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FEATURE BEARINGS & LINEAR MOTION Round oR PRoFile? WhiCh linea


Charles Isaac, Thomson Industries, and Ian Miller, Motion, look at how to choose the right linear guide for your application and environment


D


esigners of linear motion systems often find themselves weighing up the


pros and cons of round vs. profile (‘square’) linear guides. Many considerations come into play while trying to determine which technology to implement, but this is only further compounded when environmental extremes are a concern. Too often, the choice of linear guide technology is made late in the design process and based on assumptions. Taking time to perform a nuanced review can make the difference between a simple design and a complex one, affecting performance, assembly time and total cost. Round and profile rails are often


interchangeable functionally, but there are marked differences even within those categories. Making the right choice for your application can result in significant advantages in cost, performance and durability. This article discusses how those differences play out in applications complicated by environmental extremes. Issues of fluid/solid particulate egress, temperature extremes, corrosion, shock and vibration design considerations will all be discussed. It is important to remember the pros and


cons of both round and profile linear guides in an undemanding environment. Understanding these technologies and how they can be best applied to an application is always the first step in good design practice.


Round Rails Round rail has been around for about 80 years. Typically the most economical option (relative to component cost), these have a lower load capacity and looser accuracy. While the design of the


this strictness can be either a great benefit or a complicating factor for your design. The nature of this technology lends itself well to applications where extremely tight levels of precision are required, or where excessive loading is inherent to the task at hand. The compact nature of the system can help to reduce the overall size of the equipment, but higher preloads could increase in drag. Further, the nature of the profile rail mounting and installation process, including surface machining, makes it very unforgiving of poor parallelism or variation in rail height for systems that use more than one rail. Manufacturing systems using profile rails require higher precision. Their elaborate, albeit compact, profile also suffers from more complex sealing and comparably poorer contamination resilience.


round rail is very forgiving, this can be a strength or a weakness depending on your application. For applications where loose tolerances are


acceptable or where low component costs are more critical than tight tolerances, round rails are an excellent option. They are self-aligning and very forgiving of issues that arise from poor parallelism or variation in rail height for systems utilising more than one rail. This technology provides a smooth and low drag platform, and the simplistic profile boosts highly effective sealing and a natural contamination resilience. In addition, round rails do not require the time and expense of mounting surface machining, and they are the only option for end-supported applications.


PRoFile (aKa ‘squaRe’ Rails) Profile rail is the newer of the two technologies and has been around for about 40 years. Typically the more expensive of the two options, it has a higher load capacity and much tighter accuracy. The design of the profile rail is not forgiving, and


Fluid ConsideRations Exposure to fluids such as water, fog and high humidity can lead to corrosion of either rail type. Under pressure, as in washdown situations, fluids can wash lubrication away. Exposure to corrosive fluids such as chemical sprays, fuel oils, acidic food products, hydrocarbons, urea and fertilisers can introduce safety threats as well as contribute to wear, downtime and performance degradation. Both round and profile guides will differ in their fluid-handling abilities based on sealing, lubrication type, material selection, coatings, mounting and standards compliance, which is particularly important in the medical and food industries. For both round and profile guides, full-


contact seals and proper lubrication will prevent rust from developing on the raceways of steel components. Seals on round guides tend to cause less drag due to their simple circular geometry, but low-drag options are available for profile rail assemblies. Profile rail carriages may also employ longitudinal seals that protect against ingress from the bottom of the assembly. Material selection is another key component


of the fluid handling ability of a system. Round rails have an advantage in that they can use non-hardened, stainless-steel shafting. This makes them attractive for clean environments such as those found in the semiconductor and


Commonly used for hydraulic test benches, round rails position hydraulic pumps/motors for testing (Images courtesy of Motion) 2 DESIGN SOLUTIONS SEPTEMBER 2026 8 www.designsolutionsmag.co.uk


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