Bob Love, Technical Manager at THK UK, considers how linear motion guides have emerged as an important method of improving process reliability and efficiency.

Linear motion guides have come a long way since their introduction during the 1950s. Their development has closely followed the increasing automation of industrial processes in almost every market sector, satisfying the growing demand for accurate and repeatable movements and cost-efficient operation.

Linear motion guides are, in essence, transport mechanisms on which machinery and components can be moved or positioned to perform specified tasks. They can be configured to meet an array of different criteria and can provide high levels of rigidity from a minimal operational footprint. With a variety of possible options, linear motion guides can be used on level surfaces, vertical planes, inverted positions and on inclines.

Unsurprisingly, by offering versatile, high-speed performance, low-noise characteristics and compact designs, linear motion guides have been readily adopted by many different industries. For example, typical applications include machining centres, medical equipment, measuring instrumentation and industrial robots.

To satisfy this demand, ingenious linear motion systems have been engineered to secure smooth and efficient movements, as well as delivering accuracy with adaptability. Today, choice is only limited by requirements. Miniature, rotary, high-load, caged-ball, caged-roller and ballspline type guides are just a few examples of what’s available.

Current developments
Advances in control technology have, however, promoted developments that have encouraged dramatic advances in linear motion technology. For instance, some early linear motion devices relied on gothic-arch-contact or early ball-bushing designs. Although this technology is still used today, it is generally considered to be inferior to later designs.

In particular, the later circular-arc design centres on a two-point contact construction between the carriage and the raceway. Quite different from the gothic-arch principle, it is capable of self-adjustment, high capacities and consistently low friction. As the design compensates for the elastic deformation of the balls, deviations in the installation’s surface can be absorbed ensuring smooth movement without excessive forces.

This design also significantly reduces differential slip, which equates to the differential between the circumferences of the inner and outer surfaces of the ball in contact with the raceway. Since the conventional gothic-arch design meant each ball was in contact with the raceway at four points, circumferential differences were generally larger, meaning increased slippage. The circular-arc design, with only two contact points in the loading direction, reduces differential slip, thereby increasing possible load ratings and creating a smoother rolling motion.

Throughout the 1980s, demand for increased automation and machine intelligence sparked the development of even greater levels of linear-motion accuracy. For instance, the circular arc design evolved by precision grinding the circular arc grooves to 0.51 times the ball’s diameter. This enabled far greater stiffness and capacity, together with improved damping and lower profiles.

The most recent generation of linear motion products continues to build on this circular-arc principal with a host of refinements that further extend the number of possible applications by significantly reducing costs, installation times and maintenance procedures.

Factors such as noise reduction and compliance with EU directives are now fuelling these new engineering solutions. As a result, near-silent guides have been developed, many incorporating THK’s patented Caged Ballä technology, which incorporates a flexible ball retainer that prevents ball-on-ball contact and therefore significantly reduces noise during operation.

This innovative concept ensures that the linear guide’s rolling elements are equally spaced and the system’s lubricating grease is retained between them. With each row of stainless steel balls angled at a 450 angle to the guide rail, the linear guide boasts equal capacity in all four directions, greater precision and reduced variations in rolling resistance.

Another positive aspect of this design is that the retainer provides a larger area of contact between the balls and the lubricant, enabling smooth, high-speed ball circulation with significantly reduced heat generation and virtually zero wear. The result is a rolling resistance variation equivalent to 1/10 of previous models and an extension of both performance characteristics approaching maintenance free specification.

The impact of modern linear motion designs has been apparent in a number of sectors; none more so than the continually expanding food-processing industry, where speed and accuracy are prerequisites. A good example is the use of the latest linear motion guides in Apple Engineering’s high-accuracy, multi-purpose filling and depositing machines. The introduction of the linear equipment has resulted in improved the cycle times and utility of Apple Engineering’s machines.

“We changed to Caged Ball linear guides because the previous units were unable to meet the levels of accuracy and cycle times demanded by the users of our machinery,” explained Steve Smith, Managing Director at Apple Engineering. “The new guides give excellent high-speed performance, are easy to install and exhibit excellent stiffness and longevity.”

Based in Hull, Apple Engineering has won international awards for its specialised processing equipment and exports throughout Europe, the US and Asia to organisations such as Birds Eye, Wall’s, Heinz foods, McVitie Group, Oscar Mayer, and Nestlé. It uses two types of linear motion guide to control the motion of its multi-head depositing units, which inject liquid and semi-solid ingredients into a variety of food products. By delivering steady, consistent motion, as well as minimising noise, the guides are able to meet the company’s very exact demands.

More recently, Apple Engineering has used Caged Ball linear motion guides on an injection system for an American confectionery manufacturer. The linear motion guides were ideal for this application due to their robust stainless steel construction. “The guides suit the type of manufacturing. They’re easy to install, even on fabricated surfaces and in tight spaces, which is ideal for our machines as they need to be robust and are often used in factories where space is at a premium. It is essential that specialised component parts, such as the LM guides, match our quality levels,” Steve Smith commented.

Recent breakthroughs
The advent of Caged Ball technology has brought with it heightened levels of operational accuracy, robustness and high-speed performance. However, by incorporating a non-contact mechanism that eliminates ball-to-ball impact, the Caged Ball design also offers the potential for near maintenance-free performance.

This is possible as each ball is held in a position equidistant to its neighbour by a retainer. A grease pocket is formed in the space between the ball and the retainer, ensuring that the ball is subjected to a continuous film of lubricant. This configuration not only reduces ball-to-ball contact and therefore ball wear, it also greatly extends maintenance intervals by reducing lubricant usage.

This is an extremely important feature of many modern linear motion systems as it has a direct impact on long-term maintenance costs, downtime and ultimately profits. Other system components and consumables, including specially formulated lubricants and contact scrapers, have been developed to improve reliability, but the circular-arc design and THK’s Caged Ballä technology have had the greatest impact.

Loaded applications
Another important consideration is load ratings or dynamic capacity. This is especially pertinent as it indicates how long linear motion products are able to carry loads effectively. Once again this has a significant impact on life-cycle costs and maintenance regimes.

Load ratings are determined through extensive testing on life-test machines, during which products undergo thorough dynamic load trials. These tests reveal how linear motion guides perform over extended periods and provide an indication of expected life-cycle maintenance requirements.

In today’s economic climate, load ratings are an increasingly important determinant for specifiers interested in total life-cycle costs - a trend recognised by THK who have invested heavily in life-test facilities. For instance, recent trials carried out by THK in Japan have helped the company establish that its Caged Ball system has a greater dynamic capacity than initially estimated.

Future directions
The continual evolution of linear motion technology has meant the number of practical applications for these systems has become increasingly diverse. Today’s market is flooded with choice, yet in the future there will undoubtedly be many more options to choose from.

Regardless of the variety of available linear motion guides, one factor will remain constant in spite of technological innovation and change. Reliability will continue to determine cost, productivity and profits, and innovations such as THK’s Caged Ball technology and the circular-arc design will ensure that it will remain an important feature of linear motion for years to come.