Building a Better Quad Frame

rob215x · April 24, 2018, 6:30am

08 - Motor Mounts… Please Help!

I’ve looked at quite a few motor mounts and I’ve done a number of experiments. We really need more choices in this department. I have some ideas but I don’t have a 3D printer. I know some of you do, and hopefully we can work together. At the end of this post I think I’ve found a good solution, but let’s see how I got there…

This is what I found on the internet for 16mm round tubing. I selected 16mm because it seems to be the most popular. One of the problems I face is, I don’t see many choices if I want to use 18mm or 20mm tubing. Most of the mounts I saw were variations on one of these five. They’re sorted by weight.

OMG, Those mounts are HEAVY!

I designed and built a 680mm frame that only weighs 140g and now I need to add another 100g just for motor mounts? No thanks.

Mount D

I like this style mount because you can use it for an X8 and if you’re only building a quad, you can leave off one of the motor plates. One plate and 4 screws is usually 3-4 grams so 16x4 = 64g isn’t so bad. But it’s not light enough for me.

Mount E

Even though this mount is lighter, I don’t like it. You can’t use it for an X8 and in a crash, it will be weaker than Mount D. Mounts B, C, and E all have a weak point where the tube connector meets the motor plate. Even the Tarot mount has a similar weak point. Look at how little material there is between the tube connector and the motor plate. A heavy motor attached to that plate will act as a lever and guess where its gonna break?

EDIT 2018/04/26: This paragraph is about Mount E and I should have used the picture of Mount E instead of the following picture of the Tarot mount because; a) I haven’t used the Tarot mount myself, and b) According to the community, the Tarot mount is pretty strong. That being said, everyone agrees the Tarot mount is heavy and I do have lighter solutions that are just as strong if not stronger.

failure-points

3D Printed Mounts

My previous ship used 3D printed motor mounts. They were designed and printed by a friend in another group. He made several sets of them and they are quite excellent:

Each mount is glued to the tube with epoxy. Because they are glued together over the surface of the tube, they strengthen each other as one solid piece. Each mount AND the epoxy weighs 8g, for a total of 32g!

So, what’s the downside?

They were designed for a very specific, 15.26mm OD tubing. Yes, I might be able to modify the 3D file and have them printed for 16mm tubing, but then I wouldn’t have clearance for the bolts that go into the motor.
They won’t work for motors with a different bolt pattern.
They won’t work for an X8.

So, I want to make something different. But these 3D printed mounts have certainly set the bar on weight and strength!

Recent Experiments

I’ve been playing with the motor mounts that came with my H4 Alien 680mm frame. They are very similar to mount D above. They can be used for a quad or an X8. I like the separate carbon fiber motor plates because they allow for a variety of motor sizes and bolt patterns, and they can be changed if they break.

At 20g each, that’s 80g added to my frame instead of 32g for the 3D printed mounts. Let’s see what we can do with these Alien680 mounts…

Here’s all four mounts with one motor plate removed and the four screws for each plate. Now each mount weighs 15.5g. But I’m not done…

Now each mount weighs 11.7g! All I did here was switch to nylon bolts. I’ve been using nylon bolts for years and I’ve never had a problem. But I know what you’re thinking-- I won’t be able to tighten the nylon bolts enough to clamp the mount to the tube. You’re right! And, I’m not going to use the bolts to clamp the mounts to the tube. I’ll use a little epoxy between the red halves and the tube. I will never have to worry about my motor mounts rotating out of square. The nylon bolts will simply hold the motor plates. I can take the motor plates on and off without having to level the mounts each time.

Can I go further? Hmmm…

Here I am at 32g, the same as my 3D printed mounts! But wait, I don’t like it. Now I only have half the surface area to epoxy to the tubing and I don’t have the safety of the nylon bolts. Even though the bolts aren’t clamping the mount to the tube, they still go through both halves, making a solid ring around the tube. The bolts pass through one half with no threads, into the other half that’s threaded all the way through. So far, the 46g solution looks the best.

But what if…

What if I used the lighter halves of the red tube clamp (the ones with no threads) and cut some pieces of carbon fiber plate (10mm x 32mm x 2mm) for the bottom side of the tube? The answer is 8.8g each, or 35.2g total! And, now I have some safety once again because the mounts are held on by epoxy and by the nylon bolts running through my carbon fiber plates on the other side of the tube. If I want to change it to X8, I can just use the other halves of the tube clamps and I’ll be back to the 46g configuration.

If you’re worried about the nylon nuts coming loose, use a little CA glue. If you need to remove them, you can cut the nylon bolts like you would a zip tie. Personally, I don’t worry about nylon bolts coming loose. They are naturally self-locking. I’ll probably do a separate post just on nylon bolts.

What about square tubing?

