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Considerations for bearing systems with linear axes

Considerations for bearing systems with linear axes

The majority of electro-mechanical axis utilise extruded aluminium profile with a single guide rail and bearing arrangement driven by an electric motor through a belt or ball screw arrangement. However, there are physical limitations to the forces such a system can accurately and reliably guide. Festo's Nigel Dawson explores some alternative design arrangements.

There are many high load applications where the size of a single profile and bearing arrangement would be too big and expensive, and the need for a cost-effective, dynamic and easy-to-install, high load, guide unit was identified. Typical applications include use as y-axis in a multi-axis handling system.

Designers typically achieve high load capability by using two electro-mechanical guide units in parallel. They offer a higher degree of flexibility, but prove more than twice the cost and it is difficult to predict the life time of the system. The designer has to take ownership of establishing the optimum performance layout, as well as adding the additional costs of trying to align the components on a very stiff machine framework and constructing custom coupling plates and drive shafts etc. The guide arrangement selected must permit loads in all the necessary directions and deliver the systems required overall lifetime with or without pre-determined maintenance.  

Consider the development of Festo's EGC-HD electro-mechanical axis, with a wider profile than the standard EGC axis and with multiple guide rails to cater for high load applications. Initial designs of the ECG-HD called for a concept based on the same, very stiff Cupola arch aluminium profile, two guide rails mounted accurately and rigidly as far apart as possible and the well proven caged ball bearing cartridges. While this might this seem a simple task, the reality of developing such an innovative solution proved more challenging than first considered, and highlighted some useful insights for design engineers.

Durability testing was vital. Samples had a target endurance life of at least 5 million cycles equating to an estimated 5,000km. However, after just 500km, the first failure of the new axes unexpectedly occurred - when balls from the bearing cassette were ejected from the housing. During the test phase it became clear that in a dual rail system, two axes are never completely parallel - there is always some deviation tolerance. In typical systems using two separate, but alighted electro-mechanical axes, the plate connecting two bearing carriages will compensate for some of this misalignment by deforming to some degree. The issue with the new single Cupola arch system profiles was primarily its lack of compliance - too stiff structure and incoherent bearings resulted in premature wear.

The balls and rail in the bearing should be designed and selected to compensate for the alignment tolerances in the dual rail design. There are many ball bearing arrangements, each delivering different characteristics, and performance, for example, a bearing that has an O-arrangement and 'caged' from one manufacturer may not necessarily perform in the same way as a similar type of bearing from another manufacturer.

To optimise the bearing solutions, it is worth considering the number and size of balls in contact, their arrangement, contact angles and how the balls are retained. Effectively choosing the optimum bearing system can be a very complex process, but establishing close relationships with the bearing manufacturers could certainly play a vital role.

Smaller balls have more points of contact to accommodate the load and offer good stiffness. Larger balls, however, means fewer balls in the bearing cassette and therefore fewer contact points. However, with larger ball sizes, the ball itself can compress slightly, compensating for tiny but important misalignment tolerances. As the rail profile in the EGC-HD is very stiff, there is very little deflection and all the movement has to be accommodated by the balls themselves. It was therefore determined larger ball sizes would minutely deflect and be technically better for the EGC-HD system than the ones used in the single rail axis.

There are two mounting geometries - face-to-face (X) and back-to-back (O) - and the orientation of the balls inside the bearing housing also has a significant impact on the degree of defection resistance to the linear guide.

In the X-arrangement, the balls make contact to the rail in an inward-facing configuration, creating an 'X' pattern inside the rail. This narrow footprint between the centre lines of the balls provides stiffness to the guide, thus reducing the guide's ability to handle moment, or bending loads. On the contrary, the O-arrangement features an outward facing ball orientation and the footprint between the balls is much wider. Forces are more evenly distributed within the guide system and it offers a much greater resistance to applied moment-based forces than the X-arrangement, giving the linear guide better rigidity.

The X-arrangement used in the original single axis proved to be too stiff for a twin guide arrangement and so an O-arrangement is used instead.

Most engineers will assume that caged ball bearings are superior to uncaged, but this is challenged when dealing with such high loads. Much of the space inside a caged bearing cartridge is utilised by the cage itself, reducing the space available for actual balls. Uncaged ball bearings offer more space for balls and thus a higher contact area than caged designs and this higher load capacity helps in heavy duty applications. When using uncaged bearings, the maximum permissible speed of the final system is not as high as achieved using caged bearings.

The final testing phase has proven that no signs of wear were detected up to 10,000km, and the linear axis comfortably exceeds the operational safety margin required on machines which typically have an average service life of 3,600km.

In summary, designers should bear in mind that while a very stiff and accurate profile (frame) is necessary for highly precise applications, the lack of compliance in such a system has to be carefully considered. Moreover, simple linear bearing systems can be accurately modelled and their performance predicted, but the systems can quickly become complicated and existing computer models cannot factor in all the variables and accurately predict operating life. There is also no simple selection of the 'best' bearing: caged or non-caged, X or O configuration, ball size etc all have to be determined for the individual application.

To offer machine builders an accurate solution, engineers should factor the choice of guide system used into their overall design, especially in systems that carry large masses and are typically connected to a stiff structure.

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