Rotating Blades

**Emerald** · 31st January 2016, 18:16

Hey im looking at designing two horizontal rotating blades. I was wondering what I should be looking at for maximum effect. I know weight helps and so does speed. I so should I be aiming for weight over speed? also should I be looking at sharp or blunt. if sharp short or long?

sorry if its asking for a lot

Ps. I know less teeth is better so ive chosen on 3 or 4 tooth

**Danny B** · 31st January 2016, 19:40

You need to balance weight with speed, and that depends on what type of motor you're using (most of the FWs I've seen use Speed 900s).
Whatever it is, you can maximise the energy going into your disc if you bring more of the mass to the outer rim. As for teeth, chunky and blunt is your best bet. Sharp, thin blades are only good for localised damage, but thick square ones can grab onto things and fling them about.

And as for the number of teeth, you are right in saying fewer is better. The more distance between the teeth, the more time the disc has to regain speed before another one hits. If you have limited resources, go with 2 per disc, but if you really know what you're doing you can make a single-toothed counterweighted disc.

Hope this helps, ask me if anything is unclear.

**typhoon_driver** · 31st January 2016, 20:20

With a spinning blade you are looking to store as much kinetic or moving energy as possible. In a straight line kinetic energy is calculated by E = 0.5 x mass x velocity squared. When it comes to rotation the equation becomes E = 0.5 x rotational velocity x moment of intertia squared. The moment of inertia is essentially a number that is derived from the location of the mass about the central axis of rotation. The long and short of it is that you get a greater figure when you have more mass further from the central axis of rotation. There are various ways to calculate this. If you don't want to get into detail with it the team cosmos kinetic energy calculator can give you a good approximation http://www.teamcosmos.com/ke/ke.shtml (Have a play with some designs and figures to get a feel for the figures)

For reference featherweights these days can easily store a few kilojoules of kinetic energy and some are hitting 10 plus kilojoules. For reference, hypnodisc had around 3 kilojoules on it's first outing and typhoon 2 had the potential to store 40 kilojoules. Tombstone in the states can in theory go as high as 100 kJ.

You haven't stated what weight category you are considering.

You are correct that less teeth is better but not for the reason that Danny stated. The reason less teeth are important is that you get a greater cutter bite with fewer teeth. Essentially there is a longer time that the robot is moving forward from one tooth being in a location and the next tooth taking it's place as the disc rotates. You therefore end up with more of the tooth able to make contact with the opponent. You need cutter bite to get engagement with the opponent to transfer the energy from the disc. This is a factor of the number of teeth on the disc, the forward velocity of the robot and the rotational velocity of the disc.

**maddox10** · 31st January 2016, 20:32

As Danny B writes. The less teeth, the better. Easiest to make and balance is a 2 teeth spinning bar or disk.

The "striking power" of a spinning weapon is a combination of stored energy and the means to deliver as much of it in a blow.

Doubling the mass doubles the stored energy. Doubling the speed quadruples the stored energy.

To get the energy in the moving mass, the motor and battery must be able to input the power into the moving mass.

I made a series of robots with a spinning weapon, all using the same motor and pulley arrangement.
Valkiri 2, 3 and 3.5. each of 'm with a smaller disk.
Valkiri 2 started with a HUGE 395mm Single disk, weight of moving mass 4 kg.

Disadvantages:
The 8 mm thin disk needed replacement after each event.
Startup was slow and erratic.

An alternative was tried.

Disadvantage. The upside down driveability nor selfright worked. This trident went to Caliope.

Valkiri 3 , what participated at the UK champs 2015.
Disk shrunk to 356mm diameter, spinning mass still 4 kg.

This weapon is so powerfull (theoretical 7.9Kj), it outclasses series 5 heavy spinners like the one on Hypnodisk. But the weaponpower, lay out and weight distribution made it manoeuvre like a drunken whale.

Therefore, a version with a smaller disk is build. Currently named Valkiri3.5.
2.9kg spinning mass, disk diameter 280mm.

In the same periode, I did build DX11 together with Frank, a starting Dutch roboteer.
2 kg spinning mass, 2 teeth spinning bar.