TechArchive

Original article date: December 1996

The SNL bearing housing is designed on the split plummer block principle, to offer flexibility in bearing choice. Designing the SNL split plummer block housing on a modular principle allows a wide choice of bearings to be selected, including spherical roller bearings, self-aligning ball bearings or CARB.

Overall bearing performance is also increased through improved bearing housing quality and enhanced heat transfer characteristics.

Attention has also been paid to the improvement of grease and oil lubrication methods through the provision of a tapped inlet hole (protected by a plastic cap) and a grease nipple (delivered inside the housing). A standard variant of this is also equipped with an outlet hole. Oil lubrication requires running at a lower temperature when compared to the prevailing SNH design. This has been achieved in part through improved heat transfer between the plummer block and the support surface. Cross webs have been added on the base of the housing which also allows for higher running speeds and increase the stiffness of the complete structure. Webs supporting the bolt loads now avoid seating deformation.

With an increasing trend towards condition monitoring, the SNL housing has been equipped with various location dimples for fitting temperature, load and vibration sensors. Dimples for additional drillings to allow for relubricating spherical roller bearings and seals are also included.

The potential to mix housing caps is a problem that has been identified when mounting more than one split housing at a time. The consequence of such an error is premature bearing failure, because of damage to the all-important bearing seating tolerance when the incorrect housing cap and base are used. Such an eventuality can be avoided with a new marking system that pairs the cap and base together, while dowel pin locations con the feet can be used to simplify repositioning. Bosses have been incorporated in the bearing cap to improve stiffness and provide location points.

Four centre marks give simpler and more precise positioning and lead-in chamfers between the shaft bore and seal grooves have been added to ease fitting and improve seal performance.

The SNL housing has a complex geometry that requires the use of a three-dimensional design and manufacturing system. SKF Mekan now uses the Pro/Engineer software system linked to a four axis CNC milling machine. This system provides a solid prototype from which a manufacturing pattern can be created. The SNL housings have been manufactured to tighter tolerances, particularly with regards to the bearing seating diameter and seal grooves, to improve running performance. Greater accuracy of the seating bore means improved bearing roundness and load distribution under arduous conditions and allows consistent assembly accuracy.

In December 1996, operation started of a new casting facility in order to produce the first SNL units, build into a 48m production line. This produces 200 casts per hour. Depending on the size, each mould can hold up to 12 pieces. These casts are then machined to final tolerances. The range to cover shaft sizes from 20 to 160mm diameter will be complete shortly, several sizes having been exhibited in advance at the Hanover Fair in April.

Creating the SNL range has involved the design methodology known as Quality Function Deployment (QFD). The design team at SKF Mekan AB in Sweden were initially trained by Ben Bles from the SKF Engineering & Research Centre (ERC) in the Netherlands.

Essentially, this approach required that quality aspects are considered on all design, development and production activities to provide the user with an appropriate product. Consequently, the user has to be involved at the initial design concept stage.

In practice, QFD uses a matrix (called the House of Quality) to correlate a list of user needs with a number of design characteristics. This is in the form of basic “what” questions linked to “how” responses to the way in which these needs can be met. In this way, end-user requirements can be translated into the design parameters, process and production details to outline the appropriate product features.

SKF has its own technical language that is often at odds to that of the customer. For example, the user may be interested in low wear or easy mounting, while SKF engineers work with surface hardness and chamfer details. The QFD method can provide the bridge.

As the new housing has applications in a variety of industrial applications, leading users from key segments were contacted. These include companies involved in fan applications, materials handling equipment, paper machines and steel production, as well as general industrial applications. In addition, a number of distributors with a broad knowledge of industry were also involved in brain-storming sessions.

Within each end-user location, representatives from a minimum of three different departments with an interest in the product were interviewed. This included the departments concerned with quality, design, purchase, maintenance and mounting. The meetings were carried out in an informal atmosphere.

It became apparent that within each location, different departments had diverging opinions on their specific needs. Five factors, however, proved to be of recurring importance:

• simplified mounting
• housing ready for relubrication
• housing prepared for sensors for condition monitoring
• improved heat transfer characteristics
• components to build a bearing unit supplied from one source.

As crucial to the design process was the collection of information within SKF itself. Sources approached for news included the sales force, application and bearing design engineers.

Subsequently, a project team was formed to implement the new plummer block design. A constraint was that the SNL housing range should be interchangeable with the existing SNH models, which involves complying with the appropriate international standards.

Additional design goals were to ensure that all existing accessories, such as seals, were compatible with the new design, retained current designation systems and used environmentally friendly materials and manufacturing processes.

• SKF
• Tel: 01908 838305
• Fax: 01908 647849

December 1996