# Fit Selection Matrix — Function to Coupling

## Clearance Fits

### Loose clearance — free movement
| Fit | Typical application | Indicative clearance (Ø50) |
|---|---|---|
| H11/c11 | Articulated joints, cranks, very loose couplings | 0.12 – 0.45 mm |
| H9/d9 | Connecting rods, flanges with generous clearance | 0.06 – 0.20 mm |
| H8/e8 | Long shafts, joints with misalignment | 0.04 – 0.11 mm |

### Functional clearance — sliding with lubrication
| Fit | Typical application | Indicative clearance (Ø50) |
|---|---|---|
| H8/f7 | Sliding bushings, guides, pins | 0.025 – 0.09 mm |
| H7/f7 | Standard sliding fits | 0.020 – 0.07 mm |
| H7/g6 | Precision fits, rotating pins | 0.010 – 0.05 mm |
| H6/g5 | Spindles, high-precision shafts | 0.004 – 0.025 mm |

### Small clearance — centering
| Fit | Typical application | Indicative clearance (Ø50) |
|---|---|---|
| H7/h6 | Positioning, removable centering | 0 – 0.025 mm |
| H6/h5 | High-precision positioning | 0 – 0.013 mm |

---

## Transition Fits

May result in slight clearance or slight interference — the statistical distribution of
manufacturing determines the actual type on each individual part.

| Fit | Typical application | Clearance/Interference (Ø50) |
|---|---|---|
| H7/js6 | Removable pins, loose pulleys | ±0.008 mm |
| H7/k6 | Bushings, rings, removable keys | −0.002 … +0.018 mm |
| H7/m6 | Sleeves, hot-removable gears | −0.009 … +0.011 mm |
| H6/k5 | High precision, precision gears | −0.002 … +0.009 mm |

**Practical rule**: k6 is press-fit and removed with a puller. m6 requires a heavier press
or heating. Both are "removable" but require tooling.

---

## Interference Fits

⚠️ Before specifying an interference, **always calculate**:
1. Contact pressure at the joint
2. Hoop stress in the hub
3. Radial stress in the shaft (if hollow)
4. Transmissible torque by friction (verify sufficiency)
5. Assembly temperature if thermal

→ Use `fundamental-deviations.md` for exact values, then apply Lamé equations.

| Fit | Typical application | Indicative interference (Ø50) | Assembly |
|---|---|---|---|
| H7/n6 | Fixed bushings, sealing rings, bronze bearings | 0 … 0.033 mm | Press (up to 10 kN on Ø50) |
| H7/p6 | Gears on shaft, fixed pulleys, hubs | 0.009 … 0.042 mm | Press or thermal |
| H7/r6 | Medium-torque transmission hubs | 0.020 … 0.055 mm | Thermal (Δt = 80–150°C) |
| H7/s6 | High-torque transmission | 0.031 … 0.066 mm | Thermal (Δt = 150–250°C) |
| H7/u6 | Bronze/cast-iron bushings, maximum interference | 0.053 … 0.094 mm | Thermal (Δt ≥ 250°C) |
| H6/p5 | Inner rings of P5/P4 precision bearings | — | Thermal |

### Interference verification formulas
```
Contact pressure:    p = δ / [d × (C_o/E_o + C_i/E_i)]
  where δ = actual interference (mm)
        d = nominal diameter (mm)
        C_o, C_i = shape coefficients (from Lamé)
        E_o, E_i = elastic moduli of materials

Transmissible torque: Mt = p × π × d² × L × μ / 2
  where L = contact length
        μ = friction coefficient (0.08–0.12 thermal, 0.12–0.15 press)

Assembly temperature: ΔT = δ_max / (α × d)  [°C]
  where α = thermal expansion coefficient (steel: 11.5×10⁻⁶ /°C)
  + add 30–50°C for assembly clearance
```

---

> For bearings, gears, valves, keys, Seeger rings, pins, bushings, seals, and all other
> functional elements → see `functional-elements.md` (includes decision flowchart).