Experience needed regarding FW Beater and Armor

[overkill]

I have to agree with Andy, most of the weight is over the weapon end and the wheels will not have much grip. The problem with adding belt driven wheels at the front is that you need two sets if the bot is going to drive inverted. Its not impossible but the long triangular belt path takes up extra space and adds weight. The other possibility is to try non-driven wheels at the front to reduce friction - I'm not sure if they would be effective though.

When I Google 'UK silver steel rod' some of the suppliers list hardened rod - a bit of searching will turn up what you need.

With the disk, I would definitely recommend some sort of hub,rather than just spokes that attach to the pulleys at their inner ends. without a hub, the screws will be taking a huge amount of force and might shear if you don't use a large size. With the right hub and pulley arrangement, the screws will only be subject to the force from the motors spinning up the disk. I will knock up a quick sketch of a typical arrangement later.

HDPE is a thermoplastic and can be bent when it gets hot enough. I have bent 5mm and 10mm HDPE using a specialised heat gun but 20mm might be harder.

[Runsler]

i have seen it bent, just not the process.
And since i guess hitting the right temperature as much on point as possible i hoped for someone being able to deliver some numbers for that. Need some more research, someone has to have done a youtube video of it. there is one of everything.
Else it has to be trial and error.

Would be interesting to see the hub-setup, just fear it would take up more space so i'd have to push the weapon motors further to the side armor, rearrange their escs further to the front (if that is still possible then), and have a longer axle that's easier to bend on impact.

Hm, two extra wheels each side... Only place i see where i could add those would possibly be next to the weapon motors, and push the weapon escs more to the front if needed. But that won't be possible with a weapon hub, i fear. And i really have no idea i could make that within the weight, and that would still not be as far in the front as i'd need it (in the worst case, this would make the bot almost balance on these wheels, likely to tip over when the weapon hits anything and causing it to hit the ground, too. that could give me a big jump backwards after every hit and a lot of energy put into me instead of the other bot.
Further in the front the chassis is already getting thinner, and i could hardly make the pulley move inside.

Another solution i might see here: put the wheels i have now to where battery/esc are. i could push those a tad closer together (just have to make sure the battery can't crush the esc when suddenly jerking sideways or landing on the side, and turn it around so the cables go over the esc and can't get into the wheel), switch the actual wheels for pulleys (a bit further to the outside of the bot, too), and put pulleys and wheels a bit more into the front.
Not sure how much good that would do, and if it's worth the effort... and if i could make enough space for something to mount the wheels into except for the side armor. But at least would be along the battery and having the gearboxes to put some weight on the wheels.
Also seen some issues with mounting an axle to HDPE on both sides, due to almost unrecognizable bending when screwing that stuff together, putting pressure onto the axle. But guess i could use a dead shaft there.
Wanted to make the wheels from hdpe anyway, maybe i could just use 20mm instead of 10, making 10 of it the pulley and put a bearing in the middle. should be strong enough if made from one piece, just one more thing i have to ask my friend with his lathe for. If that all fits, i could possibly make the pulleys on the gearboxes a bit bigger than the one on the wheels, too (which will have to be bigger anyway) to also get a little bit more speed. Might have to sacrifice some strength because i won't be able to screw the wall between weapon and battery to the side armor to get the needed wheel size, but i hope i've got enough supports for the bulkheads even without that. Sad thing i need that wall to mount the "ground" for esc and battery there, alse i could shorten it to just the distance of the bulkheads.

Sooo... drawing again this morning instead of "just" math. maybe i'll be able to do the math, too.

[overkill]

I don't recall the exact temperature but when you heat up white HDPE its really obvious when its hot enough - it turns glass clear like this:




At that point its very soft and can be bent quite sharply, although probably less so if its 20mm thick. If you bend the plastic before it goes clear, it will still bend but there will be a huge amount of spring back. When I wanted some accurately formed parts for a recent project I made an MDF frame to bend the HDPE around. I used a high temperature plastic welding heat gun that gets much hotter that your regular hot air guns. It wasn't turned all the way up, so a normal hot air gun might work. plastic doesn't conduct heat well, so 20mm thick HDPE might need heat applied to both sides to get it hot enough to bend.

