[RimstarOrg] has brought us an oldie but goodie this week. He’s built a ball bearing motor, a design which has been causing engineers and scientists to squabble for decades. [RimstarOrg] used a microwave oven transformer with a 70 turn primary coil and a single turn secondary coil to create a low voltage, high current AC power supply. Needless to say, there’s a real risk of fire or electrocution with a setup like this, so be careful if you try this one at home. [RimstarOrg] then built the motor itself. He de-greased two ball bearings then installed them on a metal shaft along with a wooden flywheel. The entire assembly was then mounted on a board so the wheel could spin freely. Two copper straps hold the bearings to the board. Finally, the transformer is wired into the copper straps. In this configuration, the current will flow through the outer race of one bearing, through the balls, and into the inner race. The current then passes down the axle and passes through the other bearing. There is very little resistance in this circuit, so it can only be powered on for a few seconds at a time before things start to melt down.
When the current is switched on with the flywheel stationary, nothing happens. If left long enough, the bearings will overheat. The real magic starts if you give the flywheel a spin just before turning on the current. As soon as power is applied, the flywheel starts to pick up speed. Power off, and things start to slow down. This happens both with the flywheel spinning clockwise and counterclockwise.
So what’s going on here? A motor with no magnets? Is this some type of perpetual motion machine? Alien technology? We can assure you that the effect is real, but the mechanism is still unproven. [RimstarOrg] explains one common explanation, which is the motor is operating as a heat engine rather than a standard electric motor. The metal ball bearings have relatively tiny contact points with their respective races. These small points create a large resistance, which causes uneven heating of the balls. The heated balls expand to ovals. If the entire ball bearing isn’t turning, the expansion force will simply lock the two races together. If the motor is turning when expansion starts, it serves to push the outer race along.
This isn’t the only explanation of the motor though. Several electromagnetic explanations for the motor’s movement have been presented over the years. The physics behind these explanations are rather complex. The complex math and integrals can be found in this 1977 paper (pdf link) by [H. Gruenberg]. A more recent example is in this 2011 paper (updated in July of 2013) (pdf link) by [Prof. Kirk T. McDonald] of Princeton University.
[RimstarOrg] isn’t the only hobbyist to tackle this problem – [Mike] has a page about ball bearing motors as well. He concluded on the thermal explanation. Even Wikipedia seems a bit confused on the subject. The only thing everyone does agree on is the fact that the ball bearing motor is too inefficient to have any practical purpose. We may be a bit naive here, but it would seem that some simple hands on experimentation could solve this decades old question. A basic test would be to try DC vs AC current – perhaps with a common car battery as a power supply. Delving deeper, non ferrous vs ferrous balls. The real answer would lie in using ball bearings with known thermal expansion characteristics. Balls that expand more (and at a higher rate) should produce a faster running motor than “control” balls. It might be worth a phone call to your local bearing supplier to put this one to rest for good.
[via Hacked Gadgets]