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**maddox10** · 19 July 2015, 17:03

You're mad to ,start with Robot Wars whatsoever. Build something that takes a lot of time and efford, and then hope it won't get smashed in 5 seconds.
Join the assylum.

Hardox is a fine material for the spinner.
The grade, as long it's higher than the opponents armor, it's good. so aim for 500+. Valkiries disk is hardox 550, 12mm thick.

Expect everything to break sooner or later.

Brushless motors have mounting holes on 1 side.
The NTM outrunner has threaded holes on both sides. Mounting to the frame is done with the 4 M4 holes on the side with the wires coming out. The other side, on the spinning body, has 3 M3 holes to mount propellers to.
Oh, and ruggedize the NTM. The weak point are the bearings and the magnets. bearings can be replaced with decent brand bearings, and the magnets really love a lot of glue.

Hannibalito4 and Valkiri3 used a 25A per channel esc, and they moved fairly well; but with higher graded esc's it went better. the 30amps could work. But take the advice, go for the TZ85's.

Hardox, small bits of high grade Hardox I buy at a place that makes excavator buckets and simular.
Expensive, compared to buying a big sheet and cut from that, yes, but a full sheet goes a few Â£1000, and a 350mm diameter disk goes about Â£100 in raw material.(and around Â£150 in lasercutting cost)
The proven and renowed Hardox 450 3.2mm thick we buy in bulk with the DRG, and then we get a pasture sized steel plate of 2.5*6 meter (last time it was even a 2.5*8m), And then the cost is "less". If I recall correctly, divided in sheets of 1.5*1m it was about Â£160 a sheet.

**typhoon_driver** · 19 July 2015, 19:54

Carry out your bite calc at 1m/s. The vast majority of collisions are at low forward speed.

**overkill** · 19 July 2015, 20:39

That NTM motor may not have enough low speed torque to get the bar spinning up in a competitive time and will likely get dangerously hot. The NTM motors are good quality but I would try to use something with a wider diameter and closer to 4KW peak power (or at least leave room for an upgrade).

The main concern to me are the aluminium arms supporting the weapon - With the energy on tap they will very likely bend on the first full speed impact. Try to increase the wall thickness and diameter as much as you can.

**Cameron_S** · 19 July 2015, 21:37

Thanks for the advice so far.
So 30 amp ESCs out and Tz85a in. Thanks for the information on mounting brushless motors, understand that a lot better now. For hardox I might make it a problem for future me and sort it out when I need it.

At 1mps collision speed there would be about 2.4cm of bite. Much more realistic and thanks for the spot.

As for the weapon motor, shame the NTM wont do as the next step up gets a bit pricey. The outrunners seem to have a huge jump in price once in the 4k power level, so I might end up having to go down the the inrunner path. I originally didn't want to do this as the increase Kv makes single stage reduction harder, but it looks like I have little choice. Would something like http://www.hobbyking.co.uk/hobbyking...Motor_4kw.html be more suited for the job? Or I could be irresponsible and use the NTM motor and have it hate me and look to upgrade it as you mentioned. The aluminium is a bit of a shame as this was one of the few parts I've already brought. Never mind I could also just keep it for when the bot is in wedge configuration.

**maddox10** · 19 July 2015, 23:22

Valkiri uses an NTM5060 with a gearratio of 1.3. Spin up is slower than wished for but it has a very high top speed on 6s.

a bigger gearratio will solve that. On getting hot. Not really. We measured it and it stays inside any reasonable limit.

**Eventorizon** · 20 July 2015, 09:08

That is a damn good start! You have your list, you have a design and you have a plan!

"Am I completely mad to try and build a spinner with no previous robotics experience?"
- Yes, but that didn't stop me from building Conker 1 ;-)

"Is hardox a good choice for the bar and does the grade matter?"
- The higher the better, 450 minimum - 600 at Best. After tossing a basic bar design through Solidworks, you can get your 500mm bar in at 12mm thick, 60mm wide, with a 90mm dia hub in for around 2.6-2.7Kg.

