Cylindrical roller bearings break new ground

NSK's new super-high-speed Robust series of cylindrical roller bearings (CRBs) is designed to facilitate the development of the next generation of high-speed machine tool spindles with integral motors. A synergy of optimised internal design, materials and lubrication technology, the Robust CRBs combine higher speed operation up to 3,000,000DmN, with the high resistance to seizure and three- to four-times longer wear life provided by new SHX steel technology. To meet the increasing demands for higher productivity, the latest generations of machine tools are increasingly being equipped with motorised spindles. The only problem with this development is that the integral motor acts as a heat source within the spindle, tending to cause a higher temperature difference between the spindle and the housings. This increases stress on the spindle bearings, which may cause them to seise. Another consequence of the change to integral motors is that the overall length of spindles has increased. As a result, axial thermal expansion has also increased and the need to allow for expansion of the spindle has much greater significance. Generally, this is accommodated one of two ways: the outer ring of an angular contact ball bearing sliding in the housing; or the inner ring and its roller set moving axially within the outer ring of a cylindrical roller bearing. In the past, conventional belt drive spindles employed double row cylindrical roller bearings to take the belt tension - and provide support at their rear (free) end. However, as spindle speeds increased to reach or even exceed 1,500,000DmN, there was recognition that cylindrical roller bearings were not suited to such high-speed operations, because of problems of lubrication and other unstable factors. In view of this, they were gradually replaced by pairs of angular contact bearings. The subsequent development of spindles containing built-in motors has changed this situation. The facility of using a single row cylindrical roller bearing, rather than a pair of angular contact bearings, promised significant advantages in terms of simplifying the spindle slide mechanism. This, in turn, created a demand for a new high-speed, highly seizure-resistant, cylindrical roller bearing, a demand that NSK is satisfying with its new Robust CRB series. In order to achieve the desired goals of higher speed performance and lower heat generation, NSK's design for the Robust bearings had to address a number of problems encountered by conventional cylindrical roller bearings in high speed applications. These include: abrasion and seizure of the roller ends and ribs; cage strength at high speeds up to 2,000,000DmN - and 3,000,000DmN in tests; and uneven application of lubricant, leading to irregular heat generation. Reducing the risk of seizure within the Robust bearings is achieved via a synergy of materials technology and optimised mechanical design. As regards the former, the bearing inner and outer rings and rollers are made of surface-hardened SHX, a unique heat-resistant steel having excellent resistance to seizure and wear properties and an excellent life four times longer than that of standard bearing steel (SUJ2). In super high-speed operations, inner rings are generally known to have a tendency to be forced into a fracture mode by high hoop stresses. In contrast, an inner ring made from SHX has a remarkable freedom from such fractures because its internal residual stress can counter the hoop stresses. In terms of mechanical design improvements, NSK has undertaken a number of initiatives including: optimisation of the bearing rib height, closer control of the clearance between rollers and improvement of the roller accuracy. An additional benefit of this design initiative is that it can also reduce heating of the ribs and is therefore very effective. The new cage design of the Robust bearings is designed for high-speed running; it provides outer ring guidance and is built to cope with the constant heating caused by the local sliding of rollers. The high tolerance to heat of the cage is the result of using high temperature Peek (polyether etherketone) material. The polymer is reinforced with carbon fibre and offers the excellent attributes of lightness, and resistance to heat, wear and fracture. The cage pocket design also takes into consideration the material's strength, low deformation and good frictional characteristics for high-speed use. In its development of the Robust range of bearings NSK has addressed the problem, inherent in many oil-air lubrication systems, of irregular heat generation from bearings caused by uneven oil application. As a means of overcoming this, NSK has developed a unit that is able to apply a smaller quantity of oil at controlled intervals - 0.001ml of oil at intervals of 1min. By comparison, the dispensed amount of a conventional unit is 10 times higher at 0.01ml - but at longer intervals of 8 to 16min. The performance of the new unit has been subject to stringent testing which has verified the effectiveness of averaging the oil dispense rate for reducing thermal cycling. The success of the overall Robust design has been proven in tests, using a 70mm bore (110mm OD) single row cylindrical roller bearing with SHX inner and outer rings and rollers. Also tested for comparison were a similar bearing, but with ceramic rollers, and one in which the cage was of a standard roller guide type and made of reinforced polyamide. All the cylindrical roller bearings were assembled with no radial clearance and were oil lubricated. The tests revealed that the Robust bearing developed to embody the design improvements was able to achieve up to 3,000,000DmN. Its temperature rise, when compared with that of the bearing having ceramic rollers, did not show any significant difference up to 2,500,000DmN. Taking these temperature characteristics into account means that in practical situations the new Robust CRB can operate at up to 3,000,000DmN, effectively breaking new ground for a bearing of this type.