Environmentally friendly slide bearings lubricated by water
Machine bearings are generally lubricated with oil. But even today large quantities of these oils still end up in the environment. The Fraunhofer Institute for Mechanics of Materials IWM has therefore developed a method that will enable slide bearings to be lubricated with water in future.
Bearings are usually lubricated with mineral oil-based lubricants. In Germany, approximately one million tons of lubricant are used every year. The manufacture, application and disposal of this oil also impose a burden on the environment. A working group at the Fraunhofer IWM in Freiburg has succeeded in using additives to change water in such a way that it could be used as a lubricant in the future.
The researchers developed the details of their process using a slide bearing, which resembles a ring and surrounds a rotating steel shaft. The ring is made of several layers, which are as follows from the outside to the inside: a sleeve surrounding the bearing, a layer of aluminium and a layer of sintered metal that surrounds the shaft itself. The trick is that the inner sintered layer is traversed by a small channel, which lets water flow between the rotating shaft and the outer aluminium layer. This direct connection is a vital factor in the electro-chemical process.
Turning water into lubricant
The researchers use the electric voltage that arises between the aluminium in the slide bearing and the iron in the shaft to turn the water into a lubricant. "We mix what are called ionic liquids into the water," explains Dr. Tobias Amann. "The ionic liquids are fluid salts which contain anions and cations." These ions are rearranged in the electric field and then collect on the inside of the sintered metal ring. This forms a kind of galvanically generated protective layer on which the shaft can glide.
More efficient electric motors
The ion-water mixture is better for the environment than oil, and it also helps to make slide bearings even more efficient. Amann continues, "The shaft glides better when it is wetted with water. This reduces energy consumption compared to operations using considerably more viscous oil." In addition, corrosion is avoided. Normally the oxygen in the water reacts with steels containing iron, ultimately resulting in rust. The electric field prevents this from happening.
Researchers also designed a new measuring device, referred to as an in situ tribometer, capable of monitoring metallic wear and friction values directly on the slide bearing during operation. Until now, wear on a bearing could only be measured by disassembling the bearing and then assessing and measuring the surfaces. This is highly time-consuming. "Our new tribometer now makes it possible to measure wear in situ, not only making it easier to develop feasible water-based lubricants, but also to continuously monitor bearings," declares Amann.