(FW)Nibbler- Horizontal Spinner

[Eventorizon]

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]

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]

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]

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

[GlenR]

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

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.

The entire rotating mass is about 4.5kg, the bar alone is ~3.5. The original motor i had rewound for it was about 7500rpm on 6s, the one it ended up running through the event measured at 5500rpm which was a 12s motor running only on 5s. Turned out well though, will probably keep it around that speed next time and just adjust the gearing / motor wind to suit

[overkill]

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]

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.

[Cameron_S]

Good to know why the large wheels break gear boxes (analogy helped). Unfortunately there seems to be little way of reducing the size of the wheels while keeping it invertible (expect from having two small wheels on top of each other which seems difficult). Might just be a case of have lots of spares and hope it doesn't happen too often.

Thanks for the specs on decimator's bar, faster than I thought.

The design has also been changed to fit the smaller bar and the arms upgraded to 20x20x2mm steel bars. The weapon motor has also changed to this- http://www.hobbyking.co.uk/hobbyking...ize_heli_.html on 7s which I believe is the one Matt is talking about. This seems a worth while upgrade with the increase in peak power. However the increase in kv, puts having a small pulley on the motor going to large one on the bar impracticable. So would it be possible to have a small pulley on the motor going to a large pulley on a shaft near by which also has a small pulley that then goes to the bar? I think an image will be clearer. (The smaller pulleys have 12teeth-the large on on the shaft has 40-the one on the bar will have 36) Should reduce the RPM to about 3500.

Edit:Any way of making images smaller

[maddox10]

Valkiri uses a Z40 around the motor can, and a smooth ali 112mm diameter pulley on the disk.

[overkill]

The technical term for that is a jackshaft. A double reduction will work but has some drawbacks: It will add weight and complexity and takes up valuable space. There is also a potential problem when you have a small pulley close to a larger pulley. The small pulley will have very few teeth fully engaged with the belt (maybe only 4) and those teeth have to transmit all the motor power. this graphic explains it better:



When the blade hits something, there will be a huge amount of force on those few teeth and they may strip off the belt, or they might slip and start to wear down. Either way, the belt will wear out much faster than it should. The way to prevent that is with an idler or 'back-breaker' wheel, which increases the belt wrap around the motor pulley. Its also a good way to adjust the belt tension.

The alternatives to using an idler are to use a wider belt and/or larger pulleys. You can also use a smooth pulley on the weapon shaft to act as a clutch and limit the amount of force on the pulley teeth. I don't really trust this option as its hard to know what the right amount of tension is.

All the above solutions will work but the best or at least most elegant solution is to find a motor with a lower KV and have a single reduction to get a weapon speed of around 3.5K rpm, with a 4:1 reduction ratio and a 7S battery, you will need a motor with around 600 to 650KV.