Fluctuation components of contact voltage at AgPd brush and Au-plated slip-ring system with lubricant

The authors have investigated the deterioration process of the system of Au-plated slip-rings mating with AgPd brushes, which is used mainly in chip-mounters. Past tests have shown that the lubricant is very important for obtaining long lifetime. The lubricant can not only decrease ring wear but also protect the ring from oxidation. In operation the contact voltage drop is generally low and stable at the beginnings of tests and increases with the operation time. In this paper the fluctuation characteristics are observed in detail and the following characteristics are observed. The contact voltage fluctuation increases with the operation time while the fluctuation pattern is different at each ring. The minimum value of the contact voltage waveform stays almost constant around 20mV. The maximum value can become as high as 180mV on some rings. However, the surface morphology of rings with high peak values is almost the same as those with low peak value. Further, the peak value is greatly dependent on rotational speed. The peak value approaches the minimum value at low speeds. From these results it is deduced that the contact voltage increase is controlled by the lubricant thickness between brush and ring and not from ring surface itself. This indicates that tunnel effect resistance is important. The relationship between peak value and lifetime is discussed.