Well, I’ve been thinking more and more about square tubing. As it turns out, the previous solution works even better for square tubing…

There you go! 6.5g each and only 26g total! All I did here was replace the aluminum tube brackets with 10mm x 32mm pieces of 1/8" birch plywood. Sorry I didn’t drill holes for the picture but you would run the bolts through the wood, which just acts as a spacer for the motor shaft and its bolts. The mount bolts continue though the carbon fiber plates on the underside of the square tubing. The wood spacers are easily epoxied to the tube because both surfaces are flat. If you’re only building a quad, I would epoxy the carbon fiber plates as well.

Here’s where I need your help…

Looking at all of this, I think a great solution would be 3D printed “rings” that are similar in size as the red aluminum tube clamps. But instead of each side being two halves, each side would be single “ring” that slides over the tube and is epoxied in place. The rings don’t need threads, just a hole on each side. Bolts would run through the rings, then through the motor mount plate, and secured with nuts. The carbon fiber plates on the bottom are no longer needed.

Rings could be printed for different sizes of tubing, including square. If you’re building an X8, you would just add a second motor plate.

If any of you would like to help me make some of these, please let me know. I don’t have a 3D printer and I don’t have any of the software people use to print things. I can create nice looking 3D models in Blender if that helps. I think this system could work really well. It would be stronger and SO much lighter than anything else I’ve seen!

Where do I get nylon bolts?

Local hardware stores have them but you’ll pay a lot. The best place I’ve found is McMaster-Carr. Here’s an example: Nylon M3 x 0.5mm thread, 20mm long, black, $7.87 for a pack of 100.

Tobinejumi · April 24, 2018, 6:56am

There is my version of carbon fiber motor plates with 3D printed ABS mounts (motors sits on rays with inclination): http://a-photo.in.ua/albums/misc/3d/motormount-3.png

If you want, i can model your version of mounts and provide file for 3D printing.

rob215x · April 24, 2018, 7:10am

Very nice, I hope you don’t mind if I have many questions…

How much does your complete mount weigh (including bolts)?
Do you glue the ABS parts to the frame, or does it clamp?
How many degrees is the motor tilted?

I been researching about tilting the motors slightly to improve stability but I haven’t found any actual science yet, just many people guessing, lol.

Tobinejumi · April 24, 2018, 7:25am

Don’t remember weight, and hard to tell now - it’s mounted on quad. However, i can estimate weight in CAD if needed.
Mounted on thin doubleside adhesive tape, mostly to prevent rotation when tightening screws. Possibly gluing on epoxy is better solution (except possibility to unmount them back).
Tilt is 5 degrees.

fnoop · April 24, 2018, 7:29am

Certainly not where you’ve marked it, I’m guessing you’ve never actually held one of these. The only way it would break is if you crashed at speed from a height onto concrete - they’re very, very strong. They have a design flaw but it’s not there. They are very heavy and really overkill for most motors but they have the advantage that they fit over the end of the tube and extend the arm length, and they’re also good for mounting ESCs underneath the motor - ie. they’re a good integration unit. Tarot also do 16mm vibration dampening mounts made from aluminium which are nice if you have clunky motors/props.

I don’t know many people who would trust safety and potentially expensive payloads to nylon bolts, and gluing things in place is tricky and permanent. ‘Building a better quad frame’ shouldn’t mean sacrificing practicality and pragmatism for numbers on a scale. Lighter != Better.

The alien mounts look to be copies of the original tarot 650 mounts, which worked well, and I like the ideas you’ve come up with. @Tobinejumi’s mounts look really nice. Double sided tape (or even single sided) is an excellent way to stop the mounts rotating on the tubes and much easier to undo than epoxy.

rob215x · April 24, 2018, 7:35am

The double sided tape is a good idea! I will do an experiment to see how strong it is with my nylon bolts, thanks! The thing about using epoxy is it makes the tubing stronger too because everything becomes one piece. You can still remove the carbon fiber motor plates.

If I model my mount idea in Blender, what file formats can you import?

Tobinejumi · April 24, 2018, 7:38am

I’m using (and recommend to others) Autodesk Fusion 360, it’s free for hobbyist using and powerful. It’s can import STEP, IGS and variety of mesh formats, but meshes needs some more efforts to editing.
You can send me just hand drawn sketches with dimensions, thats enough.

rob215x · April 24, 2018, 7:54am

Thanks for your feedback. All good points.

That is correct. I have seen some cheaper copies, like mount “C” and that’s where they break. I just don’t want to add 120g in motor mounts to a frame that only weighs 140g.

And that’s fine. We can make lighter motor mounts and still use metal bolts. With my 8.8g configuration, changing the bolts back to the metal ones, makes it 12.4g each.

fnoop · April 24, 2018, 12:03pm

Yeah I totally agree with you there! I love my tarot 650sport frame but it’s sitting in the cupboard because it’s too heavy. I only use it when I’m going to do an important job that I need maximum reliability for, or retracts.

mike · April 24, 2018, 12:30pm

I would agree with fnoop that those Tarot plastic motor mounts are bullet proof. They are heavy tho so I don’t use them.

