Robust spindle drives with Vacrodur rolling bearing material

Current research indicates that around 80 per cent of all spindle damage that leads to machine failure can be traced back to the spindle bearing. At the top of the list are bearing wear and collisions, as well as high temperatures, insufficient lubrication and the ingress of contamination.

Developing a robust rolling bearing material that allows a significant increase in operating life and an even higher load carrying capacity under these difficult operating conditions means breaking new technological ground. Schaeffler achieved these ambitious objectives with an innovative new manufacturing method, special alloy components, and a customised multi-stage heat treatment process. The high-performance Vacrodur rolling bearing steel, which was presented at EMO 2017, possesses a range of outstanding characteristics that make it well suited for spindle bearing applications.

Unlike conventional rolling bearing steels such as 100Cr6, the hardness of Vacrodur is not reduced at temperatures above 120degC but remains stable even above 400degC. There is also no structural transformation in Vacrodur up to this high temperature, and there are only negligible changes in the material characteristics. Spindle bearings made from Vacrodur thus retain a high load-carrying capacity and remain dimensionally and geometrically stable for a long time, even under very high thermal loads, e.g. exhaust heat in motor spindles and as a result of insufficient lubrication.

Up to 20 times the operating life when lubricant contamination occurs

Ingress of contaminants is often one of the main factors that limit the operating life of spindle bearings. Several mechanisms are responsible for this: abrasion and various types of initial damage caused by particle inclusions.

Vacrodur has a very homogeneous material structure with a high proportion of very finely scattered carbides, which gives Vacrodur its extraordinary level of resistance to abrasive wear. What is more, the special alloyed material goes through a multi-stage heat treatment process that produces very high hardness values of up to 66 HRC and a tough (basic) structure that can withstand very high temperatures of over 400degC with no loss of hardness. Because Vacrodur is harder than other rolling bearing steels, smaller indentations and therefore lower ridges are produced when foreign particles are overrolled in the bearing raceway. These can easily be reformed during subsequent overrolling due to the ductile material matrix. In tests under controlled contamination conditions, the operating life up to bearing failure was increased almost 20-fold compared with the existing benchmark.

Nominal rating life increased by a factor of 13

The high level of rolling strength and resistance to fatigue due to overrolling under good lubrication conditions, means that bearings made from Vacrodur have a significantly longer operating life. A 2.4-fold increase in the dynamic load carrying capacity compared to standard 100Cr6 rolling bearing steel - which represents a 13-fold increase in the bearing's nominal rating life - has been verified in certified tests. There is even a 25-fold increase under insufficient lubrication conditions.

With its very high mechanical and thermal load-carrying capacity and its operating life that is many times longer than that of other steels, Vacrodur opens up even more new possibilities for machine tool manufacturers to significantly increase the performance and reliability of their machine tools. At EMO 2017, Schaeffler presented its first FAG spindle bearing series in premium X-life quality made from Vacrodur. Go to www.schaeffler.com to learn more.