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How It Works - Key & Keyway Stress Theory ▼

What Are Keys & Keyways?

A key is a machine element inserted between a shaft and hub (gear, pulley, coupling) to transmit torque. The keyway is the slot machined into the shaft and hub to receive the key.

  • Function: Transmit rotational motion and torque between shaft and hub
  • Types: Square, rectangular, Woodruff, gib-head, feather, spline
  • Materials: Low/medium carbon steel (AISI 1018-1045), sometimes hardened
  • Standards: ANSI/ASME B17.1, DIN 6885, ISO 2491

Failure Modes

Keys can fail by two primary mechanisms:

  • Shear Failure: The key is sheared along the plane between shaft and hub surfaces. The shear area is width (w) times length (L).
  • Bearing (Crushing) Failure: The key is crushed against the keyway sides. More common failure mode. Bearing area is half-height times length on each side.

Design Philosophy

Keys are designed as "mechanical fuses" - they should fail before the shaft or hub. This protects expensive components from damage during overload.

  • Keys are typically softer than shaft/hub material
  • Easy and cheap to replace compared to shaft
  • Allows intentional weak point in power train

Stress Formulas

Tangential Force:

F = 2T / d

T = torque, d = shaft diameter

Shear Stress:

tau = F / (w x L)

w = key width, L = key length

Bearing Stress:

sigma = F / (h/2 x L)

h/2 = key depth in keyway

Allowable Stresses

  • Shear: tau_allow = 0.5 x Sy (50% of yield)
  • Bearing: sigma_allow = 0.9 x Sy (90% of yield, compressive)

Design Guidelines

  • Key length: 1.0 to 1.5 x shaft diameter
  • If longer needed, use 2 keys at 180 degrees
  • Keyway reduces shaft strength by ~20-25%
  • Minimum SF: 2.0 normal, 3.0+ for shock
  • For high torque, consider splines instead
Key & Keyway - Geometry and Stress Distribution
CROSS-SECTION VIEW SHAFT d/2 HUB KEY w h h/2 h/2 d Torque T F Key Keyway Tangential force F F = 2T/d acts at shaft surface radius STRESS DISTRIBUTION SHEAR STRESS Shear Plane A_shear = w x L F tau = F / A tau = F / (w x L) Allow: 0.5 x Sy BEARING STRESS Bearing on hub (h/2) Bearing on shaft (h/2) h/2 F sigma = F / A sigma = F / (h/2 x L) Allow: 0.9 x Sy L (key length) Design Guidelines Key length: 1.0 x d to 1.5 x d (shaft diameter) Bearing usually governs (smaller area = higher stress)

Quick Select - Common Applications

Keyway Stress Calculator

Calculate shear and bearing stresses in keys and keyways per ANSI/ASME B17.1 and DIN 6885 standards.

Input Method
Standard Key Size (w x h) --
Loading
Material & Service
✓
Design Acceptable
Calculating...

Stress Analysis Results

Shear Stress in Key --
Allowable: -- Utilization: --
Shear Safety Factor --
Bearing Stress (shaft side) --
Allowable: -- Utilization: --
Bearing Safety Factor --
Tangential Force
--
Effective Torque
--
Shear Area (w x L)
--
Bearing Area (h/2 x L)
--
Required Length for SF = 2 --
Recommended Hub Length --
Governing Failure Mode --

Standard Key Sizes Reference

Shaft Dia (mm)Key (w x h)Shaft Dia (in)Key (w x h)
8-103 x 35/16 - 7/163/32 x 3/32
10-124 x 41/2 - 9/161/8 x 1/8
12-175 x 55/8 - 7/83/16 x 3/16
17-226 x 615/16 - 1-1/41/4 x 1/4
22-308 x 71-5/16 - 1-3/85/16 x 5/16
30-3810 x 81-7/16 - 1-3/43/8 x 3/8
38-4412 x 81-13/16 - 2-1/41/2 x 1/2
44-5014 x 92-5/16 - 2-3/45/8 x 5/8
50-5816 x 102-7/8 - 3-1/43/4 x 3/4

Metric per DIN 6885, Imperial per ANSI B17.1. Key length: 1.0-1.5 x shaft diameter.