Spinning and Winning (HW Spinner Questions)

So after watching Heat D for Death last night, and having seen the (kind of unfair) number of spinners there, I came away with a number of questions about how these things are put together. We saw Supernova's sprockets getting buckled and their chain flying off, Ironside 3's bar assembly getting bent, and Pulsar's drum seizing up. Obviously there are many different ways to build a spinning weapon, but we rarely get to see the actual inner assemblies of these bots on TV. I was hoping some of the spinner aficionados here could enlighten me:

1) What is the optimal way to power a spinner? Pulsar and Ironside 3 use drive belts (which I assume slip under excessive torque) but Carbide and Supernova use chains. If I remember correctly, Carbide has a torque limiter to prevent the full shock force of the hit from being transmitted back through to the motor, but that makes me think, surely the motor isn't actually useful at the point of impact. Robot spinners aren't like an angle grinder that requires constant torque to spin a very light blade consistently against a material. As far as I know the vast majority of the force imparted comes from the inertia stored in the disk/drum/bar. Is the motor used at all during the impact, or is it only useful for storing energy in the weapon by accelerating it? And in that case, what about powering the spinner with a powered wheel that's pressed against the drive shaft (or a connected pulley). Is there a reason most teams don't use this approach to mitigate shock?

2) What kind of shaft diameter do people use? Having seen Frostbite's puny piece of threaded bar, shaft size surely matters, but how much? Do the shear forces on a horizontal spinner have a massive effect on the shaft? Do they regularly get warped because of this?

3) Sort of still linked to the shaft thing, where do people put the bearings, and what do they use? Many robots I've seen have the bearings at the top and bottom of the shaft, but does anyone have a stationary shaft and have the weapon and pulley/sprocket mounted on the bearings? Are there some special high-force bearings that roboteers use, or are they just regular ones specced for the size of the shaft? Does anyone get away without using bearings on a spinner?

Sorry for all the questions guys, and thank you for reading and answering. Me and a friend are looking into building a heavyweight for Series 4 (if it happens) and now I've got a bit of experience building, I need to start gathering knowledge on heavyweights. Plus I'm generally curious after last night's episode.

Pretty brief reply, but Pulsar uses timing belts because they give some slip to the system and it means it can go directly to the motors without the need for a clutch, because of the size and surface area of the weapon it has the torque to use this. carbide, other robots with a weapon that size, they need more torque to spin up and a smooth pulley would slip too much so chain is better, but it means they need a clutch to stop the motor dying from impacts.

Shaft size does matter yeah, most featherweights go for around 20mm thick shafts with as much support as possible. Deadshafts (stationary) systems normally have the bearings in the hub/pulleys either side of the weapon, advantage to using a dead shaft is that the shaft can be used to support the bulkheads and be a structural component in the robot.

1) I use a V-belt on PP3D same as ironside did. B-profile. I like them because as you say they do slip but they provide a very good simple way of providing power to the weapon. No one uses a wheel pressed against a weapon because it's not a very effective method of transferring power. The best way to think about it is, with a belt profile, I have contact right around the inner surface of the belt when it's touching the pulleys. Lets assume the pulleys are the same, that's essentially a full pulleys worth of contact to transfer power. A wheel touching a disc or shaft has a small contact area where the two touch. That's it. You simply can't transfer power quickly enough through such a small area.

2) 90mm diameter titanium bar - 230mm in length. Complete overkill but it's the one part of the robot I've never had an issue with. Plus I got it cheap on ebay. You could calculate optimum shaft dia etc but quite simply, go as big as your design will allow.

3) Bearings are top and bottom of the shaft with the disc bolted onto the underside. 80mm bore ball bearings. Nothing complex about them. Cost around £25 each. You can also use bushings but you will need more energy to get the shaft moving and these are not great for high speed applications.

You can see the various bits of PP3Ds assembly on the website http://pp3d.weebly.com/

I put various CAD images through the stages, the various parts of the bot and some overall pics throughout.

We use a double V belt setup on Aftershock- never had any issues with it. We went for that setup for the reasons listed above- they give just enough slip under impact but still allow us to get up to speed in under 2 seconds.

We've got a 40mm dia EN8 shaft. Really nice stuff and still like new after all of those big hits

Aftershock doesn't use bearings- we opted to make our own Phosphur Bronze bushes- we've got a set inside the disc and the pulley and a second set inside the bulkheads. We went down this route so that we had redundancy built in in case one set of bushes failed. In the end they all survived pretty well and we'll be keeping this setup going forward.

I'll try and get some decent photos of our setup uploaded to the website.