Cut out creep

Nick Brooker, Marketing Manager for the UK Division of Actuation and Motion Control of SKF, explains how cage creep is being eliminated from the latest linear motion systems.

Linear motion devices, including slides and precision guides, are used throughout industry and are especially common in x-y-z positioning systems, where speed, acceleration and repeatability of movement, often within extremely tight tolerances, are essential for maximising the performance of production and process equipment.

Many of these devices use nonrecirculating rolling elements, contained within a cage, as the mechanism for moving and positioning a load-bearing block or carriage along a flat or V-shaped track or guide rail.

Although this method of construction is generally efficient and reliable, the rolling elements can suffer from a condition known as cage creep, which can gradually reduce the positional accuracy, repeatability and travel of the guide, eventually reaching a stage where it has to be repaired or replaced.

In a caged roller guide, the rollers are mounted between the block and rail and circulate within a metal or plastic cage, with the cage being used to separate the individual rollers to prevent rubbing, friction and drag, and in conjunction with a series of plates to ensure that rollers are held in the correct alignment; rollers are typically arranged to provide a contact angle of 45 degrees for optimum efficiency and can also be constructed in a cross-roller arrangement to increase load carrying capacities.

These caged roller units have a number of benefits; for example, as lubricating grease is contained within each cage it can be applied evenly across the face of individual rollers and retained between rollers at all times, enabling operating life to be significantly extended.

Similarly, caged systems can offer far higher levels of radial and lateral stiffness, compared with conventional roller bearings, can be used to move and position far higher loads and exhibit lower levels of friction, heat and operating noise.

Caged roller units can, however, suffer from a phenomenon known as cage creep.

This can be especially prevalent in crossed-roller devices and is often found in systems where linear guides are in continuous use, are mounted vertically or misaligned, or where there are high levels of acceleration and variations in payloads.

Cage creep is caused by the fact that the weight bearing retainer and its ball or roller components are designed to move separately to provide smooth, low-friction motion; this is not an issue under normal operating conditions but as stresses on the mechanism increase it is possible for the lack of synchronisation between the different components to move the balls or rollers slowly from their centred position in the cage housing.

This eventually results in the end of the housing making contact with the end stops of the guide rail; at this point, the individual rolling elements will begin to skid, rather than roll, thus reducing the effective stroke length, operation and lifespan of the rail guide assembly.

In addition, as the cage moves further from its centred position levels of friction increase exponentially at the limits of each stroke, resulting in uneven movement and making the condition difficult to diagnose.

Indeed, cage creep is rarely recognised by end users and is often mistaken for insufficient motor torque or guide misalignment, resulting in unnecessary downtime or system modifications in an effort to resolve the problem.

Even where the phenomenon is understood it is still necessary over time to recentre the cage components, requiring either system shut down for cage repositioning or the use of larger and more expensive drive motors to deliver the necessary power to reset the cage to the centred position.

Different manufacturers have developed a variety of mechanisms to overcome the problem, with one of the most effective being the use of a simple rack and pinion mechanism that keeps cages permanently centred by synchronising the movement of the cage and roller assembly.

In the system developed by SKF, for example, the plastic moulding that forms each individual cage cell also incorporates a small plastic toothed sprocket at its centre, with corresponding tiny notches being machined in each side of the V-shaped guide rails.

As the block moves, the roller cage automatically meshes with the rail, ensuring that each set of cage components synchronise effectively, keeping the complete cage unit correctly aligned during the lifetime of the linear guide and eliminating the need for recentring or replacement.

Perhaps as importantly, as the anti-cage-creep mechanism is integrated within the roller cage structure, there is no increase in the overall size or weight of the linear guide; similarly, it is possible to extend the application of these devices into areas where vertical orientation, high acceleration of varying payloads would previously have caused problems.

It is worth noting that the rack and pinion mechanisms are used only to guide and synchronise the cage components and are not load bearing as this is the function of the balls or rollers.

As a result, although some older systems use metal sprockets most modern units use the latest high performance engineering plastics as these are lighter, less expensive to produce and, provided they are correctly designed, will provide an exceptionally long operating life with almost zero friction or noise.

Linear roller guides offer machine builders and end users alike an extremely efficient, reliable and cost effective method of moving and positioning workloads and tool pieces, but have often been restricted in use due to the problems associated with cage creep.

Now, with the addition of the latest anti-creep technology, it is possible to expand significantly the use of these devices into an even wider range of applications.

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