Defining lines

Bob Love, Technical Manager at THK UK, clarifies linear motion guide specification and defines performance in terms of linear guide selection, load ratings and testing procedures.

Bob Love, Technical Manager at THK UK, clarifies linear motion guide specification and defines performance in terms of linear guide selection, load ratings and testing procedures.

Manufacturers of linear motion products regularly publish comprehensive specification and performance data in product catalogues and technical literature.

Although many of these figures, including dimensions, operating speeds and nominal lifetimes, are self-explanatory, there are many that are far from straight forward.

Unsurprisingly, end users don't always fully comprehend how product specifications are calculated or how to interpret these all-important statistics in real-world applications.

In the realm of linear motion guides, product specifications usually relate to one of two types of technology.

Generally, linear motion guides rely on rails, or raceways, and guide blocks, or transport platforms, along specified routes.

Various techniques are available to support the mechanism, although most systems utilise steel ball bearings held within the block running along a groove profile in the rail.

The result is a highly efficient unit that provides accurate movement and pinpoint control.

Advances in technology have, however, encouraged relatively recent and dramatic developments in linear motion systems.

THK's Caged Ball design, for example, which relies on lubricant-compatible retainers to hold the ball bearings at an equal distance from each other, has enabled smoother movements, less friction and fewer maintenance requirements.

The latest linear motion guides offer extraordinary performance levels encompassing operating speeds exceeding 300m/min and systems that travel in excess of 40,000km without maintenance.

Other statistics, including product dimensions and ratings, provide crucial product information without which correct specification would be impossible.

Nevertheless, some specifications can appear superfluous even if they provide important insights into the potential use or application of a product.

One principal criteria is dynamic load ratings (C), which are used to determine the service life of a linear motion system in response to a specific load applied from above (radial), below (reverse-radial) or from the side (lateral).

Linear motion guides can be divided into two main categories; those which have the same load rating regardless of where the load originates from (four-way equal-load type) and those specifically designed to support radial loads (radial types).

Another key measure is the static safety factor, which measures the relationship between the load-carrying capacity of a linear motion guide and an external force resulting from vibration or an unexpected impact.

In a busy factory or processing environment, this figure (fs) can reveal the ability of a linear guide to cope in particularly harsh operating conditions.

It should be noted that selecting a linear guide that can bear the maximum possible load when it is both stationary and in motion is vital to ensuring effective and accurate movements.

Likewise, data relating to radial clearance can be used to specify products and ensure that precision, performance and rigidity are maximised.

The data are subdivided into normal clearance, where the loading direction is fixed, impact and vibration are slight and axes are installed in parallel, and negative clearance, which relates to more specific applications, such as high-precision or heavy-loads.

Negative clearance is further subdivided into Clearance C1 (light preload) and Clearance C0 (moderate preload) and is determined by the internal load (preload) exerted on the rolling elements to increase block rigidity and reduce clearances.

In the present economic climate, information about life cycle costs is often the defining consideration.

Fatigue life refers to the total operating distance that a linear motion system travels before its internal metal surfaces start to deteriorate or flake.

Yet, the service lives of similar systems manufactured to the same specifications and subjected to the same operating conditions can and do vary.

Therefore, a guideline for determining fatigue life is the nominal life, which is the total running distance that 90% of a series of identical linear motion systems can achieve without deterioration, under the same load conditions.

To fully appreciate the relevance of these product specifications it is often useful to compare differing technologies.

Endurance tests, which push linear guides to their absolute limits, are used by manufacturers to establish accurate product information.

These tests involve a series of repeated movements that replicate extended use within certain performance parameters.

Often carried out on life-test machines that are designed to evaluate a number of linear motion guides at any one time, these tests form the basis of most product specification data.

For instance, a series of tests on THK's SHS linear motion guide have revealed that when carrying 50% of its dynamic load capacity, the Caged Ball product travelled 1347km with only its initial lubrication.

This compared favourably to its full-contact equivalent, which only completed 420km before flaking.

It should be noted that the conventional guide also required lubrication every 100km.

These results, and those from similar tests on other guide types and sizes, represent many years of testing and indicate that the Caged Ball design has a greater dynamic load rating than initially calculated.

Indeed, THK has recently revealed that tests carried out at its research facilities in Japan indicate that the Caged Ball design provides a greater service life than conventional full-contact systems.

THK has consequently increased its initial dynamic load ratings, measured in kilonewton units, by 30%.

In terms of fatigue life, this equates to over double the life of conventional systems.

The intricacies of linear motion guide specification reflect the important role played by these components in complex industrial, manufacturing and automotive processes.

They are required to operate quickly, accurately and smoothly, often completing many hundreds of tasks each day.

Detailed product information allows specifiers to select the most suitable product, thereby realising a system's full operating potential.

Product specifications are the guiding lights in the dark and often complex world of procurement.

They outline a system's capabilities and indicate where its limitations lie.

They are the unequivocal truths that determine which linear motion technology to opt for.

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