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Speed,Combat,Scale,Racing => Speed Talk => Topic started by: Motorman on March 05, 2019, 02:33:17 PM
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Bellcrank by Marty Higgs.
It looks like the buttons are machined with a step to act as a bearing in the bellcrank proper. The step is slightly longer than the thickness of the bellcrank. The brass washer is held tight with the screw. The assembly pivots smoothly with little or no play. Looks like a 2-56 screw. Real clean design.
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I agree with your point. It always struck me that the conventional design wears down the lines. I had one break after it passed a hefty pull test.
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I've accomplished this using several differing methods. The method I like the most, is to use 2-56, 3-48, or 4-40 blind nuts as a bearing surface/retainer nut for the pulleys. I start by removing the barbs by turning the blind nuts on the lathe to a slightly smaller diameter. While I'm at it, I true up the shank of the nut so it's diameter and roundness are consistent.
I then make up a set of pulleys from steel or brass, usually brass, drilling them to fit as snug as possible to the blind nuts shank and still turn freely. Then I carefully machine the thickness of the pulleys so that they have some end play when installed. I drill the titanium bellcrank to the body diameter of the screws I will be using, and placing the pulleys on the blind nuts, I attach the nuts to the bellcrank "T" style (shank towards the bellcrank) with button head screws. I then grind or file the bolt ends flush. Finish it off by carefully flash soldering the bolt to the blind nut, or use one of the permanent types of loctite to ensure the screw does not back out.
Les
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Been using these on combat planes. https://www.mcmaster.com/catalog/125/3324 Top left on page
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Been using these on combat planes. https://www.mcmaster.com/catalog/125/3324 Top left on page
Good find.
I see that only the first type offers a #4 size, the others are too big. The price is right.
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Good find.
I see that only the first type offers a #4 size, the others are too big. The price is right.
That's why I said top left of page. #^
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The 4/40 weld nut that Mike posted coupled with a standard brass eyelet would make a nice no-stress setup without the need for mill work.
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That's the exact setup I have been using and promoting. If you want the weld nuts to be easily removable, file them to a 5/16" hex. 1/4" button head screws hold the buttons to the bellcrank. I do this for carrier and racing.
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That's the exact setup I have been using and promoting. If you want the weld nuts to be easily removable, file them to a 5/16" hex. 1/4" button head screws hold the buttons to the bellcrank. I do this for carrier and racing.
I like the hex idea.
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McMaster Carr sells a 4-40 threaded piece of 5/16" hex stock called a coupling nut or rod coupler or something like that. Screw the weld nut onto a 4-40 bolt, and tighten it against the coupler. (You want the flange of the weld nut to be touching the coupler.) With a moto-tool and cutoff wheel, grind the weld nut down to match the 5/16" hex. Tap the bellcrank 4-40, and install some 1/4" button head screws (with Loctite) onto the bellcrank. Install/remove the weld nuts with a 5/16" nut driver or glow plug wrench. I've never had one loosen, but one gorilla handed pit man did overtighten one to the point of twisting the screw loose from the bellcrank!