Rolling Element Bearing Fits: Circumferential Load vs. Point Load
The fit is determined by the direction of the load relative to the ring—not solely by whether the shaft is rotating or stationary: For circumferential loads (where the load rotates relative to the ring—the typical case on the inner ring when the shaft is rotating), the ring requires a tight fit with an interference fit, e.g., k5/k6 or m6 for high loads and high speeds, to prevent it from moving along the shaft. For point loads (where the load direction remains stationary relative to the ring, such as on a stationary ring), a loose fit is sufficient, e.g., g6 or h6 (j6 if higher concentricity is required). The opposite applies to the outer ring in the housing: H7 or H8 (loose fit, standard for stationary, easily replaceable rings under point load) and G7 (more clearance, for loose bearings that must compensate for axial thermal expansion); if the outer ring is subjected to a circumferential load, it also requires a tight fit.
Rule of thumb: Circumferential load = tight fit with interference; point load = loose fit.
Shaft-Hub Connections: Key or Splined Shaft
The key (DIN 6885) is a rectangular driver that fits into grooves in the shaft and hub and transmits rotation via a positive connection—simple and cost-effective, but it weakens the shaft by up to 20%; when used as a sliding key with a free fit (H9/D10), axial displacement is also possible for small torques. The width depends on the shaft diameter (rough rule of thumb: width ≈ 0.3 · d; the DIN 6885-1 standard table is the definitive reference).
The straight-sided spline shaft (DIN ISO 14, with straight flanks, 6, 8, or 10 splines) or the involute spline shaft (DIN 5480, interference fit with involute flanks, 6 to 82 teeth) distributes the torque across multiple flanks rather than a single groove—offering higher load-carrying capacity and allowing for additional axial displacement (important for manual transmissions)—but is more expensive to manufacture. A rough rule of thumb for small to medium-sized shafts: for torques up to approximately 500 Nm, a keyway is sufficient; for higher torques, a straight-sided spline shaft is required—the definitive verification is performed in accordance with DIN 6892.
Reading Standard Parts
Standard designations are self-explanatory once you understand their structure: Retaining ring DIN 471 – 20 × 1.2 is a retaining ring for shafts (DIN 472 would be the bore variant) with a shaft diameter of 20 mm and a ring thickness of 1.2 mm—it snaps into a groove and prevents axial displacement. An M10 machine foot with a load capacity of 50 kN (manufacturer’s specification; no DIN standard designation) dampens vibrations under machine housings.
Mnemonic
Example: Reading a standard designation
DIN 6885-1 Key 6 × 6 × 18
- DIN 6885-1: Standard for keyways, Part 1
- 6: Width b = 6 mm
- 6: Height h = 6 mm
- 18: Length l = 18 mm
This size (6 × 6 mm) is intended for shaft diameters of >17–22 mm, in accordance with DIN 6885-1.
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