Key & Keyway Stress Calculator — Shear Stress, Crushing Stress & Safety Factor
Calculate key shear stress, keyway crushing (bearing) stress, and safety factor for parallel keys, Woodruff keys, and tapered keys under torque loading.
Quick Answer
For a 12×8mm parallel key in a 50mm shaft transmitting 500 Nm, key length 60mm: Shear Stress = 55.6 MPa, Crushing Stress = 83.3 MPa. For 1045 steel key material (yield 310 MPa), safety factor ≈ 3.7 — acceptable.
Why Key Stress Calculations Matter
The key and keyway is the weakest link in your shaft-hub connection. Get it wrong and the key shears, the hub spins on the shaft, and your machine stops. Two failure modes dominate:
1. Key Shear Stress
τ = 2T / (d × w × L). The key resists torque by shear along its length. For standard parallel keys (DIN 6885, ANSI B17.1), the key material yield in shear is ~0.58× tensile yield — but design to 0.4× for safety.
2. Crushing (Bearing) Stress
σ_c = 4T / (d × h × L). This is the contact pressure between key side and keyway wall. Exceed the material yield and the keyway deforms — slop develops, impact loading follows, key fatigue accelerates. For steel-on-steel, limit crushing stress to 130 MPa for static loads, 80 MPa for reversing loads.
3. Key Length Rule
Standard key length is 1.25-1.5× shaft diameter. Longer than 1.5d adds no benefit — the load concentrates at the ends, the middle carries little. For high torque, use two keys at 90° or 180° spacing, not one extra-long key.
Applications
- Motor-to-gearbox coupling hubs
- Sprocket, pulley, and sheave shaft attachments
- Impeller and fan hub mounting
- Pump shaft connections
Common Mistakes
- Using the wrong key size for the shaft — Standard key sizes follow shaft diameter: 8-10mm shaft → 3×3 key, 38-44mm → 12×8, 58-65mm → 18×11. Don’t guess — use the standard table. Undersized keys shear.
- Not accounting for reversing torque — Keys in reversing drives (crane hoists, shuttles) see fully reversed fatigue. Design to 50% of static allowable stress or use a tapered locking assembly instead.
- Assuming load is evenly distributed — Key load concentrates at the hub edge within the first 1.3× shaft diameter. Longer keys don’t proportionally increase capacity — the ends carry, the middle rests.
- Using a single key for high torque — Two keys at 180° balance the load. One key creates an offset load, increasing shaft bending. This is why large motors have two keyways.
- Forgetting the shaft material — Keyway crushing stress also applies to the shaft keyway, not just the key. If your shaft is softer than the key (aluminum shaft, steel key), the shaft keyway fails first.
Frequently Asked Questions
What is the difference between parallel key, Woodruff key, and spline?
Parallel key: rectangular, cheapest, most common, good for moderate torque. Woodruff key: semi-circular, self-aligning, used on tapered shafts (automotive), lower torque capacity. Spline: multiple integral teeth, highest torque capacity, zero backlash possible, most expensive. For high torque or reversing loads, splines beat keys.
How do I calculate key length needed?
L_min = max(L_shear, L_crush) where L_shear = 2T/(d×w×τ_allow) and L_crush = 4T/(d×h×σ_allow). Then add 10-20% for manufacturing tolerances and misalignment. Standard key stock comes in 20-30mm increments — round up.
Can I use stainless steel keys?
Yes, but: 304 has lower yield (~205 MPa vs 310 for 1045) and galls under fretting. Use 17-4PH stainless (yield ~1000 MPa) for high-strength corrosion-resistant keys. For general use in wet environments, 316 is a compromise.
What if my key sheared — was it undersized?
Probably yes, or: (1) Reversing load with inadequate safety factor, (2) Loose fit allowing impact loading, (3) Keyway radius too sharp (stress concentration), (4) Shaft/hub misalignment causing edge loading. Check all four before just upgrading key size.
Should I use a setscrew with the key?
Setscrews prevent axial movement, not rotational — the key handles torque. A setscrew on a key transfers some load but creates a stress riser at the setscrew dimple. For high-reliability designs, use a retaining ring or locknut instead.
What are tapered bushings and when to use them?
Taper-Lock and QD bushings eliminate keys entirely — the taper friction handles torque. Advantages: no keyway stress concentration, infinite angular positioning, easy removal. Best for pulleys, sheaves, and sprockets above 25mm shaft diameter.
How does keyway depth affect shaft strength?
A standard-depth keyway reduces shaft fatigue strength by ~25% at the keyway corner (stress concentration factor ~2.0). For high-cycle shafts (>10⁷ cycles), use a sled-runner keyway (rounded ends) or switch to a clamping hub. Check with our Shaft Stress Calculator.
What is fretting and how does it kill keys?
Fretting is micro-wear from small-amplitude relative motion between key and keyway under cyclic load. It creates pits that become fatigue crack initiation sites. Mitigation: tighter fit (H7/g6 instead of H7/f7), anti-fretting coatings (MoS₂), or eliminate the key entirely with a locking assembly.