Since the SKF acquisitionof SNFA in 2006, engineers from both companies have been working together to create a new, super-precision bearing assortment. These SKF-SNFA super-precision bearings are developed to help original equipment manufacturers and end users further optimize performance parameters of their equipment and applications.

The particular benefits of this new assortment depend on the bearing series and application but include higher speed and higher load ratings, extended bearing ser-vice life, increased maintenance intervals and reduced energy consumption.

As a result of this development, all SKF and SNFA precision bearing designs will eventually be replaced by the new gener-ation of super-precision bearings.

Machine tools require screw drives (fig. 1) that can position a work piece or machine component quickly, efficiently and precisely. To meet these requirements, screw drives are usually supported at both ends by super-precision bearings that can provide a high degree of stiffness. The bearings may also have to accommodate high acceler-ations and high speeds.

The characteristic properties of these angular contact thrust ball bearings include:

• high axial stiffness
• high axial load-carrying capacity
• very high running accuracy
• low frictional moment
• ability to accommodate high speeds and accelerations.

SKF-SNFA angular contact thrust ball bearings for screw drives are especially suitable for screw drive applications, but they are also beneficial in other applications where safe radial and axial support is required, together with extremely precise axial guidance of the shaft.

Comprehensive assortment
It is not only the design that makes the bearings special. The assortment is particularly significant because it covers virtually all screw drive requirements.

Single direction bearings (figs. 2a and 2b) enable various bearing set arrangements. Bearing arrangements can be adjusted to individual requirements and have an enhanced performance, resulting from the SKF-SNFA bearing design.

Double direction bearings (fig. 3a) reduce the number of components needed and are quick and easy to mount.

Bearings for bolt mounting (fig. 3b) and cartridge units (fig. 4) containing SKF-SNFA bearings are also quick and easy to install.

Sealed bearings (fig. 2b) provide more reliability in harsh environments.

Single direction angular contact thrust ball bearings
Single direction bearings (figs. 2a and 2b) provide all the features necessary for high-performance screw drive supports but are not limited to these applications. They are characterized by:

• superior axial stiffness
• low friction
• designed to accommodate high speeds and rapid accelerations
• excellent running accuracy according to ISO tolerance class 2 (ABEC 9)
• dimensional accuracy according to ISO tolerance class 4 (ABEC 7)
• high axial load-carrying capacity
• universally matchable in sets of up to four bearings
• a unique heat treatment for constant preload over the entire service life.

The bearings are available open or with low-friction NBR seals for operating in the temperature range –40 °C to +120 °C.

Double direction angular contact thrust ball bearings
SKF-SNFA double direction angular contact thrust ball bearings (figs. 3a and 3b) have been developed for machine tool applications where space is tight and easy mounting is required. They accommodate axial loads in both directions. Preload is applied by clamping the inner ring halves with a precision lock nut. These bearings are sealed and greased as standard. Bearings in the BEAM series can be bolt mounted to the associated component. Other features include:

• holes for relubrication via the outer ring
• either contact seals or non-contact shields.

Cartridge units with a flanged housing
Cartridge units incorporating SKF-SNFA bearings are another solution for simple and quick mounting. The main characteristics of cartridge units are:

• designed for high stiffness and heavy thrust loads
• all bearing arrangements possible (back-to-back, face-to-face, tandem and other combinations)
• sealed with laminar rings
• grease fill intended to outlast the bearing service life.

Greased bearings
Sealed SKF-SNFA bearings and units are supplied greased as standard with GMG grease. This service is also available for open bearings. The customer can define grease type and quantity. Accuracy of the quantity, even grease distribution and cleanliness are maintained in the bearing factory.

Customized solutions
Advanced modelling and virtual testing services enable the application engineering service to assist in all stages of product development. With core competencies in bearings and units, seals, lubrication, mechatronics and services, the SKF-SNFA team can help design customized solutions to meet next-generation machine tool requirements.

Application examples
Screw drives are typically supported at both ends with bearing sets in a face-to-face or back-to- back arrangement (fig. 5). With universally matchable single direction bearings it is possible to adjust the arrangement to the requirements. Sealed bearings (fig. 6) can also reduce the number of components and eliminate the need to apply grease during the mounting process. Double direction bearings (fig. 7) can further reduce the number of components. Double direction bearings for bolt mounting (fig. 8) do not require a housing and can be mounted easily.

For short screw drives an overhung support at one end is also common (fig. 9). Back-to-back arrangements are best suited for overhung supports.

For stretched screw drives, particularly stiff bearing arrangements can be designed if a tandem arrangement is used at both ends and adjusted against each other (fig. 10). Cartridge units are beneficial for these cases.

Visible and invisible features make the difference

When designing bearings, competing requirements must be considered. For angular contact thrust ball bearings intended for use in screw drives, the most important factor is to provide a high degree of bearing stiffness while keeping the bearing friction at the lowest level.

A further requirement is the ability to cope with high speeds and rapid accelerations. Maintaining an adequate load-carrying cap-acity and providing high accuracy is typically taken for granted.

The new assortment of super-precision single direction angular contact thrust ball bearings for screw drives come with a combination of visible and invisible features:

• a fine-tuned osculation
• a 62° contact angle
• a ground transition between raceway and shoulder
• a robust cage
• a unique heat treatment
• very clean bearing steel
• tightened manufacturing tolerances
• matchable execution – bearing arrangement with up to four bearings per set
• sealed execution for all bearing sizes.