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LINEAR MOTION FEATURE A wheel- guided unit


SLIDING into action


When looking for a linear motion solution for an industrial machine that will increase


efficiency and productivity, then why not consider linear wheel-guided slides? Anders Trygg, product line manager, EU Systems, at Thomson Industries, examines their benefits


I


f a sluggish linear axis slows down an industrial machine, its cycle time


and therefore its output, why not simply speed it up? The need for speed, however, usually


has to be balanced against travel length, accuracy and load requirements. Striking this balance can be particularly difficult with large-format production and packaging machines that have heavy payloads and need to operate quickly to meet cycle time goals. A solution to this can be found with


wheel-guided linear slides. In the right application, these offer the best balance of speed, load carrying capacity and travel length. Recently developed models can also offer improved positioning accuracy, opening up new application possibilities. Wheel-guided slides have a simple operating principle: Driven by a belt, the carriage rides on a set of wheels that run within a precision guideway along the length of the guide. Compared to other guides based on other types of rolling elements, the wheel guides can reach higher operating speeds and accelerations. They can also accommodate high dynamic and moment loads. Some, for example, are capable of speeds up to 10m/s and accelerations of up to 40m/s2


less than 5m/s by the recirculation speed of the balls. Furthermore, the most robust wheel guides can handle dynamic loads in travel direction of up to 5,000N and moment loads of up to 930N-m. A further benefit is that wheel guides tend to offer longer stroke lengths, with versions offering up to 11m of travel available off-the-shelf. Despite being less accurate or repeatable than a high-performance ball guide, wheel guide accuracy has not proven to be much of an issue in practice, with good versions easily achieve positioning accuracy under 1mm, more than enough for most of


the applications they target. New models with precision guideways and preloaded rollers, however, offer a positioning accuracy of ±0.05mm with solid repeatability.


WHEEL GUIDE BENEFITS Although they have been around for decades, many engineers rule out belt-driven wheel guides in the belief that their application requires a screw- or belt-driven ball guide – and sometimes they are right. There are many applications that need every iota of accuracy, repeatability and stiffness that a screw- or belt-driven ball guide can deliver. There are, however, a large class


of linear motion applications in which a ball guide strikes the wrong balance of technical features. Applications that share one or more of the following characteristics can favour wheel guides instead: • Machine productivity increases with linear axis speed


• Stroke requirements exceed a few meters


• Dynamic and moment loads are high • Accuracy requirements are moderate. Not only do filling, dispensing and


— ball guides are limited to


packaging machines fall into this category, but so do food processing machines, factory automation, material


handling, printing and scanning systems. In fact, in the right applications, wheel guides can provide a cost-effective solution.


PRISM SLIDES Another popular belt-driven linear axis uses prism slides to support and guide the carriage. Made from an engineering polymer, the prism slides run along the linear unit’s extrusion rather than a bearing. The elimination of a bearing reduces cost, especially as stroke length increases and, like wheel guides, prism guides can extend to strokes of 11m. Although prism-guided units don’t


have all the capabilities of a wheel- guided unit – in terms of speeds, loads or accuracy – they do offer advantages including low noise, shock load tolerance and contamination resistance. If an application doesn’t need all


the capabilities of a wheel guide or in applications with contamination or shock loads, prism guides can offer the lowest total installed cost of any type of guide. However, the steel reinforced ATL belt


that drives Thomson’s wheel guides can be replaced or re-tensioned externally, saving both time and expense.


Thomson Industries www.thomsonindustries.com Enter 211


Belt drive wheel guide unit


/ DESIGNSOLUTIONS DESIGN SOLUTIONS | JUNE 2014 21


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