What is the formula for key length?

L = (2 × T × FS) / (d × h × τ_allow) for shear; longer of shear or crushing.

How to select parallel key size?

Based on shaft diameter per DIN 6885 table (e.g., d=30-38mm → b=8mm, h=7mm).

Steel key/shaft: shear 70-100 MPa, crushing 140-200 MPa.

Calculate required key length or spline dimensions from torque, shaft diameter, and material strength. Accurate and reliable tool.

Required Key Length

85 mm

For 500 Nm torque, 40mm shaft, FS=2.5

Featured: Parallel Key (DIN 6885), Involute Spline

Torque T (Nm)

Shaft Diameter d (mm)

Connection Type

Material Allowable Stress

Factor of Safety (FS)

Parallel Key (Rectangular)

Standard DIN 6885, shear and crushing check.

L_shear = (2 × T × FS) / (d × h × τ)
L_crush = (4 × T × FS) / (d × b × σ)

Involute Spline

Approximate for 30° pressure angle.

T = (π/16) × L × n × h × (D + d) × τ

Enter parameters and click Calculate

Simple, cost-effective for moderate torque.

Higher torque, self-centering, misalignment tolerance.

2-3 for steady, 3-5 for shock loads.