Tackling linear motion system selection

Here we look at four main options - 'V' bearings and slides, recirculating ball bushings, recirculating ball monorails and plain PTFE bearings.

Jason Cox, Senior Applications Engineer with Hepco, explores the options Selecting the most appropriate linear motion system for your application is a bit like choosing a car.

All models can get you from A to B but each has individual characteristics which make it more or less suitable for your specific requirements.

For example, you wouldn't opt for a Mercedes CLK if you wanted to drive across rough terrain, any more than you would choose a Land Rover Defender for motorway cruising.

Similarly if pre-load adjustability is a major criterion for your linear motion application you wouldn't choose a plain bearing or a recirculating ball monorail.

However, both would be high on your selection list if resistance to shock loading is important.

So how do you know which type of linear motion system is right for you? Here we look at four main options - 'V' bearings and slides, recirculating ball bushings, recirculating ball monorails and plain PTFE bearings.

The 'V' guide principle is the most universally applicable for the vast majority of today's industrial applications.

However, applications reliant on one overriding operating characteristic may be better served by one of the alternatives.

As well as being highly resistant to shock loading, PTFE based plain bearings are also a good choice for short stroke, high acceleration applications.

They have no moving parts and operate efficiently provided they are well lubricated.

If your application is in the harshest of environments and you do not need high accuracy or zero play for example, plain bearings are eminently suitable.

'V' bearings however do come a close second in this category as unlike recirculating systems, their self-cleaning action eliminates contamination.

Highest grades of accuracy are achieved by ball monorails.

So for tolerances of 10 - 15 microns this is the best choice.

They also offer the highest load carrying capacity, space for space, against other systems.

Whilst monorails are steadily decreasing their re-lubrication frequency, there is still no comparison with the travel that can be achieved by 'V' slides with Cap Seals before re-lubrication becomes necessary.

The 'V' slide is therefore a 'fit and forget' system for many applications.

In all other categories the 'V' principle scores very highly.

The inherent low friction of 'V' systems means they positively thrive in high speed conditions especially in applications which also require prolonged accuracy.

In addition to low operating noise, low friction means that in many cases running 'dry' is possible with no lubricant needed between slide and wheel.

Ball bushings and monorails cannot operate without lubrication.

The ability to run 'dry' is particularly useful in food and clothing production systems as well as in nuclear applications for which 'V' guide wheels can be internally lubricated for life with special grease.

'V' systems are unique in possessing the means for adjusting or setting for pre-load.

This is made possible by adjustable eccentric guide wheels being opposed by fixed concentric wheels which allow the correct rigidity and running characteristics to be created for the given application.

Also, unlike ball monorails, 'V' systems are more tolerant of unmachined mounting surfaces - an immediate overall cost advantage.

The 'V' principle also scores the highest for range of options.

A wide choice of mix and match elements allow the easy creation of dedicated systems.

'V' guide options include double-edge slides and single-edge slides either spaced wide apart or combined with flat tracks and track rollers for extra compliance.

They can also have integral drives.

Combined linear and circular motion is also the sole preserve of the 'V' system.

'V' guides can be supplied as complete rings and ring segments of any angle.

They can also be joined to straight tracks to give a huge variety of 'S' bends and complete circuits for continuous motion.

Finally, cost over life of machine - again, the 'V' system is ahead on points especially where conditions are hostile and contaminated.

Low friction means lower energy consumption and long re-lubrication intervals cut maintenance costs.

By comparison, a recirculating ball monorail is highly susceptible to dirt ingressing the ball track, causing the block to fail and permanent damage to the rail.

The need for a replacement bearing is the worst that can happen with a 'V' system, a task that can easily be accomplished without affecting the slide.

Furthermore, thanks to its adjustable eccentric bearings a 'V' system will continue to work even when worn; other systems would have to be replaced.

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