Moving experience

A choice of linear motion technologies, for a wider range of applications than ever, enables machine builders and end-users to make huge cost savings. Mark Venables reports.

Today there is a huge choice of linear motion systems available from many different manufacturers and suppliers. Many of these 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.

But according THK UK‘s technical manager Bob Love, cost should be only one of a number of factors on which a purchasing decision is based. He maintains that it is important to both understand the key specification quoted by suppliers and to appreciate the fact that the same specification can be interpreted or quoted differently by separate ones.

‘In addition, although some of the information in 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,’ he said.

One of these, he said, is dynamic load rating, which illustrates the capacity of the product and ultimately helps 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. the latter type is specifically designed to handle radial loads and will have a different dynamic rating for top/bottom and side loads.

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 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.

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 theory, similar products built to identical specification should have similar service lives, making a comparison relatively straightforward. said Love. ‘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 per cent of a sample series of identical systems have achieved, under the same load conditions, without deterioration.’

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

A novel use of linear motion technology is The preservation book cradle — the brainchild of Alan Buchanan Designs — which is now an important piece of equipment in most major UK libraries, and book collections in the US, Iceland, Sweden, Ireland, Greece and Egypt.

The machine safely holds delicate books during the process of page digitisation. Traditionally, this can be a time-consuming and exacting exercise — but thanks to the incorporation of a linear drive system the latest generation of this cradle can now achieve perfect focus quicker.

Two HepcoMotion elements are featured in the cradle design — the MCS aluminium machine building system that provides a framework for the cradle and a PDU2 driven linear slide on which the camera trolley is mounted. The operation of the cradle depends on the synchronised movement of two platforms as the pages are turned.

Gentle platform movement is critical to ensure stresses are removed from the book’s spine. In earlier designs of the cradle a rack and pinion drive was used to focus the camera. This provided adjustment to within 0.5mm, leaving the operator to fine-tune the focus on the camera itself.

‘The problem is that pages very rarely lay flat — this is particularly true of parchment,’ said Buchanan. ‘That means every page may need to be re-focused. The cradle’s scope of adjustment is almost infinite.’

THK has been researching linear motion technology and the parameters within which guides and actuators can be used; prototype systems that can resist a range of temperatures and vacuum conditions that are considerably higher than anything previously achieved.

The focus of the company’s R&D programme is to evolve a new generation of linear motion technology that can be used both in the most extreme operating conditions, such as the offshore and defence industries, and in mainstream manufacturing and process applications to make dramatic improvements in reliability as well as cut operating costs.

The prototype systems incorporate a range of new properties and capabilities, including the ability to operate reliably at vacuum pressures down to 10e-3Pa or at sustained temperatures in excess of 200oC.

These properties have been achieved using the latest high-performance steels and alloys, specially developed vacuum greases, with minimal emission characteristics and perfluoroelastomer seals that are said to able to withstand chemical attack and extreme environmental conditions.

To improve functionality and reliability still further, many of the prototypes feature THK’s caged ball technology, which eliminates noise, friction, particle emission and wear associated with conventional ball race configurations, by enclosing high-precision steel balls within a separator cage, each ball surrounded by controlled density grease pockets.

This effectively prevents contact between the balls during operation, which can lead to significant improvements in both overall performance and operating life. It also makes for quieter, cleaner, smoother and higher precision movement, with a far longer service life than traditional ball raceways.

Most importantly, the technology enables systems to be used in a far wider range of applications, and to become fit-and-forget products, enabling machine builders and end-users to reduce costs.