# Expanding Flywheel

Has anyone considered a flywheel that expands as it’s rotational speed increases? Basically, the part of the flywheel that makes contact with the ball is compressed by gravity or elastics so that the weight of the edge of the wheel has less torque on the output axel. As the rotational speed of the mechanism increases, the centrifugal force allows the wheel to expand its diameter and therefore it’s linear speed while theoretically reducing the time and immediate energy required for startup and allowing more weight to be moved by the same number of motors. The expansion would have to be controlled and locked into place so that the wheel remains circular while shooting, but it may be doable. This would be a very interesting way to reduce startup stress. Thoughts?

I might be misunderstanding you, but what you’re describing sounds a little bit like a flyball governor, though I’m not sure how you could implement it to expand into a smooth wheel. It’s a fascinating idea. The angular momentum of the wheel would be less as it starts up, then increase with increasing speed because the mass would be distributed at a greater radius. It would be amazing if you could get it to work.

You might also have a look at this thread, in which somebody has a video of something like this actually happening with their flywheel…
https://vexforum.com/t/wheel-inflation-and-vibration/29598/1

Yes! That’s exactly what I was drawing inspiration from. I feel like it would be more linear with overlapping and arcs made of something flexible like polycarbonate. when the flywheel reaches full speed some kind of groove and catch joint would limit the outward expansion of the wheel so that it becomes slightly stretched and a notched standoff lock would prevent it from collapsing back to it’s starting state under pressure from balls.

It was this thread that lead me to believe that this was possible with vex motors and the rpm they can produce. I have drawn out some plans and I will be prototyping this starting today although I probably won’t attempt to use it until later. I’ll post some pictures or a video when I get finished.

And here it is:

It’s a pre-flywheel flyball governor. I attempted to add weight to the expanding flywheel, but it failed to remain stable while firing.
I’ll post some videos of this one later.

One of the most important things with a flywheel is the pressure of the wheel against the ball. Unless you have some way to maintain the gap where the ball exits, I would imagine the physics of the design would backfire.

Another thing is if you design the flywheel to be like flails, then like the weapon itself, it would have large varying impact forces. That would probably create consistency problems when shooting.

Well it was a good try at a difficult concept. With it being this early in the season, I’m sure I’ll try again.

It’s called conservation-of-energy. An ice skater speed up rotation in a spin by drawing their arms in and slows down when extending them out. You trade diameter (distance of travel) for speed (rpm) the energy remains a constant.

The video:

It runs much smoother when it’s perfectly level or at very high RPM. now I have to pick up all my locknuts.

A very difficult concept, indeed!

NBN is difficult enough, I think, but if you really must investigate this idea, perhaps consider making the rim of the flywheel out of a very stretchy material such as the black latex tubing. Then have the flyball section pry out on the stretchy material as the wheels gain speed. Of course, I think you’re right when you say you need some way to then lock the configuration in place, otherwise your flywheel diameter will always change with respect to its speed.

On the other hand, having a flyball governor might be one way of keeping the flywheels at a momentarily constant speed during a rapid firing of balls. Remember, the spring involved with the governor also can store energy, too, which can be tapped to some extent during launchings. Adding such complexity might not be worth it, practically speaking, but it would be interesting to see if it actually works.

OMG, that’s hysterical! I sure hope you’re wearing safety glasses, a face mask, and body armor! You also might try testing it away from the window and other glass in the building. :eek:

Yeah, it nicked my finger on one of the tests. These things are beasts.

I hope I don’t have to witness a whole flywheel launcher falling apart like at the ending of your video.

I hope I see people using safety glasses more often this year!

Slinging bags of spiky keps nuts around a la Mad Max Fury Road… well, what did you expect?

I do have a few thoughts on this.
Glad you are thinking of ways to shorten the flywheel spin up time. The device you have is a good first attempt. I cannot see the device touching the wheels directly but maybe attached to the gear train as a stabilizing element.

But alas… maybe to no avail.

Although you may be able to spin the expanding wheel a little faster by keeping the mass more centralized this also reduces the stored energy in the wheel at any given speed.
So, the basic objective is to get a flywheel up to a final release energy, E_f . The motors have a limited average power, p_avg, that you must stay below in order to keep from tripping their PTC fuses. The time it takes to get the E_f is
time = E_f / p_avg. This is independent of the acceleration profile which is what you are varying when you allow the moment of inertia of the wheels to change with speed.

Thought experiment: Assume you have a mass , m, rotating at a radius about an axis which is either at radius = d or d/2. The moment of inertia I corresponding to these two radi are I_f and I_f/4 where I_f = md^2 and m is the mass of the wheel. You fix the wheel radius at d/2 and spin the wheel quickly to speed ,w_initial. The energy at this point is E_initial = .5(I_f/4)*w_initial^2. You then let the wheel mass expand outward to radius d. The energy stays the same but the speed immediately reduces to w_final =w_initial/2. Since the final energy in the wheel is what is needed to shoot… we had to accelerate the mass to twice its final rotational speed in order to have the same energy with the masses extended at twice the radius. So we didn’t really save any time to get to w_final since we had to accelerate for twice the time to reach w_initial to have the proper energy when the masses were released. The faster acceleration did not help us get to the proper energy any faster since our motor power is unchanged in this experiment.

But since real motors experience the power relations seen in their torque-speed curve, wouldn’t such a flyball system help keep the motors running closer to their peak power rather than grunting so much during start-up? I think part of the OP’s motivation was to take stress off his motors during the initial ramp up to full speed, so wouldn’t his idea act somewhat like a transmission, effectively “shifting gears” as the motors pumped more energy into the rotational mass of the flywheels?

Yes, this was the intent.

What I was really going for was keeping my motors under the least possible stress when accelerating a flywheel from a dead stop, with any faster acceleration of the flywheel being a bonus. The intent was for it to act like a transmission and increase the RPM as soon as the momentum of the flywheel could compensate for the lost torque, allowing for a potential increase in the mass of the flywheel, higher RPM, or less stress on motors during startup. I will be doing trials on my prototype flywheels both with and without governors, so we’ll see soon enough if this effective or not.

More specifically, conservation of angular momentum.

Ok, my first response was not to your intention. Sorry I misunderstood.

What you are doing is ok, however, the motors are pretty tough so stressing them is not an issue for me. Tripping the PTC fuses is of course, and that’s probably what you are getting at. A simple and perhaps more reliable method is to use the current limiter function (requires encoder feedback) that Jpearman and I worked up a while ago. Setting the current limit at about .9 amps will keep the PTC fuses from tripping. Jpearman has it coded in his smart motor library now.

You inspired me to examine the dynamics of the centrifugal governor in more detail. I reported the results in a new thread. The results show that the governor that you used in the youtube video saturated a bit early and did not really limit the current response as you desired. However, I’m sure some more experimentation will improve on the design. I guessed at some of the parameters to do a simulation. Seems a spring tension on the centrifugal arms is necessary to avoid early saturation. Let me know if I made some bad guesses.