New benchmarks for spindle bearing life under poor lubrication conditions

FAG/Barden has developed 'X-Life Ultra', a new design philosophy for precision bearings. Its application results in bearings that are a unique synergy of materials, surface engineering, sealing and lubrication technology, designed to offer performance enhanced solutions to industry-specific problems. They deliver longer service life and maintenance-free operation in extreme operating and environmental conditions, plus higher load capacity, increased speed capability and excellent resistance to corrosion. The development of X-life Ultra bearings has enabled FAG/Barden to achieve a service life for machine tool spindle bearings up to ten times greater than that of an equivalent sized conventional spindle bearings. This improvement has been achieved through an efficient combination of the most advanced components in new bearing technology: specialist bearing design, advanced materials technologies, superior surface engineering technology, extreme tolerances (P4S), advanced lubrication systems and unique non-contact sealing.

The bearing rings of the X-Life Ultra bearings are manufactured from a special, high nitrogen martensitic stainless steel, which extends bearing fatigue life with its high resistance to corrosion, tolerance of higher contact pressures and insensitivity to conditions of mixed friction and contamination. Building upon this performance, ceramic (Silicon Nitride) rolling elements (notable for their considerably reduced density and mass) permit higher speeds, extend the grease service life, withstand elevated temperatures and are insensitive to lubricant starvation. Finally, specially developed lubricants such as FAG Arcanol L75 grease, and new non-contact sealing technology provide the ideal complement in achieving the excellent overall product performance characteristics.

X-life Ultra bearings prove their economic benefits in reduced life cycle costs of complete spindle systems. Due to the reduced probability of failure, unexpected spindle bearing failures are far less common, leading to reduced machine tool downtime, while service life is extended. Additionally, the costs for lubricant procurement and disposal go down; mounting and dismounting requirements decrease and the demand for replacement bearings is reduced. What all this means to the machine tool user is that within a relatively short time the precision X-life Ultra bearings will pay for themselves despite their higher purchase costs.

In both laboratory and field tests X-Life Ultra bearings have achieved considerably longer service life under the mixed friction frictions, which, more often than not, are encountered under actual machine tool operating conditions. One test conducted on multiple machining centres has been running for more than two and a half years. The machines use X-Life Ultra bearings with grease lubrication in a three-shift operation, at speeds of up to 16,000 min-1 and with two tool changes per minute. During the whole test period, no spindle failures have been experienced due to bearing damage.

As part of the test, the inner ring raceway of one of the bearings was dismounted after approximately 6000 operating hours. This revealed tracks that were hardly noticeable, indicating minimal levels of wear. Further tests revealed that under full EHD (elasto hydro-dynamic) lubrication conditions, the Hertzian pressure in ball contact could be increased from 2000MPa to 2500MPa with X-Life Ultra bearings, without affecting the material fatigue life. Subsequently, the bearing designs of some spindles are benefiting from this characteristic. In certain spindle applications, where the point of load application is at a considerable distance from the first bearing, very high contact pressures arise which can be accommodated by X-life Ultra bearings. Because of this, the traditional route of employing a larger - and more expensive - bearing with improved load carrying capacity - but reduced speed capability, can be avoided. In addition to longer life and load carrying characteristics, the higher speeds achieved in the laboratory on the X-life Ultra bearing test rigs have also been proven in field applications, particularly with FAG-Arcanol L75 grease lubrication. As a result, machine tool spindles with a [n x dm] speed index of 2 x 106 - and more, have been tested and are now in regular use.

About The Author:

Mark Harris is a successful author and regular contributor to http://www.bearing-n-bearings.com. Keep things running smoothly with ball bearings, roller bearings, ball thrust bearings and tapered bearings.