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Ann3x

Repairing Cracked Gearboxes

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I've had limited luck so far repairing gearboxes - wondering what other people do?

I've got a number of rare Super Astute gearboxes with most of the base screw mounts cracked, all the parts are there, I just need to find a way to reattach them all firmly! 

I've tried Epoxy but it just peels off the plastic.... Anyone got any good approaches? I guess:

  • What plastic is used for these gearboxes, is it ABS?
  • Anyone had any luck with any particular glue / bonding agent?
  • What about roughing up the surfaces - does it help (so far Ive been reluctant to snap / reshape as the fit will never be as good)?
  • What about reinforcement? Any approaches that work around the screw holes (wire wont work as they are not isolated posts)?

Seems a shame to waste 2 rare gearboxes.

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I had pretty good results on an ABS body with plastic cement. Seems to fuse the plastic back together rather than just bond. No idea about reinforcement.

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3 hours ago, ThunderDragonCy said:

I had pretty good results on an ABS body with plastic cement. Seems to fuse the plastic back together rather than just bond. No idea about reinforcement.

Yeah but a body is not structural.... Gearbox has a lot more point forces applied on mounting I think. I can get it looking ok with epoxy, just putting the screws in to attach it to the chassis always destroys the "fix".

Need a stronger approach I think. I found an old thread with some ideas here. In summary:

  • Tack loop of carbon thread in place with CA ensuring that the thread runs in a full loop (ie drill holes if required)
  • Rubberised epoxy over / around the thread & in the screw hole
  • Redrill/screw hole (+ retap I guess)

Might give it a go, I'm loathe to throw away TTC gearboxes - they are just too rare.

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ok, I am coming on with this:

losi_05.jpg

losi_06.jpg

Its from my Losi XXX BK1. Its total pain what they did in the design. The whole gearbox is just screwed with tiny screws from the base and another horizontal through the rear damper mount. I was really gentle driving through the pumptrack today and I cannot imagine how this model made it through competitions?! The thread is woren out and the screws came out. I am going to set a insert as usual.
 

losi_03.jpg

losi_04.jpg

Drilling and using epoxy glue + the rest from the trees of reinforced fiber plastic from Dyna Storm/TR-15T. I am not shure yet if I should drill though the holes, placing longer plastic studs to get longer screws inside. Its weird how such a thing wasnt made well in the design... :blink:

 

 

 

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That's a common problem on the astute gearbox, there's usually a big crack on screw pit at the back. 

A few ideas:

- if it's abs or ps, styrene welder is the ways to go. I don't know which material the gearbox is made of. But to use styrene welder, you need to press both sides against each other and keep them pressed for some good amount of time until the styrene hardens. So not ideal for screw pits. What you would need to do is add some new melted styrene in the crack.

-  uhu acrylit express or pattex stabilit express seem to give better results than epoxy on non-styrene plastics

-  You do need to roughen up both edges you want to glue with sandpaper. Very important, otherwise glue won't stick properly

What you could do is fill the pit with one of the above mentionned glue, drill a new 2.5mm hole and tap it at 3mm so you can use a M3 machine screw instead of the self-tapping screw.

 

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I actually went ahead already and repaired my 3 gearboxes by drilling holes, using carbon thread in a loop to secure the broken parts and then coating with semi flexible marine grade epoxy on a slow cure. Seems to be ok so far. m3 tapping is clearly safest way to handle these tho (self tappers are suicide!).

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What makes common M3 doing better then self tapper screws?

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19 minutes ago, Collin said:

What makes common M3 doing better then self tapper screws?

Lower pitch / more threads per inch so forces are lower on the plastic i guess.

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57 minutes ago, Collin said:

What makes common M3 doing better then self tapper screws?

Less stress on the plastic but also less surface holding.
The later is a bit difficult but it got discussed somewhere, difference between self tapper and tapped screws into plastics.

The self tapper have more surface on the thread to hold the screw in, works as long as the screw fits and cuts correctly into the hole and surounding plastic.

  • Thanks 1

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Ok, changed for a brand new gearbox, again with tapper screws. Hm, next time i´ll try metrics, just to see if it last longer.

losi_07.jpg

Insert is set with UHU Endfest 300 epoxy glue. Tomorrow I´ll drill the holes. seems to be okay then. Once I had one with the crack along the pit, as Ann3x mentioned on the Astude gearboxes. I throw it away caus I did not see any good reason to get it working again. Maybe the carbon thread, never saw it befor...

 

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13 hours ago, Collin said:

On the Astude gearboxes. I throw it away caus I did not see any good reason to get it working again. Maybe the carbon thread, never saw it befor..

For a normal astute gearbox I probably would have thrown away too (they aren't too rare as shared with madcap, saintdragon, hmr, kc etc) . These 3 were super astute ttc tho - too rare to waste. 

For the carbon thread I actually used narrow carbon tape (the stuff used to weave graphite parts before it's resin impregnated), twisted into a thread temporarily. Works OK but is a little brittle when not in resin which can make it hard to tie knots in. Possibly something else that's very strong and has no stretch would be better. Kevlar thread? In any case the principle is that the thread stops the fix flexing too much (which can cause the epoxy to flake off). 

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