Interactive web site of Télécom ParisTech's ELECINF344/ELECINF381 Robotics and Embedded Systems classes (a.k.a. ROSE, 2012 session).


Current through ball bearings – the final decision

Throughout the beginning of the project we have encountered various results to our endeavors of throwing current through ball bearings.

We have tried a week ago to fuel the gumstix (see our related post of 12/03/12) through the biggest of our bearings. (1 cm of diameter for its internal ring) But the mean value of the current traversing the bearing was close from zero. We concluded that this bearing was not designed to transmit current.

Then we decided to try with the smaller bearings, those that we used in the experiment related in the post of 22/02/12, in which we managed to send a signal.

But these bearing are to narrow to go around the rotor of our ventilator.

So we came back to an experiment with a 12V engine, quite similar to that of the 22/02/12 but we soldered a wire across the two bearing for a greater stability.


We send a 9V CC signal (generated with a battery) trough the two bearings and we measured the tension between the poles of the battery.

There are still some problems related to the crocodile clips that we used but if we remain careful we managed to obtain this result. The tension received has a non null mean value. It varies with the speed of rotation from 9 to 4V. There are still a lot of noise with a characteristic period of 500µs.

So we decided to observe the result through the pole or a RC circuit with a characteristic time of 100µs. ( R= 100Ohms and C=1µF )

The result totally correspond to what we expected, we transmit a signal of 4,5 V (a half of the initial value) at a speed of rotation of 1200 rpm. (measured with audacity as in the post of the 12/3/12).


We conclude that we will use this way to feed the card and these bearings. So we will have to adapt a rotor to the size of their internal ring.

We will send a 12V CC signal and our last calculus lead to a consummation of almost 10A. RoseAce is reade to shine!

Sylvain Ract

Possibly related posts:

  1. RoseAce has dirty hands

2 comments to Current through ball bearings – the final decision

  • Why did you expect half of the initial value?

  • sylvain

    Sorry for the confusion. What we expected was a signal with a non null mean value and a lot of noise around it. We had no prediction about the mean value; we only expected that it was under the nominal voltage because there are phases in which the contact is lost.