hen to comes to specifying a high-precision linear guide, machine designers often have little choice
except higher-cost profile rail. Faced with project deadlines, they may not have the time to conduct the necessary analysis to map rail functionalities to their application needs and can end up paying a premium for impractical features. Added to this, the larger the production run, the greater the potential problems. Within this component’s realm, however, diligent analysis can keep those costs to a minimum.
The primary cost factors in profile rail selection are essentially prescribed by the motion profile, but there are other areas where the design has more leeway to manage trade-offs. The selection of ball track assemblies can make a huge difference in the cost. Other factors – including material selection, maintenance, standards compliance, and training – offer additional opportunities for cost reduction, albeit with varying degrees of influence on upfront versus long-term expenses.
How the machine designer wants to position load-carrying and contacting elements in relation to each other significantly impacts cost. The three most common options are a double-faced arrangement with spherical balls, a double- backed arrangement with spherical balls and a double-backed arrangement with rollers.
• Double-faced ball tracks A double-faced ball-track arrangement is the least expensive. It arranges ball tracks with contact angles at 45 degrees, causing an X pattern (Figure 1). The X-type configuration optimises the distribution of load across all the rolling elements, enabling convergence of force vectors close to the centre of rotation. Centralising the force vectors in this way makes the double-faced arrangement much more tolerant of misalignment and mounting surface imperfection, significantly reducing installation and mounting costs. Greater tolerance for imperfection may also
allow simpler mounting, manual alignment of single rails, and use of standard components, further reducing costs. Lower-precision options may, however, contribute to higher maintenance
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costs and less durability in the future. As such, the X-type arrangement is ideal for
automation applications requiring less accuracy on assembly height and width tolerances, between ±40µm and ±100µm, such as packaging equipment, food processing equipment and medical sample handling.
• Double-backed profile rails Double-backed profile 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 (Figure 2), giving them extremely high rigidity and accuracy. The O-type arrangement, however, cannot tolerate misalignment or surface imperfection. Inadequate surface preparation will make the guide run rough and
subject it to more frequent replacement. Even tiny flatness errors can cut reduced bearing life in half, and more severe alignment issues can result in immediate failure. These potential flaws result in a higher cost of installation, which can be several times the cost of a double-faced bearing track architecture. Typical mounting surface requirements require 15-20 times more precision than the double-faced counterparts. Getting the necessary precision may require specialised equipment, staffing, customisation or other elaborate procedures, adding to both time and financial investment. Potential applications are those that require greater accuracy on assembly height and width tolerances, between ±5µm and ±50µm, such as in industrial automation, machine tool equipment, precision measuring equipment and industrial robots.
Figure 1. The X-type arrangement features a double-faced ball track with contact angles at 45 degrees. This configuration optimises the distribution of load across all the rolling elements
Figure 2. The O-type arrangement features double-backed profile rail further
from the centre of rotation for a higher resistance to moment loads. This
configuration, however, cannot tolerate misalignment or surface imperfection
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