The topic will gain additional importance in the future as the increasing use of frequency converters in modern power transmission systems for continuous speed control of motors and generators will lead to more damage caused by electric erosion. Also the current trends towards using higher voltages, e.g. in automotive vehicle power systems, will result in higher energy discharges in the future. For more than 20 years, the tribology expert Klüber Lubrication has been researching into the use of ionic liquids for innovative lubricant concepts especially in this segment. This modern concept turns the lubricant into a "lightning conductor" to prevent harmful potentials from forming. In addition to the vehicle-relevant data like speed, smooth operation and extended service life, this new generation of speciality lubricants also addresses the issue of electric conductivity.
Damage caused by electroerosion
Electroerosion in rolling bearings of electric motors or other electric components is caused by the undesirable passage of current through the contact zone between the rolling element and the raceway. Thus, heat is generated which locally melts the surface. Small pittings several micro-metres in size are formed. In subsequent operation grooves may arise across the racewaqys as secondary damage. Both types of damage lead to the generation of undesirable bearing noise at first and finally to premature failure.
There are several design approaches to solve this problem, e.g. by insulating the inner or outer rings or using ceramic rolling elements. These measures, however, do not always bring about success, despite the high costs they incur. A simpler yet more promising approach is to use a lubricant of low impedance, which is a design element of the component anyway, to dissipate the currents generated.
Basically, however, lubricants are insulators. It is precisely because they block the passage of electric currents that they increase voltage build-up in bearings components made of metal. Traditional solutions involve the use of black lubricants based on graphite and carbon, which, on the one hand, can also prevent parasitic currents. On the other hand, they contain black, solid graphite particles which impede smooth running. As such particles are displaced from the friction point over time and are subject to change by the mechanical load, electroconductivity of these greases decreases continuously.
Promising lubricant innovations
The method of making the lubricant conductive by adding ionic liquids to dissipate the currents generated shows clear avantages over the use of "black lubricants". Especially high-frequency EDM (Electric Discharge Machining) currents can be neutralised by suitable lubricant designs.
Klüber Lubrication is currently the only manufacturer who can offer such a solution and owns three patents for using ionic liquids in lubricant formulations. One of these patents implies their use as a base oil, the other two patents refer to the additivation.
Ionic liquids – background
In simple terms, ionic liquids are liquid salts. They do not have a stable crystal structure because it is prevented by charge delocalisation and steric effects. By definition, the melting point of these salts is below 100 °C and in many cases even far below this point, with the result that the salts are already liquid at room temperature. The properties of ionic liquids can be varied according to requirements by modifying the cations or anions. Thus, they are predestined for use in many areas of chemical process engineering.
Ionic liquids and their properties have already been known for more than hundred years, however then received little attention for a long time and were not used in products or processes. Owing to their electric conductivity they reduce undesirable passage of current, which make them an interesting solution for bearings in electric motors.
Klüberlectric BQ 72-72, EDM currents measured in A as a function of speed and temperature (TU Darmstadt)
With an adequate number of cations and anions ionic liquids also offer excellent resistance to oxidation. They are thermally stable and can be used at temperatures as high as 150 °C, are hardly inflammable, very soluble and non-toxic. Their extremely low vapour pressure is another advantage. Unlike speciality lubricants containing conductive particles, ionic liquids do not have a negative effect on noise. Several greases already proved their suitability for high speeds. They currently achieve speed factors exceeding 1 million n x dm. Klüber Lubrication aims at speed factors > 2 million n x dm.
Ionic liquids considerably increase the service life of bearings and therefore contribute to sustainability. As the energy impact of undesirable electric currents provokes not only damage to the bearings but also affects the lubricant's performance and decomposes the lubricant due to the extremely high peak temperatures, another positive effect is that they help extend grease lifetime.
Comprehensive test rig examinations followed by tribological analyses provide information on the condition of the used lubricant as well as that of the bearing. An important feature in this context is the interplay of results relating to the bearing and those relating to the lubricant. Due to the detailed examination of each component, any possible damage can be identified with great accuracy.
Test rig results showed that bearing and lubricant damage arising from electric currents are considerably reduced when using ionic liquids.
Ionic liquids – outlook
In cooperation with strong partners from the industry Klüber Lubrication currently works on further optimising the conductivity of lubricants while still maintaining their tribological performance by means of ionic liquids. In case of a very high level of energy passage, it is not yet possible at this stage to solve the problem of undesirable current passage merely by means of the lubricant. This, however, would be highly desirable for the industry, because it could reduce the size of bearings. An interesting approach is the combination of insulating bearings and additional discharge elements containing electroconductive lubricants.
Being a design element, the lubricant is a crucial factor when it comes to the efficiency and reliability of mechanical components. Experts like Klüber Lubrication research on continuous development as lubricants are key in extending the service life of components and increasing both the energy efficiency and sustainability.