Shafts & Bearings: Choosing the Right Fits

A shaft fit uses ISO tolerance fields (e.g. H7/k6) per DIN ISO 286 to define whether bearing and shaft are joined with clearance, transition, or interference. Fits are the foundation of reliable machine systems: whether shafts in bearings, hubs on shafts, or housing components – the correct fit determines assemblability, service life, and operational reliability.

This tutorial explains the ISO tolerance system (DIN ISO 286), presents the different fit types, and gives practical recommendations for shafts, bearings, and hub connections. With this knowledge, you can correctly specify manufacturing drawings and avoid costly mistakes.

The ISO Tolerance System per DIN ISO 286

Every ISO tolerance designation combines a letter (the tolerance field, i.e. the position above or below nominal size – capital letters for holes, lowercase letters for shafts) with a number (the tolerance grade, i.e. the width of the tolerance band). For Ø 20 H7/h6, for example, this yields well-defined limit sizes and a reproducible clearance fit.

The full system of basic deviations and tolerance grades with µm values is beyond the scope of this assembly tutorial. For a detailed explanation of the tolerance fields including reference tables, see our companion guide on ISO fits H7/H6: the tolerance system.

Three Fit Types

ISO 286 distinguishes three fit types: the clearance fit (the hole is always larger than the shaft, e.g. H7/h6 – for rotating outer rings and guide shafts), the transition fit (clearance or slight interference depending on manufacturing, e.g. H7/k6 – for rolling bearing inner rings) and the interference fit (the shaft is always larger than the hole, e.g. H7/p6 – for gears and pulleys under high torque). Which combination you choose in this tutorial is shown in the following sections on shaft-hub and rolling bearing seats.

For a full derivation of the fit types with all standardized tolerance-field combinations, read our companion guide on ISO fits H7/H6: the tolerance system.

Shaft-Hub Connections

Parallel Key per DIN 6885

The parallel key is a simple, cost-effective connection method. It is milled into a slot in both shaft and hub and prevents relative rotation by form-fit. DIN 6885 standardizes sizes: width b, height h, and depth t. For shaft diameters 12–17 mm, 5×5 mm is typically required; for 20–25 mm, 6×6 mm is standard.

Advantages: Simple, cost-effective, standard. Disadvantages: The slot in the shaft weakens it by up to 20%; parallel keys cannot slide axially.

Spline Shaft per DIN 5480

Spline shafts are a more robust alternative to parallel keys. The shaft has multiple teeth (4, 6, or 10) that fit into corresponding slots in the hub. Torque is distributed over multiple contact flanks, not over one key.

Advantages: Higher load capacity, multiple contact flanks, allow axial sliding (important for sliding gearboxes). Disadvantages: More expensive to manufacture; also weaken the shaft, but less than parallel keys.

Clamping Set

A clamping set (or quick-release clamp hub) is an axially adjustable hub with an interference fit. By axial displacement the hub is clamped onto the shaft. This enables quick assembly and disassembly without pressing or heating. Clamping sets are expensive but valuable for frequently changed hubs.

Typical Fits for Rolling Bearings

Rolling bearings (per DIN 625 / ISO 15) have standardized fit recommendations in their technical documentation. Here are the most important combinations:

Inner Ring on the Shaft

• j5 / j6: Light transition fit. The inner ring sits lightly and may develop clearance under heavy load. Standard for stationary shafts or point load on the inner ring.
• k5 / k6: Transition fit with slight interference. The firmer seat prevents creep of the inner ring under circumferential load. Standard for rotating shafts (rotating inner ring load).
• m6: Moderate interference fit. Required for high loads and fast speeds.

Outer Ring in the Housing

• H7 / H8: Clearance fit. The outer ring sits loosely in the housing and can be easily replaced. Standard for stationary rings.
• G7: Clearance fit with more clearance than H7. Standard for floating bearings, where the outer ring must be slightly axially displaceable to compensate for thermal expansion.
• J7 / K7: Transition fit. Required for high radial loads or rotating load.

Rule of thumb: Rotating ring = interference fit (j/k/m), stationary ring = clearance fit (H/G). The exact values are defined in DIN 625 tables and depend on size, load, and speed.

Fits Reference Table with Recommendations

For gears and pinions manufactured to drawing, TEA includes fit specifications directly in the production documents — see Custom Gears to Drawing.

