Above the line performance

Bob Love, Technical Manager at THK, looks at the driving forces behind the use of linear motion guides in specialised applications such as medical equipment and food processing.

Linear motion (LM) technology is used extensively in many different mainstream manufacturing and process applications as a primary method of moving and positioning workpieces, tools and other components.

Linear motion guides are now widely available and are generally acknowledged as providing fast, accurate and reliable control in many different and often demanding industrial environments.

Increasingly, however, the use of linear motion technology is spreading to other more specialised sectors of industry.

For example, guides are being used for pick and place systems in electronics production; in clean rooms for controlling the movement and positioning of laboratory test, inspection and mixing equipment; in medical systems, for rotating scanners; and in fully automated, high speed sterile food and pharmaceutical lines for ingredient delivery and filling.

This gradual change in use has both been driven by, and subsequently led to, a number of technical developments, designed to improve still further the performance and life of linear motion guides in areas where conditions can be particularly demanding, in terms of both the operating environment and the ability to provide an effective return on investment in the relevant hardware.

Although operating environments can vary widely from application to application, there are a number of requirements that are common to all; in particular, system reliability, low or zero maintenance and the need for smooth, precise motion that can easily be controlled, often to accuracies of a micron or less.

Although reliability, long operating life and low maintenance are requirements in most industries, these factors can be especially important in applications where linear motion guides are used in sealed systems or clean room environments, or where they have to be fitted in areas that are difficult to access.

The latest generation of linear motion guides, such as those manufactured by THK, which use Caged Ball technology, typically operate for up to 20,000km on a single lubrication.

This operating cycle without maintenance is considerably greater than that achievable from early guides or those using traditional ball race configurations, which are generally restricted to around 100km.

The increased life of Caged Ball guides depends on the use of high precision stainless steel balls secured within a sealed cage, with controlled density grease pockets around the complete circumference of each ball.

This effectively prevents contact between balls when in operation, giving low rolling resistance, with the added advantage that noise, friction, heat and wear are virtually eliminated, while movement is extremely smooth.

The elimination of heat, and especially noise, while being able to guarantee smooth motion at both high and low speeds, together with the ability to offer precise location, makes Caged Ball guides ideal for use in medical equipment such as state-of-the-art radiographic, MRI or CT scanners.

Thus, couches, gantries and relatively heavy scanning assemblies can easily be moved with great positional accuracy and consistency, without affecting the comfort of patients.

Smooth, controllable motion is also a requirement in other applications, including the clean room manufacture of semiconductors and LCDs.

Of equal importance, however, is the ability for the load-carrying carriage in a linear guide to move at relatively high speeds and to stop consistently in exact positions, to submicron accuracy, while also ensuring that the potential for dust and other debris, caused by the continuous motion of rotating and sliding parts, is eliminated.

The significantly reduced metal to metal contact, together with more precise manufacturing techniques, in the latest Caged Ball guides can reduce the accumulation of dust by 90%, when compared with conventional linear guides.

In practice, however, it is difficult to seal the rolling section of a linear guide completely without adversely affecting the performance of the device; there will, therefore, always be the potential for grease dispersion and gas emission through oil vaporisation.

Traditionally, fluorine based greases have been used to overcome this problem.

Although these have very low base-oil vapour pressures, are stable against heat and oxidisation and produce minimal levels of dust, they are extremely viscous with a high rolling resistance; this can in time have a detrimental impact on the operating characteristics of the guide.

A better option is a new generation of mineral and synthetic oil based greases that offer the low dust generating benefits of the earlier products but with improved resistance, torque, durability and anticorrosion properties.

Perhaps as importantly, they are easier to handle and are often less expensive.

Clean room applications can also require that equipment operates in a full or partial vacuum.

This can present a challenge to the use of linear guides as the oil in conventional lubricating mineral oil based greases will evaporate, thereby reducing the lubrication properties of the grease and contaminating surrounding areas.

Again, the solution is to use greases containing either a base oil with a low vapour pressure or inactive fluorine based greases.

Finally, applications such as food processing often require either low or high operating temperatures, that fall outside the normal specification for linear guides.

The latest guides are now available with special seals, greases, end plates and retainers that are capable of withstanding extreme temperatures, up to 150C, with heat treatments being used to overcome problems associated with thermal expansion and contraction.

The latest generation of linear motion guides is extending the use and advantages of the technology into an increasingly wide range of applications.

Continued development in manufacturing techniques will inevitably see this process continue, bringing further performance enhancements while simultaneously reducing operating and capital costs.

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