Experience needed regarding FW Beater and Armor

[Runsler]

Maybe not most knowledgeable, but the will to make the effort and take the time to share this knowledge even to such noobs as me is a really big plus

And actually i was thinking of making the tube (almost) the diameter of the discs, so it would work as a drum with teeth only on the outside. But yeah.. making it smaller and possibly just use that as the pulley also sounds great, have to do the math on it.

On another route i was redesigning the weapon end completely to put the motor on one side, have slightly angled side/front armor and two 1cm thick aluminium bulkheads to put weapon axis as well as motor and pulley on to, and only having a ~10cm long (maybe a bit more, but have to make better scetches before i can say that) and 25cm diameter place for the weapon to fill.

Don't ask me how i did that, should do some recalculations, but somehow i ended up with adding a lot of Ali and a longer circumference (if i only count those places with side armor) and still somehow saved 1.7kg o.0
So if i didn't miscalculated the needed screws (i'll need a few more now), too, that leaves me with over 6.5kg for the weapon and pulleys left. That doesn't seem to be right. Would really have to upgrade to a bigger motor with that, i fear.
There must be a mistake somehow, going to try and put it into CAD today/tomorrow to prove me wrong somewhere. Only if "C" will be Cardboard or Computer i have to decide. possibly both.

Anyway, looks a lot more like the picture you posted earlier, nick. Just not exactly 45° corners, and still more like... up to 25cm of the bot lenght is weapon/bulkhead, and the Battery, ESCs, drive system and wedge only take up a combined 17cm of length.

[typhoon_driver]

I gotta admit, I preferred the crazy two beaters either side of a solid center. There's a lot to be said for making something completely different. Take PP3D, if I'd gone conventional then I'd have carbide with a disc or that celeb bot that Suzi had. But I decided to mount the disc on the end of the shaft and design the bot around that with big exposed wheels. It's meant comprises for sure but a lot of the featherweight drum bots are all starting to look the same with little differences between them. Essentially the same as the heavyweight wedge flippers all looking the same with a different paint job.

[Runsler]

Definitely liked that one, and i really want to build something out of the box-shape
but unless i can get smaller internal parts or would go completely without top armor (only some thin plastic to keep parts from falling out) and side armor also almost gone, i would never have the needed weight to make this.
Or at least not the needed money to pay the weapon needed in that. And yeah, a wedge that would be shredded by first contact even with just another wedge... that would be pretty pointless.
The plans are still alive, just sleeping.. maybe one day.

But maybe it could be two discs instead of two beaters, making things a bit thinner and lighter. i'll try some options the next days.
But biggest problem with the "two-discs-on-a-tube"-design would be that i don't know how to properly fix the discs on the tube. would need two hubs to do so, too... maybe i could use one of then as pulley, too.
But without a mill my options here are quiet limited. Have to ask around a bit for that.

[typhoon_driver]

A solid bearing / bushing mount, pop your axle through the middle and have a threaded section on each of the axle. Put the discs on with a smooth hole in the middle of them and tighten the nuts against each other with some threadlock

[Runsler]

Well... again i did think overcomplicated.
So basically take a pipe of steel, cut threads on both ends, put bushings inside, and ready is the middle part of the weapon?
maybe need to make the middle section of that part a bit thicker, so i can keep the distance of the discs to each other.
I like that idea.
Also not much machining needed to do by myself... good thing without a mill or something like that, i can just get the discs get cut out and put them on the tube. for small parts like cutting the outer parts of the tube a bit thinner i can ask a friend with his small lathe, i think.
Assuming that part would be made from steel (no need to be hardox, i think...) how thick would that have to be? after all, with a hit on one disc, the energy from the other will be transferred into that by this middle part. that's a lot of torsion.
Starting to wonder if i actually can get a tube with those specifications from the shelf, and making one from a solid block of something could be more than i (or someone i know) could do.



Oh, now that i seem to need an axle... how thick should that be?
tried a quick search through build diaries, but honestly, since most thread titles only contain the team name or similar it isn't easy to find something that specific there.
Silver steel seems to be good enough for that, at least so much i could find.

[maddox10]

Axle. Valkiri did bend a 20mm silversteel axle in the first decent fight against a wel armored opponent, and that was a short shaft, 90mm, of wich 65mm unsupported.

Due shape and weight constriction, I replaced that with a 20mm surface hardened shaft, and that seems to hold well.

But for a drum of that construction, my gut says 25mm at minimum. And that in case hardened steel.

