"Repmag" Part II

As I said last time, I have done some initial mechanical design on my "Repmag" idea. The ultimate goal of course would be to prove and develop the design sufficiently to enable a successful physical prototype to be built. However, for now, I'll post a bit more about what I have done on it so far.

Magnetic forces replaced by spring forces

silux model with spring forces replacing magnetic forces

Here is an early silux model I made to verify the basic concept that stronger repelling forces between adjacent magnets outside of the shield, and weaker repelling forces inside the shield, would cause the rotor assembly to turn as a whole. It was assumed that the energy gained by each magnet entering the shield would be balanced by the energy lost by another magnet exiting the shield, so these entry and exit forces were not modelled. (I will have more to say about that later).

In this model compression springs, substituting for forces of magnetic repulsion, are added between adjacent objects representing the magnets. All springs have a natural length of 0.04m, and a minimum compressed length of 0.01m. Outside of the shield, the springs (black) can exert maximum force of 9N, giving spring constants of 300N/m. Inside the shield, the springs (pink) are weaker, at 6N and 200N/m.

The macro stops the model every 30º of clockwise spider rotation. Then the spring which has exited the shield is changed manually from a weaker to a stronger one, and vice versa for the spring which has entered the shield. The model is then re-started for the next 30º etc.

This model, with light magnets and rollers, but with an artificially heavy 10kg spider, is identical to the one shown in Part I previously, apart from the added springs. Starting with all components at rest, with no other torques or forces, it gave the following graph of spider rotational speed vs time:— 

Graph of Repmag spider rotational speed vs time

In this graph, the spider has undergone smooth acceleration from rest, completing just under seven revolutions, reaching 10.246 radians/sec (nearly 100 rpm) in 8.532 seconds.

The spring parameters were chosen only as "reasonable" values, and could have been a bit stronger. This "changing-spring" driven model delivers energy (½Iω²) of 0.5 × 0.0375 × 10.246² = 1.9684J in 8.532 seconds. This gives a power output of 0.231 watts, only about half the 0.507 watts obtained last time from the magnet driven model.

Next time I'll look at more details for a possible physical prototype.