Oil is normally used as a lubricant for machine bearings but it’s clear that the use of a more environmentally-friendly material would be advantageous. Researchers at the Fraunhofer Institute for Mechanics of Materials IWM, have developed a technique which uses a water-based lubricant to reduce friction in a plain bearing supporting a rotating shaft.

In Germany, around one million tonnes of mineral oil-based lubricant is used per annum. It would be more environmentally friendly if instead we could use water-based lubricants but unfortunately, many metallic materials corrode when exposed to water. A working group of the Fraunhofer IWM in Freiburg Germany have succeeded in using water as a lubricant with the help of some additives. This corrosion-free water lubrication even reduces friction because water is less viscous than oil.
A plain bearing with water-based lubricant showing the galvanic connection to inhibit corrosion. Image: Fraunhofer IWM.

The water-lubricated plain bearing consists of several concentric rings surrounding the shaft. Its structure consists of an outer-most electrically insulating sleeve around the whole bearing, then a collar of aluminum and finally a sleeve of sintered metal surrounding the shaft. This inner sintered sleeve is traversed by a small channel which lets water flow between the rotating shaft and the outer aluminium collar. An electrically conductive path is established through the lubricant between the rotating shaft and the outer aluminum collar. This is crucial for an electrochemical process as in a sacrificial anode. An electrical potential is created between a base metal such as aluminum and a noble metal such as iron, which chemically inhibits corrosion of the iron.

A water-based lubricant

The prevention of corrosion is not the whole picture; so-called ionic liquids are added to the water. These ionic liquids are fluid salts which contain anions and cations which get rearranged by the electric field and then collect on the interior surface of the sintered metal sleeve such that their ends point inward, toward the rotating shaft. This forms a kind of galvanically-generated protective layer on which the shaft glides.
Ionic fluid at the metal surface reduces friction and wear. Image: Fraunhofer IWM.

The bearing has been successfully demonstrated and industrial partners are being sought to assist optimization of the ionic liquids. One of the challenges is that the bearing temperatures can rise because of mechanical loading and this causes the water to evaporate. The team is hoping to find a mix of ionic liquids that will counteract evaporation.

Better efficiency for electric motors

The water-based lubricant is not only environmentally friendly, but also reduces friction of the plain bearing. Compared to oil it has lower viscosity so a shaft spins with less friction. Electric motors equipped with these should, in principle achieve better efficiency.

In projects funded by the Ministry of Economics, Labor and Housing Baden-Württemberg, IWM has developed other tools in cooperation with the University of Freiburg. They have designed a new measuring device, called an in-situ tribometer, capable of monitoring metal wear and friction values directly on the plain bearing during operation. Up until now it has only been possible to assess bearing wear after first disassembling and inspecting the bearing. In-situ measurement not only facilitates time-economic development of water-based lubricants, but also allows continuous monitoring of bearing condition.