Electrical breakdown in rolling bearings and other machine elements

EDM spark erosion is a huge problem in electric drives and windturbines

Inverter-controlled motors are used in numerous technical areas. The generated electric field influences the capacitive-coupling bearings of the drive shaft. The lubricants used in these bearings are therefore stressed not only mechanically but also electrically.

In order to select a suitable lubricant, knowledge of the electrical fluid properties is required. The electrical conductivity and hence the dielectric strength are of great importance here. The latter parameter indicates the external voltage needed to cause current flow in the medium. This current flow causes damage to the bearing raceways (see below) and also contaminates the lubricant. The electrical load on the lubricants thus leads to premature aging and increased maintenance requirements.

Figure: Typical electrically caused wear patterns on the bearing raceways – (1) craters (pitting damage), (2) fluting, (3) grey frosting

The bearing currents that occur can be categorized as capacitive reloading currents, discharging currents (so-called “electrical discharge machining currents” or “EDM currents”), rotor-ground currents and circular bearing currents, which are all known to have a damaging effect on the bearing.

In the type of damage phenomenon that occurs, the interaction between the tribological system, defined by the lubrication/friction condition, and the resulting electrical behavior plays a crucial role.

The causal relationship between the lubrication condition present in the rolling or ball bearing and the type of bearing currents that occur has already been investigated by MEGT scientists at the Technical University of Kaiserslautern [Maschinenelemente und Getriebetechnik Berichte Bd. 20/2016, ISBN 978-3-95974-029-6] and can be illustrated by the following figure:

Figure: Lubricating conditions and EDM discharge frequency in a rolling bearing [“How Shaft Voltage Causes Bearing And Lubricant Degradation”; Hausner, Simon; Introducing flucon’s Dual Approach @ STLE Annual Meeting 2024, Minneapolis, USA]

This clearly shows the temperature dependency of the frequency of discharge currents (red) with otherwise constant mechanical boundary conditions.

An increase in temperature in the otherwise constant system is accompanied by a reduction in lubricant viscosity and thus a reduction in the lubrication gap height. As a result, the investigated rolling bearing changes from the full lubrication condition to the mixed friction condition, which is illustrated by the exemplary lubrication gap profile (blue).

In addition to the temperature and viscosity, the speed and the load are also relevant influencing variables for the breakdown frequency.