Round Rail vs Square Rail

  • Round Rail vs Square Rail

    In the universe of machine tooling there are different shaped rails. Is there one shape that’s better than the other? What are the advantages to round rail and what are the advantages to square and where are the different shapes best used? To better answer that question, let’s look more closely at round vs square.

    Round rails are used in lab automation, printing, assembly and inspection stations, welding and wash down. There are definitely advantages to the round shaped rails. Bearings can rotate around the axis, allowing for slightly different elevation. The round rails can be self-aligning and can shift up to a half degree which can fix up to a 25um parallelism error. They also can be end supported, allowing gap spanning of 12-24x the shaft diameter.

    The original invention of the square rail was for machine tool. They have high precision rates, can handle heavy loads and high rigidity. Square rails have more surface contact which equates to higher load capacity for similar size. They can hold tolerances between .0002”- .001” over 10 feet compared to .01” on round.

    Some other considerations when comparing the two technologies:

    Quiet – Plain bearing technology along with round is usually the best choice

    Impact (shock) loads – Many applications will have a shock load, sometimes customers will not be aware of this.  Plain bearings have the greatest surface contact, and handle this well. Generally speaking when expecting a shock load, size for 3x the expected load.

    High load applications – Square technology becomes 5x stiffer when properly loaded, compared to when under loaded.  (common mistake)

    Smoothness – Generally speaking round technology will have a smoother feel to it


    What we found at the end of the day, there is no best choice for everything. For some applications multiple products could work, for others the choice will be more definite. What do you think? Is there one that works best for you?