Barden’s new hybrid cylindrical roller bearings meet exacting demands

FAG/Barden has employed innovative materials and design technology to overcome the traditional problem of limited speed capability when using conventional cylindrical roller bearings (CRB) in machine tool spindles. This problem has been solved with a new hybrid series of cylindrical roller bearings that employs FAG/Barden's unique Cronidur 30Ô steel and ceramic rollers. The hybrid series achieves new levels of high-speed performance (n x dm >2 x 106) and offers the properties required to meet the demands of high speed machining at the upper limits of performance with rolling bearings.

The limitations of conventional cylindrical roller bearings for use in machine tool spindles has meant that when the term "spindle bearings" is used today engineers overwhelming understand it to relate to angular contact ball bearings. This belief is not misplaced as angular contact ball bearings have almost totally displaced cylindrical roller bearings in recent years, the former units becoming the focus for R&D activities on machine tool spindles and, hence, have benefited from great improvements in the fields of attainable speeds and grease lubrication.

In contrast, the development of cylindrical roller bearings has been, at best, limited, reflecting their declining use in high-speed applications. Several factors have influenced this decline, but, ironically, the majority of these result from advantages inherent in the design of the bearings. To illustrate this point: the excellent "floating" property of the cylindrical roller bearing has to be weighed against the fact that the bearings can transmit forces in one direction only.

In addition, the easy installation of the separable cylindrical roller bearing results in a somewhat complicated clearance adjustment, presenting the engineer with problems of correct installation. Despite these drawbacks cylindrical roller bearings still offer significant advantages for machine tool applications. One clear point in their favour is rigidity. The radial rigidity of a cylindrical roller bearing fitted with zero radial clearance can be up to 50% higher than that of a spindle bearing pair with all balls loaded. This means that just one CRB can be employed to transmit radial loads in applications where, otherwise, at least two angular contact bearings, preloaded against each other, would be required to do the same job.

In today's intensely competitive cost conscious climate, the benefits of using one bearing instead of two are obvious and have led to developments aimed at eliminating or minimising the disadvantages of CRBs, especially in terms of their speed range. The objective is to increase the bearing's speed capability to keep pace with the bearing at the work end of a machine tool spindle.

FAG/Barden have been in the vanguard of these developments, the company building upon its success with hybrid ball bearings. What experience with the latter bearings has demonstrated conclusively is that the greatest increases in spindle bearing speed performance can be achieved with a combination of silicon nitride balls and FAG/Barden's unique Cronidur 30Ô bearing ring material. Based upon this experience, FAG/Barden have now adopted the same technology to produce the first hybrid cylindrical roller bearing, complete with silicon nitride rollers and Cronidur 30Ô rings.

Higher Speed For Enhanced Spindle Performance

Originally developed for aerospace applications, Cronidur 30Ô is a new bearing steel that exceeds the speeds and fatigue life available from other bearing steels, with the added benefits of superior anti-corrosion and wear resistant properties.

The importance of the higher achievable speeds with Cronidur 30Ô is that they provide machine tool builders, rebuilders and machine users with a relatively simple and low cost method of achieving greater performance from their machine spindles. Added to this, the even temperature operation at higher speeds of the Cronidur 30Ô bearings provides opportunities for substantial cost savings on spindle systems, simply by changing from air-oil to grease lubrication.

Optimising the performance of the Cronidur 30Ô rings for high-speed operation are ceramic rollers. The case for using ceramics is a strong one, borne out by empirical testing. It is well known that the density of a rolling element material takes effect during dynamic processes, e.g. accelerations and high speeds. While, in a spindle bearing, the centrifugal forces, which act on the balls, change the contact angles and thus the entire kinematics drastically, this does not affect the cylindrical roller bearing. Instead, the effect is restricted to the individual rolling element loads and the load distribution within the bearing. Furthermore, due to the lower centrifugal force encountered with ceramic rollers, the load-carrying arc is somewhat larger and the ends of the rollers are subjected to a lower load than in an all-steel bearing. As a result, the transition from an unloaded to a loaded zone is that much smoother. The procedure, however, is more critical with a roller than with a ball, because, generally a roller does not run in an unloaded zone without a certain amount of skewing. Also, at the transition to a loaded zone it must be aligned. This creates sliding motions, which stress both material and lubricant. Under these operating conditions, therefore, the favourable frictional behaviour and the low mass of the ceramic rollers can only have a beneficial effect on bearing performance.

The second major argument in the case for ceramic rollers concerns their relative levels of rigidity. Since the radial elasticity in a cylindrical roller bearing changes noticeably with the speed only if the number of load-carrying rollers decreases there is no great difference in rigidity between a hybrid cylindrical roller bearing and an all steel bearing. Consequently, the high levels of spindle stiffness, which are so essential for final surface finish, are maintained.

In addition to offering high levels of rigidity, a hybrid bearing will react less critically to a somewhat too high preload than an all-steel bearing, which may meet with problems. The reason for this is that the coefficient of thermal expansion of silicon nitride is smaller than that of steel. As a result, the radial bearing clearance or preload of hybrid CRB bearings changes with temperature, whereas an all steel bearing does not. The change in preload of the hybrid is only small however, and takes place at a low level of contact pressure. Moreover, the fact that the change is actually a decrease in preload with the operating temperature means that it presents the spindle user with improved levels of safety in clearance adjustment.

The final major benefit of FAG/Barden's new hybrid CRB design is the favourable tribological behaviour of the steel/silicon nitride material. A major operating characteristic of CRB bearings is the pure sliding motion, which occurs at contact areas between the bearing roller end faces and lips. Under these conditions friction is reduced considerably with silicon nitride material. Consequently, hybrid bearings offer a clear improvement under these conditions, helping to reduce wear and improve operating life Another consequence of favourable tribological behaviour is that friction and adhesion between the ceramic rollers and steel bearing rings (and, consequently, wear) is very low, especially at cold starts. Added to this, the superior tribological properties of the ceramic material enable "cool running" which can extend the working life of the grease, and hence, the life of the hybrid bearing considerably.

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.