To use or not to use the Strong Arm Reinforcement Plates?

M_W

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I just got my P3S back from having the shell replaced for the second time due to stress cracks and I'm wondering if I should get a set of those UAV Bits Strong Arm reinforcement plates. There are a bunch of threads on here with differing opinions about them. Some people say they seem to help. while others say that they don't do any good at all.

The first time I sent it in to have the lower shell replaced they did it free of charge, but this time it cost me $80+ so I really don't want to get any cracks on this new shell. I've always flown it moderately with the braking turned down and have never crashed it, so I can't imagine that it's due to user error.

Is there any "risk" involved with using the Strong Arms? My main concern would be over tightening the screws. Do most people who install them before cracks appear usually have good luck with them?

Are there any other viable options for preventing stress cracks? I see two other brands of reinforcement plates listed on PhantomHelp, but the UAV Bits ones seem to be the most popular.

There's always the chance that this new shell won't have the cracking issue, but I'd rather not risk it.
 
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To be honest, I'm questioning myself the same thing.
I bought cnc machined plates, and use them.
The problem is, - I had no stress cracks before on my P3A, and got the first one by overtightening the anti-stress plate :( (on lower shell, not the motor screw, but one one of the smaller ones).
And, those plates give additional weight, so - a bit less flight time (not easy to judge because of battey wear., but, I think, I can see the difference in my log files before and after plates).
 
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I installed a set on my P3S because within 2 weeks of owning it brand new, I saw a crack develop on the front left arm between motor screws. removed the plates today to check and so far so good no additional cracks. I repaired the crack with 2 part epoxy before Installing the plates. The process for that was to take a tiny Jewelers file triangle shape. I used it along the crack to cut in a shallow V shape then filled in the void with epoxy sanded it flush and installed the plates. I think thick CA would have also done the job, but I'm partial to epoxy.
 
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I also did the 2 part epoxy glue repair after I saw a small crack.I applied the method to all 4 arms,putting the same amount of glue in each one.No crack 6 months now.
 

M_W

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Well I got the Strong Arms and installed them today. I tightened the two torx screws to about 7 turns as recommended in the install video and the other 4 screws to "hand tight", however in order to tighten them snugly I had to screw them down to where they dug into the Strong Arms plastic a bit. It was like this on all four arms and I'm a little concerned about it, but if I had tightened them any less they would have been a bit loose. The paper manual that came in the box said to be careful not to over tighten them, but the video said you don't have to worry about it. :confused: The manual also didn't mention tightening the 2 torx screws 6.5-7 turns or using nail polish. A bit confused about the conflicting information here.

Has anyone else had a similar experience with the Strong Arms?
strongarm.PNG
 

M_W

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And then there's this one torx screw that went well past 7 turns before tightening up:
strongarmtorx.PNG
 
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I would suggest making them snug but not over-tightening, then go back to them after flying a couple of times and re-tighten, maybe even go back to them yet again later.
 
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Well I got the Strong Arms and installed them today. I tightened the two torx screws to about 7 turns as recommended in the install video and the other 4 screws to "hand tight", however in order to tighten them snugly I had to screw them down to where they dug into the Strong Arms plastic a bit. It was like this on all four arms and I'm a little concerned about it, but if I had tightened them any less they would have been a bit loose. The paper manual that came in the box said to be careful not to over tighten them, but the video said you don't have to worry about it. :confused: The manual also didn't mention tightening the 2 torx screws 6.5-7 turns or using nail polish. A bit confused about the conflicting information here.

Has anyone else had a similar experience with the Strong Arms?
View attachment 88063
this was why I went with the aluminum ones even though I have a 3D printer and printed a set for my Nephew in Nylon. I just didn't like the idea of trying to tighten to the plastic. The ones I printed out had a beveled flush fit hole so would be easier to tighten to then just a flat surface provided the proper length screws were used. definitely use some form of thread locker there are some plastic safe ones as well as nail polish. I used Blue Loctite on mine but I don't use it like everyone else does I put it on the threads at the start of the threading up ten threads and let it sit till its tacky then install them.
 

