How to specify linear motion guides - Part 2

In the second instalment of his series of educational articles, Bob Love, Technical Manager at THK UK, aims to demystify the jargon associated with linear motion guides.

In today's industrial marketplace there is a plethora of linear motion systems available from many different manufacturers and suppliers.

To the uninitiated, many of these products and systems can be seen as similar in terms of performance and functionality, making it difficult to select an appropriate solution on any basis other than cost or possibly a headline performance figure.

In practice, however, cost should be only one of a number of factors on which a purchasing decision should be based and it is therefore important both to understand the key specification quoted by suppliers and to appreciate the fact that the same specification can be interpreted or quoted differently by different suppliers.

In addition, although some of the information contained within linear motion product catalogues is generally detailed and accurate, there are often terms or definitions required for accurate product specification that are not always clear and this article therefore seeks to provide a brief guide to the common terms and what they mean.

Dynamic load ratings illustrate the capacity of the product and ultimately help to determine the service life of a linear motion system when a specific dynamic load is applied from above (radial), below (reverse radial), or from the side (lateral).

Linear motion guides can be divided into two categories: four way equal type and radial type.

The former is capable of handling loads from all directions and has a dynamic load rating that is standard or consistent regardless of the direction of the applied force, while the latter type of guides are specifically designed to handle radial loads and will have a different dynamic rating for top/bottom and side loads.

Product selection can therefore be optimised against the load direction to give the best level of overall performance.

The load factor (fw) indicates the relationship between the load-carrying capacity of a linear motion guide and an external force generated by vibration or an unexpected impact.

In a demanding factory or processing application, this figure can be used to modify the ability of a linear guide to respond under particularly harsh operating conditions, where environmental factors can have a negative effect on a nominal fatigue life calculation.

It should be noted that selecting a linear guide that can bear the maximum possible load, both when it is stationary and in motion, is vital to ensure consistent and accurate movement.

It is normal therefore to consider the static safety value (Fs) to safeguard against shock loading, as well as determining fatigue life expectancy under dynamic conditions.

Radial clearance data can be used as a guide to help maximise the precision, performance and installed rigidity of linear motion products.

The term covers both normal and negative clearance.

The first applies where the loading direction is fixed and where impact and vibration are slight and the axes installed in parallel, while negative clearance relates to more specific applications, such as high precision or heavy loads.

Negative clearance is determined by the internal load, or preload, exerted on the rolling linear guide elements, to increase block rigidity and reduce clearances and is often subdivided into clearance (C1) for a light preload or clearance (C0) for a moderate preload.

There are also a number of application related criteria that can influence the overall performance of a linear motion system.

These include rail and carriage spacing, the number of carriages within the system, the level of exposure to high acceleration and deceleration forces when loaded, the use of bump stops, system cleanliness and the correct use of lubrication.

In effect, fatigue life is determined by all of the factors described above, being the total operating distance that a linear motion system travels before its internal mechanisms start to deteriorate or surfaces start to wear or flake.

In theory, similar products built to identical specification should have similar service lives making a comparison relatively straightforward; in practice, however, this is rarely the case as differences in production techniques, choice of materials and finishing processes will all affect long term reliability.

Perhaps the simplest guideline for determining fatigue life is the general figure quoted by manufacturers for nominal life; this is generally taken from standard test data showing the total running distance that 90% of a sample series of identical linear motion systems have achieved without deterioration, under the same load conditions.

Naturally, if higher levels of reliability are required, these can be built into the selection process, often by correct selection of product rather than simply increasing the sise and capacity.

The data and information presented to customers by manufacturers of linear motion products can often be complex, diverse and sometimes conflicting.

Gaining an understanding of the basic terms used in this industry will help design and mechanical engineers develop more efficient and reliable machinery.

It should also be recognised that many linear motion guide manufacturers are more than happy to guide customers when it comes to specifying such equipment.

The earlier the manufacturer can be involved in the machine design and build cycle the more effective a solution is likely to be created, in terms of performance, functionality and cost.

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