MACHINE-BUILDING n with acceleration to 50m/s2 – about the same as you would get from rollers instead of balls. Like profile rails, round rail systems consist of a shaft, at least one pillow block (containing ball bushing bearings) and end supports (Fig 2). A key difference is that round rail is more prone to inaccuracies due to deflection or misalignment. This is addressed by using either end-supported or fully supported configurations and selfaligning bearings. Round-rail systems are simpler and less expensive to produce than profile rail systems. Designers often specify them when cost control is top priority, and accuracy and high load-handling are not. Speed is largely a function of whether they use standard ball bushings, self-lubricating ball bearings or rollers. Round-rail ball bushing assemblies are limited in speed by their recirculation system. Their maximum speed is 3m/s with acceleration up to 50m/s2, making their speed comparable to profile rail using rollers or caged balls. Round-rail system rollers are convex and chain-driven (Fig 3) and can achieve much higher speeds. This roller assembly does not require an end-cap, which gives the rollers more freedom to recirculate at higher speeds. Travel speeds of up to 31m/s with acceleration up to 138m/s2 are possible with high load capacities comparable to profile rails. Standard or self-lubricating bushings? Designers looking to achieve smooth operation in harsh environmental conditions might use round-rail systems with selflubricating bushings. These need no maintenance and can be used at speeds up to 43m/s. Of course, your final linear rail guide selection will depend on a variety of motion parameters, such as load size, envelope and environmental factors. But when it comes to speed, the first question to ask is whether you will be designing your solution for use in a challenging environment. If the answer is yes, and speed is a priority, start by evaluating a round-rail system with chaindriven, convex rollers. If speed is not your top priority, look at round-rail systems with selflubricating ball bushings. If meeting challenging environmental conditions is not a top priority, but speed, accuracy and load-handling are, you should evaluate profile rail systems with ball bearings against your motion parameters. If you want high accuracy and load handling along with smoother operation, then look at a caged-ball configuration, keeping in mind that you will lose any speed advantage from the profile ball recirculation technology. If high load handling and accuracy are less important than the initial costs of your chosen rail, and you are comfortable with speeds up to 3m/s, you will find many options among round-rail systems using standard ball bushings. Taking the time to consider speed-related factors that impact your applications will help ensure your loads reach their destinations on time. n Fig 2. A round-rail system consists of a shaft, at least one pillow block, and end-supports. Image: Thomson Industries Fig 3. Thomson’s RoundWay linear roller bearings provide smooth, accurate and highspeed operation in high-load, heavy-duty applications. Travel speeds can reach 31m/s, with acceleration up to 138m/s2. Image: Thomson Industries
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