Interference Fits for Bearings, Pins & Bushings
A press fit (interference fit) is when the shaft is slightly larger than the hole, requiring force to assemble. The interference creates a clamping force between the parts — no fasteners needed. Getting the interference right is critical: too little and the part slips, too much and you crack the housing.
Common Fit Classes
| Fit Class | ANSI | ISO | Interference | Use Case |
|---|---|---|---|---|
| Loose Running | RC9 | H11/c11 | Large clearance | Free-running with dirt/debris tolerance |
| Free Running | RC7 | H9/d9 | Moderate clearance | Bearings, shafts that need to slide freely |
| Sliding | RC4 | H7/g6 | Small clearance | Precision slides, spigots, locating |
| Location Transition | LT | H7/k6 | Zero to slight interference | Locating pins, light press fits |
| Light Press | LN2 | H7/p6 | Light interference | Bushings, dowel pins, light-duty press fits |
| Medium Press | FN2 | H7/r6 | Medium interference | Bearings in housings, gears on shafts |
| Heavy Press | FN3 | H7/s6 | Heavy interference | Permanent assemblies, heavy-duty bearings |
| Force Fit | FN5 | H7/u6 | Maximum interference | Permanent — requires hydraulic press or heat |
Practical Interference Values
| Nominal Size Range (in) | Light Press (FN1) (in) | Medium Press (FN2) (in) | Heavy Press (FN3) (in) |
|---|---|---|---|
| 0.24 – 0.40 | +0.0002 to +0.0005 | +0.0004 to +0.0010 | +0.0006 to +0.0012 |
| 0.40 – 0.71 | +0.0002 to +0.0006 | +0.0005 to +0.0012 | +0.0007 to +0.0015 |
| 0.71 – 1.19 | +0.0003 to +0.0007 | +0.0006 to +0.0014 | +0.0009 to +0.0018 |
| 1.19 – 1.97 | +0.0003 to +0.0009 | +0.0008 to +0.0018 | +0.0012 to +0.0022 |
| 1.97 – 3.15 | +0.0004 to +0.0010 | +0.0010 to +0.0022 | +0.0015 to +0.0028 |
Bearing Press Fits — Quick Reference
Most ball bearing manufacturers (SKF, NSK, NTN) recommend these fits:
| Application | Shaft Fit (bore) | Housing Fit (OD) |
|---|---|---|
| Inner ring rotates (most common) | j5 or k5 (light interference) | H7 (clearance — slides in) |
| Outer ring rotates | g6 (clearance — slides on) | M7 or N7 (interference) |
| Both rings stationary | h6 (transition) | H7 (clearance) |
| Heavy loads or vibration | m5 or n6 (heavier interference) | P7 (interference) |
Assembly Methods
- Arbor press: For light press fits (FN1). Manual, controlled, good for small parts.
- Hydraulic press: For medium/heavy press fits (FN2–FN3). Monitor force to detect problems.
- Thermal expansion: Heat the housing (or cool the shaft with dry ice/LN2) to create clearance. Do not use a torch — uneven heating warps parts. Use an oven.
- Loctite retaining compound: Augments a light press fit with adhesive. Useful when interference alone isn’t enough but heavy press risks damage.
Critical Rules
- Never press a bearing through its balls. Apply force to the ring being press-fit. If pressing onto a shaft, push on the inner ring only.
- Chamfer the shaft and bore. A 15° lead-in chamfer makes assembly dramatically easier and prevents scoring.
- Monitor press force. A sudden spike means misalignment or bottoming out. A gradual increase is normal.
- Thin-walled housings reduce effective interference. The housing expands. Use FEA or published correction factors for wall thickness < 2× bearing OD.
Related: Tolerance Stack-Up Analysis | Surface Finish Guide | GD&T Reference