Well first of all thanks for the comments, I describe the solution to my problem. The fault was that the drum turned once to the right and once to the left, the tachometer rotor was loose, but the tachometer had run backwards, because as the rotor was loose every time I wanted to centrifuge, I pushed the tachometer backwards so it had run from its place and did not fulfill the function, thanks from the South and I hope the solution is useful.