M_W

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I would suggest making them snug but not over-tightening, then go back to them after flying a couple of times and re-tighten, maybe even go back to them yet again later.
The problem I had is that some of them didn't really snug down until I got them to the point they're at in the picture. It didn't take much force at all for the screws to go down in the plastic and then they tightened down.
 
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Yeah that's what I was worried about with the plastic ones. its hard to know what size screws are needed for these plates also, as to long you damage the motors not long enough they don't get a good enough bite to hold well. even the metal ones I bought had screws that were to long supplied with the kit for the motors and the ones for the body weren't long enough. I ended up pulling a motor out and measuring the thread length then adding the plastic from the body hole to that and then the aluminum plate thickness to get a total length then ground down the screws supplied. I had to buy the body screws. a good way to do the plastic ones IMO if the screw mounts flush to it is to use a small washer to create more surface area to prevent digging into the plastic, but then that also requires a slightly longer screw. Can you tell if the printed Parts are solid prints? a lot of times People don't use solid to save plastic filament that will make crushing the plastic very easy over a solid plate.
 

M_W

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Yeah that's what I was worried about with the plastic ones. its hard to know what size screws are needed for these plates also, as to long you damage the motors not long enough they don't get a good enough bite to hold well. even the metal ones I bought had screws that were to long supplied with the kit for the motors and the ones for the body weren't long enough. I ended up pulling a motor out and measuring the thread length then adding the plastic from the body hole to that and then the aluminum plate thickness to get a total length then ground down the screws supplied. I had to buy the body screws. a good way to do the plastic ones IMO if the screw mounts flush to it is to use a small washer to create more surface area to prevent digging into the plastic, but then that also requires a slightly longer screw. Can you tell if the printed Parts are solid prints? a lot of times People don't use solid to save plastic filament that will make crushing the plastic very easy over a solid plate.
They seem solid other than all the lines and grooves in them of course. It sure didn't take much force for the screws to go down into the plastic though.
 

M_W

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Shoot... I hope this won't cause any issues later on... I shouldn't have gone over 7 turns but it just didn't feel all that tight. :(

strongarmtootight.PNG
 
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there are quite a few Plastics that can be used for 3D printing some are better then others. I find ABS fairly stiff and takes tightening down to pretty well within reason of course and only if its printed as a solid ABS is what most people print in, but there is also a lot of printing being done in PLA which is a softer plastic. as far as the print lines go those are present even in some of the highest quality prints some folks will acetone wash the part to remove them and make the part look smooth. As much as they look sunk in Id say one of two things are for sure 1 they aren't printed as a solid or 2 they don't have a built in standoff like the picture of the ones I made below
plate.jpg
 
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Metal for me thanks - I have a 3D printer but not interested in plastic plates.

Plates do not stop cracks - but certainly slow down their formation / development.

The material DJI use for the shell is not actually suited for Epoxy, that is why bevelling the crack etc. is needed to get a hold. Better is to get a bottle of Plastweld ... generic MEK .... liquid solvent used by the plastic model builders. Applied by thin brush or even melt some plastic in a small amount as a paste.

Nigel
 
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Metal for me thanks - I have a 3D printer but not interested in plastic plates.

Plates do not stop cracks - but certainly slow down their formation / development.

The material DJI use for the shell is not actually suited for Epoxy, that is why bevelling the crack etc. is needed to get a hold. Better is to get a bottle of Plastweld ... generic MEK .... liquid solvent used by the plastic model builders. Applied by thin brush or even melt some plastic in a small amount as a paste.

Nigel
same here Brother. I believe in the parts I make on the Printer, But I felt using metal to reinforce plastic would be greater then another plastic and I wanted a rigid mount for my motor screws to seat to. hopefully DJI will find a solution to this Issue for folks. I've already bought a spare body just incase the plates didn't work. The cavity under the motors lends itself to a more permanent solution. I think roughing the inside around the motor standoffs and use a lay up resin with Kevlar or carbon fiber mesh would be great reinforcement to the lover body section where the motors mount. it would certainly make that area rigid enough to resist stress cracks. I'm collecting the materials for that trial already. Ill reinforce the new body ahead of time in prep for a swap out. I have a full set of motors that were damaged by the previous owner of my P34K can use those to help mock up the whole deal.
 

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