[Runsler]

Damn, and i got black one^^

Anyway, i guess i'll have some visual guidance to do so then. if it doesn't work well, i'll just get white one.
And making a frame would definitly be needed, would try to make the angel of the frame just a tad sharper than i'd actually want it in case i con't get it hot enough on the inside and have some springback.

In other news: managed to squish ESC and battery together a bit, allowing for a wheel, a pulley and a 10mm thick mount for both right next to them, pushing the point where the wheels touch the ground about 5cm to the front of the bot and almost doubling their size. plus the pulley on the wheel being a bit smaller, i might reach a calculated top speed of about 11mph, that's good enough for me.

But got literally no clearance around the wheel/pulley, will have to carve out just enough material from wherever i have to to make it roll. (assuming a 10mm pulley here, that has to be enough for the drive.

Also finally got a name, since the one i wanted to take originally would only have fit with the double-beater-design.
New name: "Zahnfee", literally meaning "Tooth Fairy".
Representing my job as well as what it could do with this big spinner hitting your cheek^^

And beside a name, i did some math: with new drive system, i am at 8885g (including hardox top/bottom).
not included: the actual spinner blade, bolts and screws, and maybe some small electronic parts i have to change to make everything fit.
Guess that's pretty doable.

[Runsler]

Did you manage to make a sketch for that hub yet?
will start doing the side armor tomorrow, so would be good to know if i have to make a bit more place for the hub there...
Also, how does the hub actually work?
I did believe it would also put bolts on the axis and the disc, so where is the real difference between a hub and the way i wanted to do it first?
is the disc hold by the pressure of the hub on it? That's the only thing i could imagine...


And here are the new pictures of my drawings.
Made the wheels a bit more to the front and bigger, driven by 10mm wide pulleys and fitting belts. Hope that's wide enough, since i just don't have more space. even so i have no planned clearance in this and will have to carve that out of the side walls. Or just use 8mm Pulleys, but i guess that would be too thin for the drive system.

IMG_3461k.jpgIMG_3462k.jpgIMG_3463k.jpg


(Also, anybody knows why space suddenly doesn't work in this forum? had to write this in an editor and copy-paste it)

[leorcc]

test test.

Space works fine.

[overkill]

Here is a hub and disk arrangement using your dimensions from a few days ago. Its a pretty standard set-up with an extra pulley. The advantages are that the bearing is fully encased in aluminium so it can't bend or distort. Impact forces are transmitted from the disk through the hub without involving the screws, so they can be smaller.





It may look complicated, but the individual parts are fairly easy to make and only need a small lathe - I made a larger version of this on a tiny Sieg C4 model. I picked an 80mm diameter for the pulleys based on a 2:1 reduction as you can definitely get away with a lower deduction with two motors. I went with a 20mm axle as its completely encased in the hub & bearing - it would be best if the mounts are as close as possible to the disk to avoid any bending.

Edit: just noticed that I had to increase the pulley diameter to 90mm so the pulley groove and the screws didn't intersect. One idea to reduce the pulley diameter and width is to change to a micro-V pulley. A 'J' sized micro-V (also called multi ribbed) belt is only 3.8mm thick and would get the pulley diameter down to 80mm or less. With two belts you could use a very narrow 4 rib belt to make the pulleys narrower.

[Runsler]

So, just to get this right...
There is the disc, the hub, two pulleys, a few bolts and bushings.

It seems the hub and one pulley are made from one part (is there any problem with making them from two parts? guess that would be easier/cheaper), while the disc and the other pulley are placed on that hub, and then squeezed together by the bolts. Bushing into the inside of the hub, and ready to go?
So the hub basically avoids the needing of a tube-part where everything is fixed on and the bushings come in, right?

So we have two kinds of forces involved when the disc hits:

1. the disc suddenly stops, potentially sheering off the bolts when the momentum of the pulleys (and the motors) want to keep them rotating, but the disc can't. This will be countered by the clamping of the disc between the pulleys, so the force will be distributed through friction more than just the bolts in the way. (should the hub be just a tiny bit shorter than the disc is wide, so i can really tighten those together?)