[Runsler]

Ah well, will have to see if i can find a suitable design for the discs, anyway.

Solid they would be a bit more than 4.5kg each, so more than 9kg total, assuming 12mm hardox as earlier in this thread was mentioned.

Well... the first beater design would come in lighter, so this would be quiet pointless, and only make the work a bit easier, but still not practical at all.

Maybe if i try something along what Wrex for example uses, more a spinning ring with lets say three spokes. might need a bit more than 12mm thickness then. Have to try that and see how heavy it is. If not, i might have to go for the single disc or beater in the middle solution, or take a double bar spinner instead. Although these seem to be extremely unpopular as vertical spinners. Is that just because they have inefficient weight distribution, so they are usually just used with bigger diameters (so: horizontal spinners)?
Can only remember two verticals with a bar, one of them being "Huge", and that one is special, anyway.

[overkill]

The reason there are few vertical bar spinners is that a short bar is basically a solid disk with a bit clipped off the sides and it is less weight efficient than a disk with spokes. Its not until you get to around 350 or 400mm diameter that spinning bars make sense. At that diameter you have to go to crazy lengths to make a vertical bar spinner work. You can still do a ~250mm bar but it won't have as much KE as other shapes with the same weight. There some ways to improve that, like an aluminium bar with tool steel tips, or fancy CNC shapes but they will add cost & complexity.

Most builders end up going with the cheapest, most KE efficient design that fits into their weight and space restrictions. In your case the ~250 diameter is between the comfort zones of drums & beaters and not quite large enough for a spoked disk. If I had to build a vertical weapon that diameter, I would go with a Hardox disk 25mm thick, with thin spokes and the smallest possible hub to keep most of the weight out at the rim.

[Runsler]

My motor seems to be a bit of a problem, anyway...
So even if i could store a lot of energy in the weapon, i am unsure the motor could provide that energy in a reasonable time.

Searching for other motors of a more "flat" build, since the diameter of the motor regardless what i will actually build seems to be no problem, but the length will be. That would even fit into the middle of the double-weapon-design, but it is quiet hard to find in that size, they only seem to come really big or quiet a bit smaller (mostly for quadcopters, it seems)

Solutions for the single-weapon build would be:
A longer motor, and put the weapon more to one side (would like to avoid that, the expected gyroscopic effects would be even harder to handle if i tip to one side easier than to the other).

Two of the motors i already have, but i doubt i could drive two brushless motors with one ESC, even if it is two times the same right? even if it has enough power.
That meaning one more ESC, too, and finding a space to put that into and secure it at.
Also i'd need twice as many pulleys.

Or also a more powerful, but still at most 5cm (without shaft) long motor of a "flat" design i have still to find.


If i don't find a practical way to do this, i might as well just go with a bar as spinner, since ineffective energy storage also means it is easier to spin up for my motor, if i won't have the time needed to spin up fully, anyway. Could make that one really solid so i won't have to do it again soon, and might also be the cheaper solution. But a disc is cooler...

[maddox10]

Driving2 brushless motors on 1 esc, forget it.
Sensorless brushless motors use the ripple/back emf to "read" the position of the magnets compared to the windings and adjust the cycle to that.

Sensored ESC/Motor combinations are a lot more accurate in that reading process and can achive better startups due that.

But introducing 2 sets of magnets and winding not in sync.... That's a a recipy for disaster.

[overkill]

I could be cheeky and suggest this motor:





It definitely has enough power and it is only 53mm without the shaft.

Using a bar is fine, just try to get as much weight out at the tip as possible for more KE.

[Runsler]

thought so, but well, would be a shame if it was possible somehow (maybe by synching both motors or stuff like that), povide a somewhat "easy" solution and i won't use it just because i didn't think someone could find a way to make things work i can't. that would be stupid.

Would it also be a problem if i would use two ESCs, two Motors, and one Pulley on either side?
Just found out i'll get a space just big enough for the ESC right next to my weapon motor in the front part that's now angled. So won't be a problem to do this on both sides.

While the NeuMotor looks great, it is specificated for much more than 5s, and honestly... getting my current motor/ESC-setup twice would be cheaper. But i bookmarked it, so it definitly is an option, and if not for this build maybe for a later one. Always could use flat, powerful motors, to find long ones is way less trouble.
Also, with only 5S i don't know if the kv isn't too low on that one, would maybe even need a smaller pulley on the weapon than on the motor to get a good speed.


