Electric Discharge Machining Damages in Bearings

Introduction

Electric discharge machining (EDM) is a manufacturing process of machining electrically conductive materials mechanically irrespective of their mechanical properties. It is used in various applications such as automotive, aerospace, medical implants, etc. in this technique material removal takes place due to potential differences between the electrode and the conductive material. The schematic representation of EDM is shown in Fig-1.

Fig-1 Schematic representation of EDM [1]

Reason for the Damage

EDM currents pose a significant threat to bearings due to the arcing that occurs at certain voltages. This arcing results in uncontrolled discharging with high ampere values, which can alter the structure of the steel bearing rings and balls. As a result, the material melts and re-solidifies on the metal surface, causing ripples several microns deep to form on the raceways of both the inner and outer rings. The first sign of this issue is usually excessive noise, which is particularly problematic for electric vehicles (EVs) that are becoming increasingly silent with advancements in technology.

Fig-2 Damages on Bearing surface [2]

Damage analysis

Electrical discharges within the bearings can damage the rolling components, raceways, and lubricants. Signs of this damage, such as fluting, pitting, burned grease, and vibrations, can be detected later on through the use of tools like stethoscopes, vibration analysis, and grease testing. However, early detection of this issue has been challenging, and the only solution has been to attach an oscilloscope to the motor and inspect for unusual motor currents. This approach requires expensive equipment, specialized probes, specialized knowledge, and a significant amount of time.

Fig-3 Schematic representation of the vibrational analysis [3]

Solution

Nowadays, the issue of electrical discharges in bearings is better understood and can be recognized, monitored, and addressed. One of the main challenges has been to identify it early, as many factors such as voltage, load, speed, and lubricant type can affect electrical erosion. There was a need for a rapid method to detect electrical discharge currents in bearings, as they can lead to electrical erosion over time.

Reference:

[1] Palanikumar, K. and Davim, J.P., 2013. Electrical discharge machining: study on machining characteristics of WC/Co composites. In Machining and machine-tools (pp. 135-168). Woodhead Publishing.

[2] https://vfds.com/blog/bearing-currents/

[3] https://ivctechnologies.com/2021/05/26/identifying-the-root-cause-of-electrical-discharge-machining-with-vibration-analysis-and-shaft-to-ground-voltage-current-analysis/