Drives & Controls Magazine May 2023

40 n LINEAR MOTION May 2023 How to optimise round-rail performance and costs When designing linear motion systems, the initial decision between round or square rails can impact cost, performance and durability significantly. Square rails, which are typically more expensive , have a higher load capacity and tighter accuracy. Round rails offer distinct cost and reliability advantages for certain applications, but maximising those advantages depends on specifying the right bearings to minimise friction. Designers must often choose between plain bearings (also called bushings) and ball bearings. Within each type, there are more options that impact price, performance, maintainability and durability. Understanding these options early in the design cycle will help to optimise the performance and costs of linear motion applications. Because plain bearings have no balls or other rolling elements, they cost less. They also out-perform assemblies with rolling elements in terms of smoothness and quietness and, because they have fewer moving parts, they are less susceptible to dust and dirt. Plain bearings with elastomer liners amplify these benefits. These are available with highperformance, bearing-grade materials applied to aluminium or other substrates to provide good load capacities (20% those of a ball bearing) and a reasonably low friction coefficients – typically 0.05–0.25. Because the contact area of a plain bearing is distributed over a large area, these bearings can also be used with unhardened round shafts, giving desgners more flexibility to choose materials for other requirements – such as corrosion protection or weight. Some manufacturers make plain bearings from self-lubricating polymers that minimise the need for maintenance. The cost savings and smooth operation of plain bearings, however, come at the cost of load handling, precision and speed. They are best for applications in which alignment and moment load are non-critical, or those that require shafts that are incompatible with traditional ball bushings, such as aluminium or 316 stainless steel. Plain bearings typically handle static loads up to 15kNm, and dynamic loads up to 2.75kNm, and speeds of up to 0.5m/s. When those parameters are met, plain bearings – especially those using polymers – are beneficial in both clean and harsh environments. The main applications, involving light loads, lower speeds and precision, are found in packaging, factory automation, robotics, woodworking, food and beverage, and vending machinery. Applications that might involve ultra-clean, toxic or harsh environments include semiconductor production, medical automation and food production. Scaling up Applications needing higher load-handling, speed or precision will need bearings with internal rolling elements. The type of bearing specified will vary according to the number of balls used, directly impacting the allowable load and speed. Precision steel designs use hardened steel linear ball bearings, which have point-to-point contact on a fixed point. They can handle dynamic loads up to 6.8kNm nd diameters from 6.3–100mm. The next performance level up embeds a single ball track in a conforming groove on a floating plate. These bearings have a low coefficient of friction and can triple the loadhandling capability over fixed-plate versions and increase bearing lives up to 27 times longer than precision bearings. This configuration is optimal for rails from 4.8–50mm in diameter and dynamic loads up to 4kNm. Eduard Schweinfort, manager of linear bearing and guide products at Thomson Linear Motion, offers advice on which type of bearing to choose for round-rail linear guidance systems. With no balls or rolling elements, plain bearings offer a smooth, quiet and economical option for light-load linear motion applications. All three linear bearings shown use recirculating balls, but the one on the left has a hardened sleeve with a bearing surface ground into it, while the other two use floating bearing plates.