In class the other day I had an idea for a top which would maintain a constant rotational speed using a large interior mass which would move closer to the center if the top slowed down. The mass moving inward would change the moment of inertia, causing the top to speed up, similar to a figure skater pulling in their arms and legs to spin faster.

The basic idea is to have a mass travel on a shaft, directed toward the axis of the top, and some method of monitoring the top’s rotational velocity. You could do this a number of ways, one of them involving some sort of stepper motor and an accelerometer at the edge of the top: When the accelerometer detects the top slowing down, it would make the mass move slowly toward the center of the top, thereby increasing speed.

However, I had an idea that you could also accomplish this in a purely mechanical fashion, so I sketched it out in my notebook while the teacher was talking about something unimportant, and I just modeled the main mechanism in SolidWorks. (I’m probably going to do this more frequently to give myself more practice with SolidWorks)

The main idea is as follows:

The mass is pressed toward the center axis by using a very strong spring, strong enough to overcome the centripetal acceleration. The mass is attached to a geared shaft, which, if you stopped the shaft from rotating, would cause the mass to stop moving. The geared shaft goes to a mechanism at the outer edge of the top, where it can be most accurately affected by the rotational velocity difference.

The mechanism which keeps the mass from moving inward is rather simple: A mass placed at the edge of the top will be accelerated outward by the centripetal acceleration, so you have this acceleration create braking force. The more mass, the heavier the braking per angular velocity. You can also balance this out a bit by putting a spring to oppose the masses inclination to move outward.

I have made a SolidWorks model and generated a PDF file for your more thorough viewing: Governed Motion of Top.PDF [The file has been misplaced, it is probably on my computer somewhere.] In this rough diagram, the large mass is on the side of the top’s axis, while the small mass is on the side of the top’s outer edge.

I apologize for the crudeness of the draft, I was impeded by a redraw lag caused by using an integrated video card. I think the method I used to draw a helical geared shaft also slowed it down, but I’m not sure of a more efficient way to model it.

Here are some screen shots of the mechanism, maybe they will be more informative than the PDF:

The braking mechanism, up close.

An even closer view, maybe it will give you a better idea.

This is the braking mechanism without the secondary friction disk

These helical geared shafts killed my redraw rate, very badly.

What would this mechanism be good for? Not much, I’m afraid. It would be an enjoyable toy, I think: Wind up the mass, spin the top and it could go for hours, potentially. That sounds like a potentially fun little toy. Maybe the idea could be used for a governor on a motor, since it would need a similar mechanism. If you have any ideas, please post them in the comments.

As I have said here, I don’t like the idea of government enforced patents. However, because of the strange laws of the country, if I don’t patent something, another corporation could do it and restrict use of it. Because of this, any idea I distribute online is distributed as an “Open Patent”, that is, anyone can make, modify, sell, distribute, etcetera, any of my published ideas without my permission, and without the requirement of noting who the idea came from.

I’m not so prideful that I think this silly mechanism will be worth any money, but I thought I would bring it up, in promotion of discarding current patent laws.