Ultra high speed bearings for machine tools

NSK's ROBUST series of ultra high speed bearings employs a larger number of smaller diameter balls as their rolling elements, enabling them to operate at rotational speeds some 20 percent higher than conventional designs.

The ROBUST range retains the same external dimensions as conventional angular contact bearing designs and, as a result, is directly interchangeable. However, the lower mass of its smaller rolling elements results in lower ball centrifugal force, and hence lower friction and heat generation.

Ball centrifugal force is reduced at high speeds and internal stresses are lower, enabling speeds of 3 x 106 dmN to be achieved with air/oil lubrication.

Within the ROBUST range there are a number of options available to customers who may need to improve further on the standard configuration. These bearings can have ceramic (silicon nitride) balls rather than the standard steel option with rings manufactured from SHX, a new steel developed by NSK.

SHX offers ultra high speed performance and extended fatigue life, making it particularly suitable for use in machine tool spindles, which must run at ultra high speeds with a minimum supply of lubricant.

Its heat resistance is nearly equal to M50 steel, a material which is used widely in bearings for jet engines, while its wear resistance, resistance to seizure and durability are all of a high standard.

Development of materials for use in ultra high speed bearings has also included the use of alternatives for the cage construction. With the base specification using a phenolic cage, those requiring a higher specification would be advised to choose the polyamide design. For ultimate performance, requiring speeds in excess of 2.5 x106 dmN, the PEEK cage is available which has excellent temperature characteristics.

Bearing design for such high speed applications must also focus on proper lubrication in order to deliver high levels of performance and durability. Some applications require oil jet lubrication and this requires the machine tool design to accommodate such a system, as well as suitable quantities of high grade oil together with adequate filtration capability.

An alternative is oil-air lubrication, which is a method of spraying oil by turning it into a mist using compressed air. This method requires a less complicated machine tool design and also greatly reduces the quantity of oil required. In addition, the bearing design is also simplified as there is no need for oil ports to be incorporated into the bearing ring.

The ROBUST range is also available with conventional grease lubrication or as a sealed version, which makes for easy handling and also improves grease life time. For a higher specification, the SpinshotII, an exclusive oil air lubrication system developed by NSK, can be requested.