The problem with the square tube is that I think you will find that none of the typical plate mounts you showed with your wood backing plate are actually wide enough to straddle a 20mm tube. I also would never use nylon bolts they just can’t handle the load. Also I would be concerned about slippage if trying to straddle the tube.

I use the Tarot anti-vibration mounts which are surprisingly light, except the X8 anti-vibration mount which are surprisingly heavy, and I just bolt them to the tube. But the square tube will deflect when compressed in the middle making that attachment a problem.

ambattuhari · April 24, 2018, 12:58pm

and Sir, epoxy- ing or bolting the motor mount, before checking the motor balance: ? your comments.

Hugues · April 24, 2018, 4:56pm

OMG how did I completely missed this blog until now ? Excellent. Keep up dispensing the right knowledge.

fffrantz · April 24, 2018, 6:03pm

Actually, quite a few of us that are in the know do use non-steel bolts in most all applications. It’s a matter of understanding the properties of Nylon. We use two types of materials to mount motors and props and structural components: Nylon or Aluminum. Never use steel unless you need ferrous properties. It weighs too much … and the red and blue Aluminum parts are rather pretty.

When to use Aluminum.

high motor torque exceeds nylon shear/vibration capability given available bolt holes
high motor temperatures would degrade the shear capability of nylon

When to use Nylon

when not using Aluminum (typical four-hole moderate size motors)
when shearing away the motors (heavy part of ship) can save the ship structure in a crash
when on a ship where duration is important
for most all spacers
for most all washers
for most all nuts
for most all connections when joining structural elements
on lock nuts

With nylon, check tightness prior to flight. You can also use nylon like a locking nut by using Imperial sizes on metric. But use care.

Off the top of my head, I can think of only one reason to use steel … in the motor housing to contain the magnetic fields. But I’m sure there are others as well (but many of those can be fulfilled by Ti).

QuadStarDrones · April 24, 2018, 10:49pm

Great information! Perfect timing since I know a guy that is just getting into this!

Coby · April 25, 2018, 4:22pm

Cost, availability and overall ease. For some the extra weight is not as much of an issue as being able to easily source the parts and/or worry about integrity of build after hard “landings”. Again, we get back to defining the use case and context. If you are worried about weight to the point of using Al or Ti then you are likely spending some cash, and building a wood frame may not be a match to Al and Ti fasteners.

Also setting and checking torque on a nylon fastener if you are into such things. Most DIY builders probably not using a torque wrench and/or production quality assurance practices. Probably overkill anyway. Unless you are concerned about your nylon fastener being over torqued and stretched too far. Or if it’s been in a crash and taken beyond yield but not ultimate, so now it’s deformed and you do not know it. I guess you could institute an NDI program.

Coby · April 25, 2018, 5:28pm

Difference between bending and torsion. It all comes down to where the material is in relation to where the force is applied. Round tubes put the material right where you need it for torsional loads. Square tubes do the same for bending loads. I beam even better for bending but they suck at torsion.

You might be able to get rid of your internal cross braces if your ply is thick enough. Full wood monocoque.

Coby · April 25, 2018, 5:34pm

And get rid of requirement for motor mount since you should be able to drill out end for direct motor mounting.

Also makes the build easier.

EDIT: Not so sure. Finding thin walled square tube in a good size seems harder than it ought. Don’t like the idea of one vendor in China being the only source. In the US it looks like 1in square is pretty common. A bit big for this application I think.

fffrantz · April 25, 2018, 6:02pm

You are correct Coby.

As you can probably tell, many of us are not your average builders. We build high performance ships where 5 grams:

begets an additional 1 minute of flight
if placed 10 cm from CG adds 500 g-mm^2 to the MOI of the ship so it doesn’t turn as fast
in a 10 g corner or takeoff is actually 50 grams.

You are right. Not everyone cares about that. Some just build for fun or the education, which is great.

The only reason some of us build ships is to end up with something better than what you can buy commercial or to set world records, or to teach the commercial world how to build high performance drones, or to make money building quality/performance not offered elsewhere.

FYI, aluminum parts are inexpensive and easy to get (like buying from Amazon). The probably add 20% to your motor/prop costs. If you like the colorful ones, like I do, try (I don’t think that McMaster sells colored)

http://www.probolt-usa.com/aluminium.html

Coby · April 25, 2018, 6:32pm

Looks like a great resource!

Yeah, I tend towards truck vs race car so nylon usually does not work in the long run for me. Al could though. Just what I need , another excuse to buy more crap for the shop.

fffrantz · April 25, 2018, 6:50pm

OOOh! Trucks … so mud and lots of water??? If so, stick with steel. Dissimilar metal thingy will cause rust.

You should post a photo of your beast sometime.