2. the disc wants to go down while the enemy gets send up, potentially bending the axis.
This will be countered by the hub with the bushings sitting tight on the axis, which is as short as possible so those forces don't get much leverage.

Somehow still unsure about this, sorry... can't seem to get the hang of this concept.

Is Ali as material for all those parts okay, or should the hub be from steel?
The way i would do this would be to just get two Ali discs and an Ali tube, and throw them in the lathe to make two pulleys and one tube that should fit tightly inside the pulleys and disc as well as having bushings fit inside it. So... after all, the hub is something to hold the bushings in and distribute the second kind of forces from the disc to a wide part of the axis? (guess for that last task it should be steel, not Ali then...).


On a second similar issue:
Since i will have to use belts for the drive train now, too...
Any ideas on what would fit there?
Pulley width is an absolute maximum of 10mm, really can't fit more in there.
8mm would be better, but not sure if that is enough for any usable belt.
Also wanted to make those pulleys from HDPE, since it's lighter, i could make pulley and wheel from one 20mm thick disc, and i believe those don't have to be as sturdy as the weapon system. If just a HDPE-wheel direct driven on a gearbox works, HDPE-pulleys should be just fine, right?



A lot of possibly stupid questions, just want to make sure and not causing any avoidable problems on these important parts.

[overkill]

The tube through the disk and one of the pulleys has to be one piece; if the tube is separate then the disk and pulleys can be pushed sideways off the tube. Here is a simplified list of steps to making the hub assembly:

Buy a piece of round aluminium stock a little wider than the pulley diameter and 25 to 30mm longer than the two parts so there is something for the lathe chuck to grip. I recommend at least using 6061 alloy or 2024 if you want a really tough hub.

Face one end of the stock and turn it to the diameter of the pulleys.

Turn the first pulley (the one without the tube) on the end of the stock.

Cut the pulley off the stock and face the other side. The central hole will be cut later.

Face the remaining stock and then turn the tube section to diameter, checking to make sure its a tight fit inside the ID of the disk.

Drill and then bore out the centre of the stock for the bronze bushing. The bushing should be a press fit, so under size the hole by around .02mm.

Take the part out of the lathe and cut it roughly to length.

Put the part back in the lathe, held by the tube end and make sure it is running true.

Face the pulley end to final length and then turn the pulley profile.

Put the first pulley back in the lathe and get the face running true.

Drill & bore out the pulley so its a close sliding fit over the tube on the other part - that's all the lathe work done.

You can now use the holes in the disk as a guide to drill and tap the hub and you are all done. It sounds like a lot of work but trust me, that is the way a machine shop would tackle it. Depending on how fast your friend is on a lathe, it can be made over a weekend from start to finish.

The hub would be extremely heavy if it was made of steel, virtually all builders use aluminium. Due to the thick cross section of the parts, they will be more than strong enough. There are a couple of reasons to start with a single piece of round bar rather than two disks and a tube; as mentioned having the tube as a separate part means the pulleys and disk could be pushed sideways off the tube, even if its a press fit into one of the pulleys. Buying a tube remotely near the right dimensions will be impossible. Holding a thin disk in a lathe while you turn the entire outside diameter is a real pain! It can be done but your friend would need a large diameter expanding collet and they are expensive & rare in a home machine shop. Cost; buying a single piece of stock may actually be cheaper as the supplier will likely charge a cutting fee to chop two thin disks off a length of bar stock. You might also be able to pick up the bar stock as an off-cut on eBay.

For the drive pulleys, timing belts are the usual choice. Making the pulleys yourself isn't an option but the commercial aluminium ones are light to begin with.

[Runsler]

Okay, i guess i understood that now...
Will have to ask him then, but that sounds really doable.

Hoped for the driving pulleys from HDPE mostly because i would (now) know how to put a HDPE-Disc on the gearbox hub, and how using a lathe to make a 1cm pulley and a 1cm wheel from a 2cm disc of hdpe seemed like a solid plan

But gonna search for those pulleys now, only found steel ones yesterday. Hard to find something with the imperial thread of the gearbox here, might need to cut that.