Angular-contact rolling bearings are compact,, light and precise

AXS (pictured right) and SGL bearings are lightweight bearings that can support significant loads and high tilting moments. The reduced design envelope of the bearings also opens up new opportunities for more compact machine builds and automation systems.

Limitations of conventional rolling element bearings

Today's engineering applications often demand smaller, lighter, higher-precision bearings with good load-carrying capacity. In order to fulfil these requirements, improved designs of ball bearings, angular contact ball bearings and four-point contact bearings have been developed. Where extremely high or combined loads occur - comprising axial, radial and tilting moment loads - tapered-roller bearing sets or crossed-roller bearings are normally specified.

Thin-section ball bearings enable the design envelope and mass to be reduced; however, the small cross-sections of these bearings and the rolling contacts between the balls result in lower basic load ratings. The load-carrying capacity, rigidity and rating life are also reduced significantly. These bearings also require high-precision ground bearing seats and involve considerable investment in terms of both production and assembly.

Technological progress has now enabled the development of even smaller diameter bearings, which are of great value in metrology, laser technology, semiconductors, aerospace, precision engineering and optics. Corrosion-resistant hybrid bearings are also opening up new applications in clean room and vacuum environments.

However, in many machine designs, these types of bearings often reach their design limits and can only meet the high requirements under certain conditions. Examples here include industrial gearboxes, automation systems, robotics, conveying equipment, hydraulics and medical equipment.

New alternatives

AXS and SGL angular contact rolling bearings are claimed to offer designers a more cost-effective alternative to conventional ball and rolling bearings. The new bearings are cylindrical rolling bearings with raceways that are oblique to the bearing axis.

Similar to thin disc springs, AXS bearings have a conical bearing ring that is produced by a forming technology. A roller and cage assembly runs between these two hardened rings that have a contact angle of 45 degrees, though bearings with a 60-degree contact angle can be used for higher axial forces and high tilting moments. Bearing diameters are currently available from 8-175mm.

The high precision of the SGL bearings is achieved by the machined, ground bearing rings, which have a triangular profile. A roller and cage assembly runs between these two hardened rings. The cylindrical rollers are guided by a cage, which is made from a wear-resistant polymer.

The contact angle of an SGL bearing is normally 45 degrees, while AXS bearings are designed with a 45-60 degree contact angle, depending on the load conditions and application. SGL bearing diameters are currently available from 8-200mm.

AXS - lightweight with high load carrying capacity

The AXS angular-contact rolling bearing combines low mass and a compact design envelope with impressive basic load ratings. With a bearing ring thickness of just 1.2mm and radial section heights of 7mm and 10mm, the AXS offers great potential for design miniaturisation.

Schaeffler explains that the roller and cage assemblies comprises cylindrical rollers that are snap-fitted into high-strength polymer cages with the smallest possible pitch. The high basic load ratings enable additional preload - for conditions such as swivel operations - in order to increase further the tilting stiffness. Maximum use of the design envelope and the use of cylindrical rollers increase the load ratings and rigidity compared to thin-section ball bearings. For example, an AXS angular-contact rolling bearing with a nominal diameter of 140mm has a roller and cage assembly that contains 90 cylindrical rollers in a 3.5mm x 3.8mm cross-section.

AXS bearings can be mounted very easily and securely. As the bearing rings are geometrically identical, there is no risk of them being mixed up during fitting. The bearings can be fitted easily with the roller and cage assembly without requiring specialist skills or knowledge. Turned bearing seats are sufficient for the formed bearing rings; there is no need to grind the seats.

SGL - accurate and rigid

SGL series bearings are recommended where high load-carrying capacity, accuracy and rigidity are required. The precision-machined, ground, triangular-profile bearing rings - in combination with the line contact of the cylindrical rollers - result in highly precise, rigid bearings.

SGL bearings are said to have significantly higher tilting rigidity than conventional angular-contact ball bearings. The SGL series corresponds to dimension series 18 and so are directly interchangeable with series 718 bearings. Other sizes can be produced as required.

The use of quadratic rolling elements provides significantly improved kinematics. The rolling behaviour of SGL bearings is almost ideal. The bearings run uniformly with low friction and so are suitable for high-speed applications.

SGL bearings are used predominantly in 'O' arrangements; the resulting large support distances increase the rigidity of the bearing design. Depending on the requirements, the bearing arrangement can be easily adjusted from bearing clearance to bearing preload. As a result, the bearing achieves significantly higher precision. The bearing cross-section, which is significantly larger compared to AXS bearings, provides higher radial and axial load carrying capacity.

The SGL bearing is best used where the rating life and tilting rigidity in the application need to be increased further. The SGL is therefore a suitable alternative, for example, to crossed-roller bearings under higher requirements. When using a pair of angular-contact rolling bearings, the number of load-bearing rolling elements is therefore doubled.

Applications

Used in the 'hand' of a paint robot, AXS bearings open up new possibilities in terms of cost optimisation for supporting several axes in extremely thin-walled designs. The small bearing cross-section enables compact designs to be implemented. The low weight of the bearings, combined with the relatively large diameter, enables lightweight designs with high load-carrying capacities to be produced, which, in turn, leaves more space for the passage of paint hoses. The bearings are maintenance-free, being lubricated for life with special grease.

AXS bearings are already being used in tracking systems on solar power plants. Here, the small cross-section, high load capacity and high rigidity of the bearings are critical. These characteristics ensure high design flexibility and enable more compact gearboxes to be designed, which offer reliable, smooth and low-vibration support for solar mirrors. This cost-effective, optimised drive design weighs significantly less but is also able to withstand the high loads that can occur in high wind conditions.

AXS bearings are suitable for applications requiring high load-carrying capacity and tilting rigidity, as well as low weight and a compact design envelope. In solar tracking systems, speed and precision play a secondary role - the decisive factor is that the bearing should support high loads and tilting moments and should also allow smooth, quiet movement.

In medical equipment, AXS bearings are being utilised in ceiling-mounts for operating theatres, where they are replacing wire raceway ball bearings. The AXS bearings provide smooth, precise positioning of medical apparatus. The small cross-section increases the open cross-sectional area of the support arm in the ceiling mount, enabling the passage of numerous feed lines for water, electricity, oxygen, IT cabling and so on.

High-precision gearboxes are constantly undergoing design refinements and optimisation. Due to increasing demands for reduced design envelopes, less friction, lower temperatures, smooth running and high tilting rigidity, the capabilities of the bearings within gearboxes are subjected to constant analyses. The performance limits of standard bearings are often reached or exceeded in these cases. Increasingly, conventional angular-contact ball bearings or crossed-roller bearings used in gearbox output bearing designs are being replaced by SGL angular-contact rolling bearings. The rigidity and accuracy achieved with line contact and an 'O' arrangement make a strong case and also increase the rating life.

For more information on Schaeffler's AXS or SGL series angular-contact rolling bearings or for a copy of catalogue TPI 185, call the Marketing Department on +44 (0)121 313 5870 or email [email protected].