Higher-speed cutting and increased machine accuracy require higher spindle speeds, so right at the heart of a machine's accuracy are the spindle bearings. These bearings, more than many other components, dictate the upper limit of spindle speed. To ensure the demand for higher speeds can be met, NSK has invested in new materials and production techniques to deliver a wide range of super precision bearings that allow machine tools to operate at higher speeds and help deliver greater accuracy and performance.
To enable machine tool spindles to achieve higher cutting speeds while maintaining a high degree of running accuracy, super precision bearings have to be used. These are most common of the angular contact ball bearing type. From its collaborations with machine tool companies and spindle manufacturers, NSK has developed a product portfolio of angular contact bearings that covers machine tool spindle applications across the widest range of operating speeds.
NSK's whole range is regarded as the most comprehensive available from one manufacturer, and is backed-up by the research and development facilities of NSK's European Technology Centre (ETC), one of the most advanced units of its type in Europe. The ETC conducts in-depth testing of angular contact bearings with new types of grease and with AIR MIST lubrication. The purpose of these tests is to establish how the physical properties of the lubricants used relate to bearing performance.
The frequent requirement for tool changes on modern machine tools, such as machining centres, means that to optimise production time not only are higher maximum spindle speeds required but also, critically, faster acceleration and deceleration. However, during periods of rapid acceleration or deceleration, spindle bearings are subjected to severe thermal load conditions. These are caused by drastic changes in the ambient environment that result from heat generated by the motor, housing cooling and other factors. NSK's solution to these problems is the ROBUST series of bearings, designed for ultra-high-speed spindles.
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 per cent 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 × 106 dmN to be achieved with air/oil lubrication (dmN = bearing PCD (mm) × spindle speed (rpm)).
Such high levels of performance owe much to the substantially improved rolling contact conditions within the bearings. These ensure reduced heat generation and considerably lower bearing operating temperatures than can be achieved with the standard design of angular contact ball bearings. Of course, reduced friction and cooler running add up to increased bearing reliability, a benefit further enhanced by the provision of a single shoulder which allows better lubricant distribution within the bearing.
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 is a result of special proprietary materials and heat treatment technology, which offers ultra-high-speed performance and extended fatigue life, making it highly suited 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 setting new industry standards.
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 ×106 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 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.
For more details on the super precision bearings or to discuss the specification of bearings for ultra-high-speed applications, visit the NSK website at www.nskeurope.com.