Lubricating greases for oscillating movements
Rolling bearings can operate under extreme condi- tions, such as oscillating movements. Permanent start/stop operation prevents the development of a stable separating layer between the friction partners, leading to what is known as boundary friction. The wear risk increases and the lifetime of the rolling bear- ing can decrease as a consequence.
In oscillating rolling bearings mounted on a horizontal shaft, the effect of gravity draws the lubricant gradually downwards. If the oscillating amplitude is small, the upper part of the rolling element may not be sufficiently covered with lubricant. Additionally, the amount of fresh lubricant entering the space between the friction partners is very small, leading to difficult lubrication conditions for which a special lubricant is required. Test results obtained with rotating movements cannot be applied to oscillating movements.1)
If oscillating amplitudes are constantly very small, the rolling element cannot rotate to redistribute the lubricant. The resulting effect may be bearing damage with ball or roller-shaped brinell marks occurring to the loaded bearing rings. This effect is referred to as the False Brinelling Effect and is a typical wear pat- tern of tribocorrosion.
Roller bearings are a particular lubrication challenge due to their tribological characteristics, as they often operate under extreme conditions like high pressure at low rotational speeds.
Lubricants for these conditions need to have special characteristics to cope with rolling friction in the linear contact zone between the roller and the raceway and, in some cylindrical and tapered roller bearings, sliding friction at the flange. On the one hand, high pressure resistance is required, which can be attained with higher base oil viscosities and/or special additives. On the other hand, sufficient oil release must be provided to the sliding contact between the face of the roller and the flange. Synthetic base oils com- bined with special soap-based thickeners like lithium, lithium complex, calcium complex or barium complex have proven their effectiveness in practice.
The wear resistance of such special lubricating greases is normally analysed with the DIN standard testing machine FAG FE8, testing the wear resistance of the lubricant in the raceways, rolling elements and the cage.
Klüber Lubrication has developed speciality lubricants to meet these requirements, with tried-and-tested solutions for a range of roller bearing applications.