Shafts and Bearings: Understanding and Choosing the Right Fits

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

The ISO tolerance system standardizes how deviations from the nominal size are defined. Each tolerance designation consists of two parts:

• Tolerance field (letter): Defines the position (above or below nominal size). Capital letters (A–Z) for holes, lowercase letters (a–z) for shafts.
• Tolerance grade (number): Defines the width of the tolerance range. Grades 4–7 are fine, 8–12 are coarse.

Example: Ø 20 H7/h6

• H7: Hole with base position H (positive deviation) and tolerance grade 7
• h6: Shaft with base position h (negative deviation) and tolerance grade 6
• This results in a clearance fit with well-defined limits

The ISO system saves enormous administrative effort: instead of millions of different tolerances worldwide, there are standardized combinations that have proven themselves in practice.

Three Fit Types

1. Clearance Fit

The hole is always larger than the shaft. Clearance is guaranteed even in the worst case. Typical combinations: H7/g6, H7/h6, H8/h7. The shaft can rotate. Applications: rolling bearing outer rings, guide shafts, axles intended to rotate.

2. Transition Fit

The hole may be larger or smaller than the shaft depending on manufacturing tolerance. Either clearance or slight interference can result. Typical combinations: H7/k6, H7/j6, H7/m6. The condition is between clearance fit and interference fit. Applications: rolling bearing inner rings, couplings, gears with moderate retention requirements.

3. Interference Fit (Press Fit)

The shaft is always larger than the hole. A compressive stress is generated (form-fit). Typical combinations: H7/p6, H7/s6, H7/u6. The press fit creates friction and prevents rotation under load. Applications: gears with high torque, flywheels, pulleys, hub components with high retention requirements.

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-clamp hub) is an axially sliding hub with interference fit. By axial movement 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

Assembly Guidelines and Best Practices

Assembling Interference Fits

Interference fits can resist cold steel components with 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.