Got on with the weapon design, did a spoked disc inspired a bit by Valkiri, i have to admit. Diameter 250mm, at 24mm thickness, with the outer ring as well as the spokes (four of them) being about 20mm wide (a bit more for the ring on the tooth and counterweight), coming in at about 3000g if made from Hardox.

That's looking quiet doable, also having no problem to get it fit into the rest of the build. Motor and pulley on one side of the disc in this version, the axis will stay static and only disc/pulley on it will rotate (allowing some big bushings). Static axis is put between two 10mm Ali bulkheads, supported from the outer sides by 2cm HDPE coming at about 43-45° at it (have to fine-tune there a bit, depending on the actual motor i will use this angle will change a bit).
Motor will also be mounted on the same bulkheads (on one side of them), the shaft going through the bulkhead and then the pulley on it. got about 3mm clearance between pulley and disc as well as pulley and bulkhead, just gonna need a longer weapon shaft, i fear. the original one is just 20mm long, and would only stick out 10mm from the bulkhead. not a good thing to mount a 14mm pulley on to, even less with the clearance. (and i'd have to figure out how to put the pulley on that, without making everything wider. maybe just put holes through the hole diameter of the pulley, make them threaded and bolt that thing on the flat part of the shaft.

Would that be possible, or again put too much stress on the Motor?

[overkill]

I was just joking about the Neu motor when I noticed that it fitted the space you had - its more at home in a heavyweight .

Running two brushless motors on one shaft is fine, its just the ESCs that have to be separate for the reasons Mario mentioned.

The disk design sounds fine, you could probably thin the spokes a bit as they are quite short, or even go down to three spokes to get more weight on the rim. If you can spin that up to a high RPM, it will be very effective.

You can replace the shaft in an outrunner motor fairly easily. There is usually a circlip on the shaft at the bottom of the motor; popping this off with a small screwdriver lets you pull the entire endbell off the motor. The shaft is held in the endbell with two tiny grub screws. Loosen them with an allen key and then gently tap out the shaft with a punch and a hammer ( I usually end up using a long screw as a punch). Buy a length of silver steel the same diameter as the original shaft , cut it to length and do the reverse of the disassembly procedure. You are better of without the circlip groove in the new shaft, its a weak point where the shaft snaps.

If you are mounting the motor on an aluminium bulkhead, there is a great opportunity to add a support bearing to the shaft. Here is a photo of a motor mount I made with an extra support bearing:



The motor mounting holes limit the size of the bearing you can fit, but I discovered that a needle roller bearing is perfect for the job; it takes high side loads and has a small diameter. If that sounds useful, we can work out exactly what bearing to get.

[Runsler]

It indeed sounds useful, and i already had the idea of the extra bearing. just dropped it, because the original shaft of the motor is flatted at exactly that point, so a bearing there would possibly cause problems.
But with a new shaft anyway, that's looking really good what you have there.

What is the "flattest" way you know to mount a pulley onto said shaft?
Was thinking about just thin holes from the outside through the complete radius, put a thread in, and bolt a... let's say M3 straight through that onto the shaft. Maybe two, one from each side, and flatten out the shaft there just a tiny bit or drill tiny crater (or what is a good word for it?) in it to give the screws something to hold onto.

An additional hub would really widen the construction quiet a lot.
I will try to get my drawings done the next 1-2 days and post them here, so it's easier to communicate everything (and maybe you'll spot something else i did wrong).

Also need to think of a way to put the two pulleys and the disc onto a tube, without needed wielding (i just can't do it, and would prefer to make as much as possible on this bot myself and with tools and equipment so i can repair it myself if needed). Possibly screw the pulleys sideways in the disc (so better not thin out the spokes, at least not there^^), and do with those pulleys the same as with the small ones to connect them to a tube connecting them all. Only more sturdy, since there will be more energy involved.

[overkill]

If you need to save space with pulley mounting, about the only possibility is grub screws at the bottom of the V. "Grub screws suck" is a bot building truism but sometimes there are no alternatives. You can improve their reliability by grinding flats on the shaft, using Loktite on the threads and using the largest possible grub screw. Drilling a recess for the point of the grub screw is another option but harder to do with a silver steel shaft.

I am not sure about how your hub arrangement looks, but its really hard to put screw threads into Hardox. Its not totally impossible but will need a specialised tap, ultra-heavy cutting oil and a precisely sized hole. Most builders use an aluminium tube or block and tap into that. By coincidence Ellis just posted some inspiring examples:





You probably won't need quite as many bolts as Ellis used