Numerical values: Limit deviations per ISO 286

The following limit deviations (lower/upper deviation in µm relative to the nominal size) apply to the common rolling bearing fits per ISO 286:

k6 produces a light transition/interference fit at the rolling bearing inner ring to prevent creep; n6 creates a firmer interference for shocking or high circumferential loads. The tolerance of the bearing bore itself follows ISO 492 (tighter than a standard bore per ISO 286) — as a result the effective fit k6/bearing bore is tighter than k6/H7. All values are reference figures from ISO 286; the binding standard and the bearing manufacturer's specifications take precedence.

The notch effect of the keyway: strength per DIN 743

The statement “a keyway weakens the shaft” can be quantified. DIN 743 (“Calculation of load capacity of shafts and axles”) captures the local stress increase at notches via the notch effect factor β. The keyway is one of the most critical cross-sections of a shaft, because it introduces a sharp-edged longitudinal slot precisely where the torque is transmitted.

Stress concentration factor, notch effect factor and notch sensitivity

The stress concentration factor α describes the purely geometric stress increase (maximum stress at the notch root relative to the nominal stress) for purely elastic behaviour. The notch effect factor β, which governs fatigue strength, is always smaller than α per DIN 743, because real materials only partly “feel” the stress peak:

The supporting factor n (also called support factor) accounts for the relative stress gradient at the notch root: at a sharp notch the stress drops steeply, so only a small material volume is highly loaded – the material “supports” itself. n increases with a sharper notch and with lower material strength. The reduced notch effect factor β of the keyway lowers the fatigue strength of the component compared with the smooth specimen:

For the keyway per DIN 6885 the notch effect factor for bending is typically in the range β​_b ≈ 1.6 … 2.4 and for torsion around β​_t ≈ 1.3 … 1.8 – depending on shaft diameter, slot form (side-milled vs. end-milled slot) and material tensile strength R​_m. Higher-strength steels react more sensitively (higher β), because their supporting effect is smaller.

Reference values for the notch effect factor of keyways

The values are orientation figures in the order of magnitude of the diagrams from DIN 743-2 or Roloff/Matek and the FKM guideline context; the binding figure is the standard-compliant individual calculation using the diagrams provided there for the stress concentration factor and supporting factor.

Worked example: permissible torque of a shaft with a keyway

A shaft ⌀ 40 mm of quenched-and-tempered steel (e.g. C45E, torsional fatigue strength of the smooth specimen τ​_tW ≈ 150 N/mm²) carries a keyway per DIN 6885 with β​_t = 1.6. We seek the permissible alternating shear stress in the slot cross-section (simplified, without further influence factors such as surface or size factor):

With the polar section modulus of the solid shaft W​_t ≈ 12,566 mm³, the fatigue-resistant permissible alternating torsional moment (without safety factor) is:

Without a notch (β​_t = 1) the same cross-section would be at τ​_tW = 150 N/mm² and thus T ≈ 1,885 Nm. The keyway therefore lowers the permissible alternating torsional load to roughly 1 / 1.6 ≈ 62.5 % – a reduction of about 37 %. This shows: for dynamically highly loaded shafts it is worth moving to a spline or serration connection or to an interference fit, which transmits the torque without a sharp longitudinal slot.

Note on the scope of validity: the calculation is deliberately simplified and serves to indicate the order of magnitude. A complete design per DIN 743 additionally takes into account the size influence factor, the surface and surface-layer factor, the mean stress and the required safety against fatigue failure.

Assembly Guidelines and Best Practices

Assembling Interference Fits

Cold steel interference fits can resist press forces up to approx. 50 kN. For higher torques: hydraulic or thermal assembly. Thermal assembly: heat the hub to 100–150 °C, slide it onto the cold shaft – as it cools the interference fit forms automatically. This is gentler and more reliable.

Adjusting Clearance Fits

Clearance fits (e.g., H7/h6) should be adjusted before operation. Radial clearance in bearings can be set using locknuts or locking washers. This optimizes service life and reduces noise.

Surface Finish Considerations

Rough surfaces can weaken interference fits. After fine machining, surfaces should have Ra < 0.8 µm. Special coatings (e.g., nickel, chrome) can alter the fit – always clarify with the manufacturer.

Inspection and Testing

After assembly, fits should be checked with plug gauges or calipers. For critical applications: X-ray or ultrasound testing to detect internal stresses.