The Gear-to-Shaft Connection
A gear is only useful if it can reliably transfer torque to or from its shaft. The gear-to-shaft connection must transmit the full operating torque, maintain angular positioning, resist axial movement, and survive millions of load cycles. Three primary methods are used: keys, splines, and interference fits.
Keys and Keyways
A key is a small piece of metal (usually steel) that sits in matching slots (keyways) cut into the shaft and gear bore. The key transmits torque through shear and bearing forces on its sides.
- Square keys: Width equals height. Standard for most applications. Sizes are standardized based on shaft diameter (DIN 6885, ANSI B17.1)
- Rectangular keys: Width is greater than height. Used for larger shafts and higher torques
- Woodruff keys: Semicircular cross-section that sits in a curved pocket in the shaft. Self-aligning, used for tapered shafts and light-duty applications
- Gib-head keys: Have a head that protrudes above the hub for easy removal. Used where keys must be regularly installed and removed
Key material is typically plain carbon steel (AISI 1018 or 1045). The key should be slightly softer than the shaft and gear to ensure that the key fails first — it is the cheapest component to replace.
Key Design Calculations
Keys are designed against two failure modes:
- Shear failure: τ = 2T / (d × w × L) where T = torque, d = shaft diameter, w = key width, L = key length. The shear stress must be below the allowable shear stress of the key material
- Bearing (crushing) failure: σ = 4T / (d × h × L) where h = the effective height in contact. This is often the limiting factor
If a single key cannot transmit the required torque, use a longer key (up to 1.5 × shaft diameter) or switch to a splined connection.
Splines
Splines are essentially multiple keys formed integrally with the shaft. They provide much higher torque capacity, better centering, and allow axial sliding when needed:
- Straight-sided splines: Rectangular teeth similar to key profiles. Simple to manufacture but limited torque capacity per tooth. Common in automotive drivelines
- Involute splines: Use involute tooth profiles (like gears). Better stress distribution, self-centering, and higher load capacity. The modern standard for most applications (ANSI B92.1, DIN 5480)
- Serrations: Fine-pitch triangular teeth for precise angular positioning. Used for adjustable components
Interference Fits
For high-speed applications where keyways create stress concentrations, interference (press) fits can transmit torque through friction alone. The gear bore is slightly smaller than the shaft, and the gear is heated or pressed onto the shaft. Common fit: H7/s6 for heavy press fit. The torque capacity depends on the interference, contact length, friction coefficient, and shaft/bore diameters.
Selection Guidelines
- Single key: Standard choice for moderate torque, easy assembly, and low cost
- Two keys at 120°: When a single key cannot transmit the torque
- Involute splines: High torque, high speed, or when axial sliding is needed
- Interference fit: High speed (no stress concentration from keyway), high reliability, permanent assembly
- Interference fit + key: Belt-and-suspenders approach for critical applications