Thorp Air Command - T18.net

Well...during the annual I found a very interesting DoDa ! The Usher carb heat box thru shaft bearings have failed ? Excessive play . They appear to be a very thin bonze type bearing sleeve ? Brazed onto a mounting ring ? Carb heat box is a Spruce part number . The only time this carb heat has been used is during the run up checks . Anyone had this type of problem and what was the cure ? Looking at what is left is a VERY thin bronze sleeve that has actually split . You can take a pair of needle nose pliers and reform the "bearing."

I guess my solution is to cut the shaft to get the butterfly "flapper" valve out (they used a crimp method to secure the shaft to the butterfly ) Find new upgraded bearings and reinstall the shaft and attach the butterfly with screws ? This heat box only has about 175 hours on it ? No BOOM , just tired !

RB

Pics?

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http://www.dixiefriedfabrication.com/

Lou : It would be difficult to get pics of the "bearings ." I just never notice the "slop" in the bearings during the last annual ? Like I said they only get exercised once a flight during the run up . I was hoping to finish the annual this weekend , but this will set me back a few hours ? I'll take pics tomorrow morning , but like I said it will be difficult to see the problem . Cutting the shaft and removing the butterfly and reinstalling a new shaft with better bushings seems to the solution ?

RB

Are you going to Tony's at the end of the month ? Rosamond Sky Park

Rich - I made my own box, basically copying a design that was used on many T-18s. It's made of welded up aluminum, & the diverter butterfly valve consists of a 1/4" aluminum tube shaft with a phenolic butterfly plate. The shaft rides in/on simple rubber grommets. Those are my bearings. No lubrication is probably necessary, but I used a spray-on bonded dry lube on assembly. No appreciable wear in 23 years, & I use carb heat during run-up, basically for every landing, & otherwise as conditions warrant.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Shaft/butterfly vibration is the culprit. Been a problem since the '40s. They used to sell ball bearing kits. Didn't solve the problem, however. I like JE's solution.

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Bob Highley
N711SH
SN 835
KLAL

Ditto on what Bob said. That part takes a beating from vibration. Mine was a mess too. John's solution is the way to go.

A buddy of mine is building an airplane and he had a good solution to this problem too. He made the air box part of the cowl and put the flex joint between the carb and the air box. I think that is sort of what JT originally drew up with that big round air-cleaner and a deep cup around the carb, but you don't see his solution on many thorps today.

RB, look closely at the butterfly. If it is like mine, the butterfly is welded/brazed to the shaft. Mine failed this annual as well. Spruce had new ones on back order for a month. I just cut a ring from some scrap steel and welded it the the outer case. It looks like crap but is functional. Besides, I didn't want to pay 250 bones for a cheap piece of bronze that was going to fail again soon.
Cubes

John probably has the most effective solution to the problem, the rubber will move. I used bell crank bearings, but I load the control cable with a little tension when the carb heat is off, in other words there is no Room to have slop as it has been taken up with cable tension. I used a button lock cable, the through shaft for the butterfly is .250 4130 tube, the butterfly itself is .025 304 stainless. Just an opinion, if thing are not allowed to rattle or slop against one another the wear factor goes way down as where there is no movement it is very hard to generate wear without movement. McMaster Carr sells some 660 flanged bronze bushings, this material is slick, and very tough, might be a good solution, although the rubber seems to be the easiest. I used to have photos of my carb heat box, it is less than 2" from top to bottom as I was short of some space due to the scoop I used........Dan

Cubes : First thing I saw when the filter came off was the butterfly valve "crimped"/welded to the shaft . Got out the Dewalt Demo saw with a metal blade and cut the shaft to remove the BF valve . I like John's idea of the rubber grommets . Right now I just need to drill out the rivets holding the bronze bushings in then I can use the existing holes for the grommets . Rest of the day will be spent making a new BF valve . Monday off to the metal supply house to get a new "rod ." The new BF valve will be attached to the shaft via mini bolts and high temp lock nuts .

I wish I had adequate aluminum welding skills to make a new box , but my limited welding skills with the soft stuff is limited to wrapping it around a potatoe ! Attached are 2 pics of the bushings . Both were FUBAR ! Pretty piss poor material to only last 175 hours .

RB

As Dick stated, my aircraft has the "banjo" air box, with a big honkn auto filter, attached to the lower cowl and the carb heat butterfly shaft uses grommets as bearings, been this way for 44 years, still works fine. I believe this is JT's original design.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Well...the old bushings are out along with the BF valve . Also drilled out the old shaft on the control arm to reuse . New BF valve has been bent up and attached to the 1/4" shaft with 4-40 heat treated cap screws and high temp locking nuts . Tomorrow will get the rubber grommets and cut the shaft to length and weld the control arm on . The heat box is in good shape other than the FUBAR bushings . There is enough room to attach the BF valve in the heat box .

New BF valve in with AN 931-4-7 rubber grommets to replace the bronze ones . BF valve is attached to the shaft using heat treated cap screws and high temp lock nuts should I have to remove the shaft to replace the grommets ! Sad that these heat boxes are not made so you can replace worn components ? I guess the good news is this is the weak point in the design and can be replaced with a little work ! Total cost...about $3.00 for the parts . Much better than buying another short term carb heat box for $200 + saw bones ! BATA BING, BATA BOOM !

RB