"Could I get away from only having spares of the cheap things e.g bearings/drive motors? What parts are prone to breaking? Do brushless motors have mounting holes at the bottom? Is it a good idea to mount only to the bottom, or is supporting both sides a better idea?"
- In an ideal world you would swap out the stock bearings in the brushless, glue in the magnets, put in a Titanium shaft and then have a bearing to support the back of the motor as well. That said many machines run stock motors and just have lots of spares. The NTM's are so cheap that its more effort/cost to reinforce them than replace them.

"Is the stall current of a drill motor over 30amp? Is the ESC iâ€™ve mentioned be able to handle the stall current or is the Tz85a a better choice?"
- TZ85A every time. If you look after them they will last you several years and several robot iterations. Just check the insides every so often for swarf and loose components

"Where do people normally buy hardox from? Struggling to find any online sellers that sell something other than digger wear plates."
- That is a tricky one. I got the Hardox for my first weapon from JL Steel in Scotland. They were very kind and provided me with a plate for my weapon (20mm Hardox 500) and my armour (4mm Hardox 450) and asked I donate Â£60 to MacMillan Nurses rather than paying them. However - http://3.bp.blogspot.com/-65GE6TQ2E_.../s1600/041.JPG - That is what I did to a 12mm thick bar spinner last time I met it in combat so you will want a spare.

**GlenR** · 20 July 2015, 11:26

Hiya, Nice start. You are crazy indeed, but get stuck in and make it, then find what fails and improve it till the whole lot eventually works! That's half the fun.

For a bar spinner like that, 500mm is in my opinion too big, the robot will be huge and in the robochallenge arena at least, will barely be able to get up to speed + will throw itself out immediately if it ever does. Decimators was about 350mm effective diameter for reference

The aluminium section on the front is way too weak sorry, that will crumple the first time it hits something 25x25x1.6 or 2mm thick steel would do though, very cheap too.

The NTM is fine to get you going with the aforementioned mods for reliability as long as the weapon sits on efficient bearings. They have decent enough torque especially the super low Kv ones like that. Check out the SK3 motors too, they are substantially better made and tend to have more power IMHO.

Probably have way too much battery as well. Lots of weight can be saved by using 1x for the entire lot, or at least a way smaller battery on the drive. 4s 2000-2200mah is plenty, the drive will be fast especially on 125mm wheels (they might be too big btw, probably will break the gearboxes) and you shouldn't burn out motors. Or go for a bigger 6s pack and run the weapon and drive off it.

Absolutely go for the TZ85s over the 30As on drills, trust me on that one.

Good luck

Keep us posted

**harry hills** · 20 July 2015, 11:38

For the record, that bent bar wasn't hardox, it was a different kind of steel which turned out to basically be only just better than mild. It was also 8mm not 12mm. So, don't build your bar from 8mm mild steel and you'll be fine. Also wasn't that the hit that slightly bent your 20mm hardox beater which caused you to shit even more bearings up through the rest of the comp, Alex?

Echo Glens comments, esp the ones about the tz85. I was an idiot and designed myself into a corner meant I had to use 30a. Don't do that lol.

**Eventorizon** · 20 July 2015, 12:10

Originally posted by harry hills View Post

For the record, that bent bar wasn't hardox, it was a different kind of steel which turned out to basically be only just better than mild. It was also 8mm not 12mm. So, don't build your bar from 8mm mild steel and you'll be fine. Also wasn't that the hit that slightly bent your 20mm hardox beater which caused you to shit even more bearings up through the rest of the comp, Alex?

I hit you so hard I bent my own beater, which was under designed because I had estimated at top speed of 8000rpm with losses, not 11'000 :P And I was using tapered thrust bearings which had to be in compression (something I didn't know at the time) so that's why they kept exploding. If I ran that again I would use regular deep groove and I doubt it would break/explode. As for the thickness, I just looked at the Pic and guessed

And I failed :P

**typhoon_driver** · 20 July 2015, 12:15

I'd be careful reducing battery capacity. The C discharge rating is a multiple of the capacity of the battery. Smaller battery = lower available current.

I would also recommend running the brushless at as high a voltage as it will allow. Going on the black box data from boner when I ran it on 6s and then on 10s later, the current draw difference was enormous. On 6s the motor drew easily around 80a if not more. On 10s the motor was only drawing 10a peak at any time. The 10s was run on a larger motor so that may have had something to do with it but always run brushless at their top rated voltage.

