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System rigidity

System rigidity in machine tool applications is extremely important because deflection under load has a major impact on machining accuracy. Bearing stiffness is only one factor that influences system rigidity. Others include:
  • shaft stiffness
  • tool overhang
  • housing stiffness
  • number and position of bearings and the influence of fits
Some general guidelines for designing high-speed precision applications include:
  • Select the largest possible shaft diameter.
  • Minimize the distance between the tool end bearing position and the spindle nose.
  • Keep the distance between the two bearing sets short (fig. 1). A guideline for the spacing is:

    l ≈ 3 … 3,5 d

    l=distance between the first tool end bearing row and the rearmost non-tool end bearing row
    d=tool end bearing bore diameter
Diagram 1 provides an overview about the relative stiffness of different bearing arrangements. For details about the bearing series, refer to Designation system in the relevant product section. The comparison is based on preloaded bearings with 100 mm bore on the tool end and 90 mm bore on the non-tool end. These guideline values cannot substitute for precise system rigidity calculations. For advanced system analysis, contact the SKF application engineering service.
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