38 n LINEAR MOTION May 2025 www.drivesncontrols.com How to optimise the costs of profile rails The main costs when choosing pro le rails are determined by the motion pro le, but there are areas where there is more leeway to achieve trade-o s. Choosing ball track assemblies can make a huge di erence to the cost. Other factors include materials, maintenance, standards compliance, and training. They o er opportunities for cutting costs, albeit with varying degrees of inuence on upfront versus long-term expenses. How a machine designer positions loadcarrying and contacting elements in relation to each other a ects costs signi cantly. The three most common options are: doublefaced arrangements with spherical balls; double-backed arrangements with spherical balls; and double-backed arrangements with rollers. A double-faced ball-track arrangement is the least expensive. It arranges ball tracks with 45-degree contact angles, resulting in an X pattern (Fig. 1). This con guration optimises the distribution of the load across all of the rolling elements, allowing force vectors to converge close to the centre of rotation. Centralising the force vectors in this way makes the double-faced arrangement much more tolerant of misalignments and mounting surface imperfections, reducing installation and mounting costs. Greater tolerance for imperfection may also allow simpler mounting, manual alignment of single rails, and use of standard components, reducing costs further. Lowerprecision options may, however, result in higher maintenance costs and reduced longterm durability. The X-type arrangement is ideal for automation applications that require less accuracy on assembly height, and width tolerances between ±40μm and ±100μm – such as packaging equipment, foodprocessing machinery and medical sample handling. Double-backed rails All else being equal, double-backed pro le rails have a higher resistance to moment loads. This is because the O-type arrangement of vectors resisting the moment load is further from the centre of rotation (Fig. 2), giving them extremely high rigidity and accuracy. However, the O-type arrangement cannot tolerate misalignments or surface imperfections. Inadequate surface preparation will make the guide run rough and subject it to more frequent replacement. Even tiny atness errors can halve bearing lives, and more severe alignment issues can cause immediate failure. These potential aws result in installation costs which can be several times higher than those of doublefaced bearing track architectures. Mounting surfaces typically need to be 15-20 times more precise than double-faced versions. Getting the necessary precision may require specialised equipment, stang, customisation or other elaborate procedures, adding to both time and costs. Potential applications are those that need more accuracy on assembly height, and width tolerances of between ±5μm and ±50μm – such as in industrial automation, machine tools, precision measuring equipment and industrial robots. When applications require maximum rigidity, designers may opt for rollers over balls. Rollers provide more stability and greater load capacities with minimal rolling friction, thanks to their larger contact surfaces. However, pro le rails with rollers come at a higher cost and intolerance for installation variances, making them coste ective only for the most demanding applications in terms of accuracy and moment load capacity. These include machine tools, precision measuring, and robots requiring the highest possible accuracy on assembly height and width tolerances, of between ±5μm and ±20μm. Material choices While the rail track architectures have the greatest impact on costs, other choices a ecting project budgets that are within the designer’s control include material selection, maintenance, standards compliance, and training. Material selection can be critical in cost management, inuencing both the initial and operational costs of pro le rails. Aluminium is the lowest cost option, but is best for lower accuracy applications. Fig. 1: The X-type arrangement uses double-faced ball tracks with 45-degree contact angles. This conguration optimises the distribution of loads across all of the rolling elements. Fig. 2: The O-type arrangement has a double-backed prole rail further from the centre of rotation for a higher resistance to moment loads. However, this conguration cannot tolerate misalignments or surface imperfections. Machine designers needing precision linear guides usually have to specify high-cost prole rails, but diligent analysis can reduce those costs. Too often, designers do not have the time to optimise rail functions for their applications and end up paying a premium. But, as this article from Thomson Industries explains, focusing on key elements can help to reduce costs considerably.
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