your hand pressure) elongate the hole by removing material from just one side - you're trying to gently remove a little bit of material from all around the inside of the hole. Every few swipes, check the fit of the inner bushing. Rinse and repeat until you have a nice smooth inner surface that the inner bushing can just slide into. Check the fit of the whole assembly. If it is still tight to rotate the bellcrank, the spacing between the bronze bushing flanges is likely the cause. Again take some fine-grit sandpaper and a flat surface (like a stick or a thin piece of aluminum) and lightly remove a little bit of material from the face of the bushing flange. You need to be careful to hold everything square so you evenly remove material from the face of the bushing - one way to do this is to disassemble the bellcrank bracket and put the sandpaper on your workbench, then put the bushing (and the aluminum angle that holds it) face-down onto the sandpaper so you can easily rub it while holding it flat/level to the bench. A few swipes on each of the bushing flanges should be all it takes to enable free movement!

Enjoy,

--Noel
Sonex #1339

Chris,
I would not be happy with that amount of slop. The bellcranks should be loose. To loosen up that second one, some polishing compound on a bolt or steel rod will do the trick. Chuck it up in a drill, spin it and move it back and forth. I have done the final reaming on a bunch of bushings that way. I guess it is not reaming but it works. After polishing take the rod out and clean up the bushings.

Here is what I did to the control stick bushings:
viewtopic.php?f=39&t=578&p=23081&hilit=control+stick+bushing#p23081

Here is another technique, similar to what Noel recommended.
viewtopic.php?f=39&t=578&hilit=Mixer+bushing&start=830

Hi Spaceman
With controls there is a fine line between too tight and too sloppy. A shim will fix #2 in your video.
Both sets of bronze bushings are definitely over reamed and too sloppy. You could either replace the bronze bushings or make your own slightly oversized steel bushings.
For the bronze bushing I personally would only drill or use a reamer and not sand or use grinding paste on any bronze bushings. It may increase the wear rate.
Steve
Sonex 892

Steve -

I'm curious: Why do you think the wear rate would increase if sandpaper or another abrasive material is used? I should have mentioned that the items should be cleaned with WD-40 or a rag with thinner to get the grit out; but are you thinking that something else may be a problem, with the use of an abrasive to clearance the bushing?

--Noel

Thanks for the knowledge guys!!!

I thought that slop was a little alarming, but I wanted to make sure I wasn't being to nitpicky. New bronze bushings on the way!!

By the way, what exactly does the term "line ream" mean anyhow? The plans do say to line ream the bushings until everything fits, so I figured it meant something like carefully enlarge the whole a little at a time. Is there some specific procedure they're referring to or is that pretty much it?

Noel wrote:

I'm curious: Why do you think the wear rate would increase if sandpaper or another abrasive material is used? I should have mentioned that the items should be cleaned with WD-40 or a rag with thinner to get the grit out; but are you thinking that something else may be a problem, with the use of an abrasive to clearance the bushing?

OK, I know I'm being a busy-body here. I think Steve's alluding to the bushes being sintered bronze. They are made by pressing powdered bronze into shape with heat to make a porous material so that it will hold oil. If you use an abrasive on it some of the abrasive is likely to get trapped in the little holes. It will come out later and increase the wear rate on the steel shaft that is running in the bush.

Peter
Sonex 894

peter anson wrote:Noel wrote:

I'm curious: Why do you think the wear rate would increase if sandpaper or another abrasive material is used? I should have mentioned that the items should be cleaned with WD-40 or a rag with thinner to get the grit out; but are you thinking that something else may be a problem, with the use of an abrasive to clearance the bushing?

OK, I know I'm being a busy-body here. I think Steve's alluding to the bushes being sintered bronze. They are made by pressing powdered bronze into shape with heat to make a porous material so that it will hold oil. If you use an abrasive on it some of the abrasive is likely to get trapped in the little holes. It will come out later and increase the wear rate on the steel shaft that is running in the bush.

Peter
Sonex 894

Spaceman
I believe line boring is assembling the parts to be a complete assembly beforehand. Then drilling through the paired bushes in one action, so the holes in are in line and true with each other.

Noel. Peter has described what I was thinking. I read it somewhere when I was building mine in 2009. That some fine abrasives may be impossible to remove from the pores. Sorry cant remember the source, but it made sense to me at the time.
In the Symmco sintered bronze catalog ( see link below) there is a small section on machining at the end. Reaming is not recommended as it seals the pores?
http://media.wix.com/ugd/57a430_4d2f640 ... c2eab8.pdf
Anyway as John Monnet says we are not building watches.
Steve
Sonex 892
Jab 3300 242 hrs

I remember having to fit an extra bolt in those things to stop them binding.
http://www.mykitlog.com/users/display_l ... 021&row=22