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What locking compound should I use?
342 8 2018-7-30
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Chris_A
Second Officer
Flight distance : 6142 ft
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Hi all, due to a little plastic cracking on my P3A (around the motor arms), I decided to fit reinforcement plates (one to each motor arm). However, on doing so I have to fit replacement securing bolts, so my question is, what would you recommend as a locking compound? The original securing bolts had (and I can only assume) blue Loctite on each of the threads.


Any recommendations would be helpful (I was tempted to just use a little epoxy, but thought I would ask here first). The image below shows the plates that I have fitted (including replacement securing bolts).

Many thanks

Chris


P3A_3.peg.jpeg
2018-7-30
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solentlife
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Flight distance : 1087530 ft
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Blue Threadlock - Plastic Safe variety.

Specifically Blue. Because that is not permanent and you can always unscrew later. If you use RED - you will have problems as thats designed as a permanent solution.

The amount needed as you see from DJI fitting is tiny. You basically just get it into the valleys of the screw thread and that's all thats needed. No humping great blobs as you see some people !!

Note you will have some people advise you not to use any locking compound ... well my view on that is DJI do - so I do !

Nigel
2018-7-30
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Chris_A
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solentlife Posted at 2018-7-30 06:54
Blue Threadlock - Plastic Safe variety.

Specifically Blue. Because that is not permanent and you can always unscrew later. If you use RED - you will have problems as thats designed as a permanent solution.

Many thanks Nigel
2018-7-30
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Chris_A
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Flight distance : 6142 ft
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Nigel et al.

Daft question (and difficult to put into words), but how tight should tighten each of the bolts (at the moment they are just finger tight as I'm awaiting delivery of blue thread lock). I'm guessing, finger tight, then nip up a fraction?? I really don't want to add more cracks by overtightening. Any thoughts?

Many thanks

Chris
2018-7-30
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Nigel_
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Chris_A Posted at 2018-7-30 23:08
Nigel et al.

Daft question (and difficult to put into words), but how tight should tighten each of the bolts (at the moment they are just finger tight as I'm awaiting delivery of blue thread lock). I'm guessing, finger tight, then nip up a fraction?? I really don't want to add more cracks by overtightening. Any thoughts?

Just enough that they don't come undone again!

If you use a marker pen and draw a line across them onto the plate then you will be able to check if they are unscrewing very easily, if they are then tighten a little more.  If you do this and check regularly over the first few flights then you can stay on the safe side of too tight.

You also want to ensure that your plates are not loose enough to move about through vibration, if you have plastic safe threadlock then maybe put a couple of drops on the plates to stop them sliding around, better than extra tightness on the screws.

2018-7-30
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Chris_A
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Nigel_ Posted at 2018-7-30 23:32
Just enough that they don't come undone again!

If you use a marker pen and draw a line across them onto the plate then you will be able to check if they are unscrewing very easily, if they are then tighten a little more.  If you do this and check regularly over the first few flights then you can stay on the safe side of too tight.

Thanks Nigel, I actually used a little dab of epoxy under each plate (permanent I know, but I've decided not to obtain an official repair, so I don't really have anything to lose). Once I've added the thread lock, I'll run a few low level flight tests and see what gives (hopefully nothing lol)


Thank again for your advice.

Regards


Chris

2018-7-31
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solentlife
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I tighten mine quite hard to be honest as they are screwing into metal base of the motors.

The trick of the line across screw to surround - is similar to trucks to give visual check that wheelnuts stay good !

Nigel
2018-7-31
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Chris_A
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solentlife Posted at 2018-7-31 08:22
I tighten mine quite hard to be honest as they are screwing into metal base of the motors.

The trick of the line across screw to surround - is similar to trucks to give visual check that wheelnuts stay good !

Good idea, I'll do that also. Thanks again.

Regards

Chris
2018-7-31
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solentlife
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I don't personally agree with the over-tightened view some propose. I agree that every fastening has an ideal torque setting, but here we are talking screws through plastic into metal base of motor.
I think common sense says not to 'graunch' them up so tight you stress or crack the plastic. But you need to be reasonably tight and using a standard HEX driver ... (A hex driver is more like a screwdriver)... you cannot over-tighten so easily as you can with Allen key.

You need to make sure its reasonably tight because that motor will 'bed in' and if you are not tight .. any flex will create cracks.

If I was looking at modifying a shell - instead of plates - I would like to see CF run along the arms and supporting the motor mount. I think the general design where the arm slims down dramatically before the motor and then widens out is part of the problem. Take that shell apart and that bottom arm is extremely flexible - the top shell providing the stiffness. No wonder cracks appear. DJI do not use what is called 'Engineering Plastic' - more expensive and needs specific moulding processes etc. Eng P would be a better material to use.

My 2c's

Nigel
2018-7-31
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