In Focus

Preventing discharge damage

Preventing discharge damage

Conductive rolling bearing grease provides inexpensive and efficient solution

Whether in the plastics industry, in paper-making machines or industrial electric motors - the issue of damage caused by electric discharge has been known for a long time, and today it is more acute than ever. It affects primarily rolling bearings in machines that are susceptible to static charging. In many cases, a conductive grease especially made for these applications can provide an inexpensive and efficient solution to this problem - while at the same time ensuring optimum rolling bearing lubrication.

Static charges may have a variety of causes. In film stretchers, for example, plastic material is conveyed on steel rollers, leading to electrostatic charging. In tumble drier drums, the plastic content of the laundry, such as nylon, may be the cause. Rolling bearings operating under high loads have a particularly high risk of damage, since there is frequently partial direct contact between the rolling elements and the raceways, leading to sudden discharge, not unlike an electric arc. As the metal-to-metal contact is restricted to a very small area, even currents well below 1 ampere can cause the contact points to weld or fuse together. Typical discharge damage shows in the form of plates, craters or grooves on the bearing.

electrostatic charging in film strechers

Craters result when surface melting takes place on the raceways due to the electric potential. Molten metal par ticles may also be carried off and deposited on the raceway, where they are rolled down. Grooves form when current flows while the rolling elements and the raceways are under load. This causes the rolling elements to vibrate, and over time the typical grooves form on the inner and/or outer ring. Repairing such damage costs a lot of time and money - in extreme cases the bearing as a whole has to be replaced.

The severity of the damage depends on a number of factors, e.g. the current intensity, the time of exposure, the bearing load, the speed and the lubricant used.

Preventing flashover

This was the starting point of the development of a conductive grease by Klüber Lubrication, enabling the problem to be solved in an efficient and inexpensive way before damage is caused. The conductive rolling bearing grease Klüberlectric BE 44-152 conducts the electric charge continuously through the bearing, thus preventing point-to-point flashover in the first place. At the same time, it is a rolling bearing grease offering a long lubricant life and high reliability. In order to demonstrate how
Klüberlectric BE 44-152 makes a difference compared with conventional greases, Klüber determined the electrical resistance of greases according to DIN 53482. This standard describes how the electrical resistance of non-metallic materials is measured. It can be done by measuring the resistance between two plates between which the grease is contained in an insulated hollow cylinder. The distance between the electrodes can be anything from one millimetre up to several centimetres. In addition to this standardised test setup, Klüber Lubrication has developed its own dynamic test rig, which was used to develop and optimise the special grease.

Resistance reminiscent of semiconductors

While the specific resistance of standard rolling bearing greases ranges from 10^11 to 10^14 Ω x cm, the special grease Klüberlectric BE 44-152 has a specific resistance of a just 10^5 Ω x cm, which is within a range typical of semiconductors.

Of course, in all applications of conductive greases in rolling bearings the maximum expected current intensity must be taken into account. If it exceeds 1 ampere, insulating the bearings with ceramic, plastic or other materials might be recommendable. Another possible solution is the use of bearings with ceramic rolling elements. However, such measures entail high costs and may make substantial design changes necessary. At low current intensity, in particular, the use of Klüberlectric BE 44-152 can be an inexpensive alternative to efficiently prevent discharge damage. Besides its "electrical" performance, a rolling bearing grease is of course also expected to offer good service life and high reliability. So when selecting a lubricant, its suitability for the particular rolling bearing application should also be checked. Reliable information in this regard can be obtained for example through test runs according to DIN 51821, part 2, testing the upper temperature range and the lubricant life. In addition, performance characteristics like corrosion protection or the speed factor should also be taken into account.

Klüberlectric BE 44-152 ...

... has proven successful in numerous tests as well as in practical use. It can be used over a temperature range* from - 40 to +150 °C.

On rolling bearing test rigs determining the grease life and speed factor, Klüberlectric BE 44-152 performed "very successfully". In the FAG-FE9 test (acc. to DIN 51821, part 2, Fa 1500 N, n = 6000 rpm, at 150 °C), for example, the grease attained an L50 life > 200 hours.

Also notable is its very good corrosion protection: in the Emcor test (DIN 51802, 1 week, distilled water), a corrosion degree ≤ 1 was attained. Klüberlectric BE 44-152 covers also speed factors** up to 1 000 000 mm x min-1, which are encountered in many applications.

Klüberlectric BE 44-152 is a fully synthetic grease based on a synthetic hydrocarbon oil, lithium soap and dark solid lubricants.

* Service temperatures are guide values which depend on the lubricant's composition, the intended use and the application method. Lubricants change their consistency, apparent dynamic viscosity or viscosity depending on the mechano-dynamical loads, time, pressure and temperature. These changes in product characteristics may affect the function of a component.
** Speed factors are guide values which depend on the type and size of the rolling bearing and the local operating conditions, which is why they have to be confirmed in tests carried out by the user in each individual case.