I would also echo the comments regarding the A frame. Aluminium box will just crumple and send the bar into your own machine. A 500mm bar is doable but you would want a much larger motor behind it to get up to speed.

**maddox10** · 20 July 2015, 13:25

The spinning bar of massacre was Creusabro 8000, and should have been the equivalent of hardox 500. Unfortunatly, the ones selling it to me didn't know what we do with it. Now they do.
It's perfectly possible that the Creusabro in real world wear resistance applications will perferm like advertised.

And perfect bearings. Every spinner I have build uses sinterbronze bushings. Seems to do the job.
Advantage. Smaller, lighter and cheaper than the equivalent in bearings.
Disadvantage, it gets messy with the extra grease after each fight.
But for every bearing/bushing, installation by the book, or else you get rubbish.

**Cameron_S** · 20 July 2015, 18:11

As said before huge thanks for all the info it's a big help.

I think you're right Glen about the bar size, I was being a bit optimistic with the length. Reducing it should also help the spin up time, power needed and weight. With the only downside being the reduced energy which, as you said, might not be a bad thing. The dimension Alex talked about also seem a good alteration too (reducing the thickness to 12mm and increasing width to 60mm). This thickness appears to be easier to find and the increase in width should make it stronger (hopefully not having the same fate as Massacre's bar- nice to know that was 8mm and a weak metal

). Out of interest what sort of RPM does the bar spin at on Decimator and how much does the bar weigh (sorry if these are answered elsewhere)? I ask this because it has a great balance between delivering good hits and not pinging itself out the arena.

You all are unanimous in thinking the tz85a is a worth while upgraded and Aluminium not being up to the task, so these have been changed. Probably go for the 25x25x2mm steel arms as talked about by Glen.

With a few more of yous opinions on the motor choice, I think I'll stick with the NTM50-60. If it sufficient for the disk on Valkiri, it should be okay for me even if the spin up is a little slow. The reduced bar size should also help with this. I'll also try to design it so it can be upgraded in the future. After checking out the SK3 range they definitely look like a good upgrade for the future.

Might spec down the battery capacity a bit too (thanks again Glen). Also thanks for the reminder about increasing the discharge rate Gary. Something like this
http://www.hobbyking.com/hobbyking/s..._5S1P_40C.html I think would do with the increase to 50c and decrease to 2200Mah (drive battery). [Edit: ignore the following bit(miss read the post)- like the idea of having one battery, but don't think it's easily possible due to the different voltage between drive and weapon motor and the huge peak current that would be drawn from it].- Thanks for the information Gary about increasing the voltage, but I'll like to keep it 7s or lower. This is solely down to the limit of cheap brushless ESCs seems to be 7s.

As for the bearings vs bushings debate, I think I'll stick to bearing as I vaguely remember reading somewhere they don't have to line up as well (could be wrong about this).

Also out of interest why do larger wheels break gearboxes? Is it because of the increase in stresses from faster acceleration? Finally is they any way of reducing the likenesses of them breaking e.g metal gears?

The information so far as been brilliant and gives me a much better chance of making it an arena with a working robot. So once again thanks

**overkill** · 20 July 2015, 23:46

Larger wheels can break gearboxes as larger radius provides more leverage on the pins and gear teeth. Think of it this way: if you have a stuck bolt, its hard to shift it with a short spanner. Get a long enough spanner and the bolt will shift or the head will break off.

Bearings VS bushings is a complicated comparison. they will both wear faster if they are misaligned but bearings will usually take longer and fail more gracefully (unless you are Alex). The big problem with bushings (which many people don't know about) is their maximum speed rating called the PV value - here is a quick explanation: http://www.mcmaster.com/#about-plain-bushings/=y529dd

The other problem with bushings is that they can't be sealed and pick up grit, leading to wear & jamming. For the speed you are planning, you can use sealed bearings that will be dirt-proof.

**maddox10** · 21 July 2015, 01:43

The bushings I use are on the edge of their spec in Valkiri3.
But that's a monster in weaponpower; Diameter 356, 4 kg going 5.5Krpm. (theoretical 6300).
Start up amps, around 120, running amps, 20.

The grit issue is solved like any good bushing in a backhoe, bulldozer or any other heavy equimpent dealing with massive amounts of dirt and sand. Grease it so the grease pushes out the crud after each use.

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