How to specify linear motion guides

In this, the first instalment in a series of educational articles, Bob Love, Technical Manager for THK answers the question: what are linear motion guides?

Linear motion guides are used throughout industry to move or position tools and workpieces, parts and assemblies, and complete machine systems and other large structures.

Linear guides can range in length from just a few centimetres to several metres, can move at speeds of over 5m per second and be positioned to levels of accuracy that can be better than 1m.

Choosing the best linear guide for each specific application is not always a straightforward exercise as different manufacturers use various methods and materials of construction, and the range of specifications used, often with little commonality between suppliers, leads to further complexity.

In this series of articles we will describe the basic construction used in many types of linear motion guides and then move on to look at the key specification used, understand what they mean, and finally summarise the main technical and commercial issues that need to be considered when specifying, installing and operating these devices.

Linear motion guides are generally constructed of a profile rail over which a block or carriage runs.

This is designed to carry a workpiece or tool, and is supported by bearing rolling elements that allow it to operate smoothly with high levels of precision and repeatability.

Although the idea of using rolling element mechanisms has been used for thousands of years - for example, the builders of Stonehenge and the Pyramids moved large blocks of stone on timber rollers - the development of the modern linear guide can be traced back to 1944 when a round shaft linear bush guide was first manufactured in the USA.

This basic design gradually evolved through ball splines with straight-ridged shafts, to angular-contact ball splines with fixed shaft supports and then in 1973 to a profile rail linear guideway, with two-point circular arc contact, that forms the basis of today's foremost linear guides.

In essence, a modern linear guide consists of a rail, carriage, balls, end caps and ball retainers, plus the associated seals and where appropriate lubrication and mounting mechanisms.

Operation is relatively simple, with the rail and carriage being separated by a caged series of steel balls.

These are generally mounted within raceways and enable the carriage to move smoothly along the rail, with the series of balls being circulated by return mechanisms in each end plate as the carriage moves.

The raceways are shaped using either gothic arch or circular arc structures.

Although the former provides higher individual rigidity with four contact points for each ball, rather than just the two of the circular arc, its construction leads to higher levels of differential slip, especially under heavier loads, which leads to greater friction, noise and heat and effectively impairs smooth, consistent and repeatable motion.

Gothic-arch raceways are generally used for miniature types of linear motion guides, where loads and frictional forces tend to be far lower.

For all other applications, the circular arc raceway is preferable, especially in a configuration where four raceways are used with each carriage, as this provides far greater load carrying capacity, is effectively self-adjusting and offers significantly higher levels of stiffness and reliability across the full operating range.

In addition to the shape of the raceways, one of the key factors for ensuring efficient operation is the design and construction of the ball mechanisms.

Traditionally these have been arranged without cage retention in a continuous loop through the raceway, with lubricating grease being used to minimise the noise and frictional effects of ball to ball contact.

This construction does, however, suffer from inherent drawbacks as no matter how much lubrication is applied there will still be relatively high levels of friction between balls and between the balls and the walls of each return hole.

This increases operating noise and heat and, perhaps more importantly, reduces smoothness, repeatability and operating life.

To overcome these problems the latest linear Caged Ball guides incorporate mechanisms that effectively seal each row of precision ground steel balls in a cage, where each ball is locked in with an independent pocket of grease.

This construction requires almost zero maintenance and, by eliminating ball to ball contact, produces an exceptionally low level of friction, allowing each linear guide to move smoothly, with minimal noise and heat and at extremely high speeds, often in excess of 300m per minute, with levels of positional accuracy that can be better than 1m.

In addition, reliability and operating life are dramatically improved, with many devices being capable of travelling the equivalent of more than 40,000km without maintenance or relubrication.

Modern linear guides have evolved to become extremely versatile, high precision devices that are used in applications ranging from electronic pick and place and semiconductor manufacturing, medical equipment and machine tools, heavy duty palletising and handling, and even civil engineering to provide protection to buildings in earthquake zones.

Using linear motion guides effectively depends on a good understanding of how they work and of how to specify them correctly - a topic that can be fraught with difficulties, and which will be the subject of a subsequent article.

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