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I lost the screw that holds the motor bell to the base of the motor. I replaced it and flew, but it came loose and I lost it again. I used blue thread lock to secure it. Do I need to use red thread lock? What's the trick to keeping the screws on?

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Use red or better. Blue and sub-red are explicitly designed to loosen with low to medium heat. (in the manual)

FWIW, thread lock is little more than an increasingly heat resistance paint, close in composition to latex paint (depending on the brand). I have used latex paint and also nail polish in a pinch (an old trick from a car mechanic).

If you want a screw to stay, and are okay with possibly never getting it out again, there are a number of advanced and possibly no bueno tricks. All of these are from decades of mechanic work and caution is advised.

  • Toothpick: Shred it, thin it out, stick it in and screw away and grind the fibers in.
  • Tooth pick with thread lock, glue, or paint.
  • Metal shavings, grind them in
  • Damage the correct end of the screw or thread so you get a fused connection (may never come loose >> aka >> danger zone).
  • In some cases, you can cover the screw with a proper cap to stop it from spinning and falling out, similar in theory to the use of a c-clip. For example, drop solder on the top and let it "cast" into the head, then put electrical tape over it (not recommended, many reasons like motor may be off balance).
  • Copper braid or similar, just jam it up.
  • Re-tap and get a new screw

Your problem is most probably a combination of these factors:

  • heat
  • hot/cold expand and contract cycle
  • vibration
  • possible weakened threads (cut wide from friction).

The trick is context, experience, knowing the tools and materials, and not buying cheap stuff.

Now to the meat of the issue - BLDC motor, that is a broad designation (https://en.wikipedia.org/wiki/Brushless_DC_electric_motor), so you could be using an in- or out-runner. Big/low Kv versus smaller high Kv demands a different solution possibly. Each motor has a unique footprint on vibration, heat, operational range, expand/contract margins, etc. So there are a number of factors that will help you decide what is best, I would contact the company if possible (probably Chinese and ZFG, but always an early consideration if possible).

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According to Wikipedia, threadlockers like 3M Loctite work by using a special fluid that resists movement of the bolt to which it's applied by increasing static friction. They're produced in different strengths, often denoted by color codes by purple → blue → red. (in ascending order of maximum strength) Here are a couple of things you could try that come to mind:

  • Cover more of the thread surface in threadlocker. Intuitively, the greater the surface area exposed to the threadlocker, the better the torque-out strength will be.
  • Consider using a threadlocker with a higher load specification. BLDC motors and drones are quite mechanically noisy environments with a lot of intense vibrations. I don't know what your motor looks like, but presumably, the bolt in question rotates together with the shaft it's attached to while rubbing against the inner race of the bearing at the bottom of the motor. The issue may be that the intensity of the vibrations and/or force of the changing speed of the motor exceeds the practical limits of the threadlocker you're using.

We might be able to ascertain more about what's going on if we had images of the motor in question.

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