Parallel Key Torque, Shear & Crushing Calculator (DIN 6885)

Why Hub Flank Crushing (Not Key Shear) Almost Always Limits DIN 6885 Keys

When sizing a shaft-hub connection, machinists first look up the keyseat width b and milling plunge depth t1 + hc in the ANSI B17.1 & DIN 6885 keyway dimensions calculator. However, standard rectangular keys above Ø22 mm are wider than they are tall (for example, 10 × 8 mm on a Ø35 mm shaft with shaft depth t1 = 5.0 mm). That leaves only h − t1 = 3.0 mm of key height projecting into the hub keyway to resist bearing compression against b = 10.0 mm of shear width!

Furthermore, on a Form A round-ended key milled with an end mill of diameter b, the two semicircular ends (radius b/2) do not carry full uniform bearing contact against the hub wall. Standards published by DIN German Institute for Standardization (DIN 6892:2025-10) and Machinery's Handbook (31st Ed., Industrial Press) therefore subtract one key width from the nominal length to obtain the effective load-carrying length:
leff = L − b (Form A round ends)  or  leff = L (Form B square ends).

When Should You Switch from a Single Key to Twin Keys or a DIN 5480 Spline?

Increasing key length gives diminishing returns, because torsional wind-up along the shaft concentrates bearing pressure at the drive-end edge of the key. If the required key length Lmin exceeds your hub width, you have three engineering options: (1) step up the hub material from cast iron or mild steel to heat-treated 4140 Q&T; (2) machine two keys in the shaft and check their load sharing separately, since this calculator models a single key; or (3) replace the keyed shaft with an involute spline sized in the DIN 5480 metric involute spline calculator or a keyless interference fit in the press-fit interference calculator.

Worked Examples: Sizing Key Length on a Ø35 mm Gearbox Shaft & Ø50 mm Alloy Hub

Worked Example 1 (Ø35 mm Shaft at 400 N·m with K_A = 1.25): A Ø35 mm shaft uses a 10 × 8 mm DIN 6885 key (t1 = 5.0 mm, hub bearing height 3.0 mm). With a short L = 50 mm Form A key (leff = 40 mm), design tangential force is F_t = 28.57 kN, giving acceptable shear stress τ = 71.4 MPa (SF = 1.26) but excessive crushing stress p = 238.1 MPa (SF = 0.55 < 1.0, max torque 218.4 N·m). Lengthening the key to standard L = 90 mm (leff = 80 mm) cuts crushing stress in half to 119.0 MPa (SF = 1.09) and raises capacity to 436.8 N·m.

Worked Example 2 (Ø50 mm Shaft with 14×9×80 mm Key in 4140 Hub): At T = 900 N·m (KA = 1.25), the engine selects a 14 × 9 mm key (t1 = 5.5 mm, bearing height h − t1 = 3.5 mm as the conservative design note, leff = 66 mm), yielding τ = 48.7 MPa (SF = 1.85) and p = 194.8 MPa (SF = 0.92) against 180 MPa allowable. The 80 mm key does not pass: crushing pressure p = 194.8 MPa gives SF = 0.92 < 1.0. With L = 90 mm (leff = 76 mm) the key passes at p = 169.2 MPa (SF = 1.06) and τ = 42.3 MPa (SF = 2.13). Size the broaching tonnage for the hub in the keyway broaching force calculator and check shaft torsional notch stress in the shaft torsional stress calculator.

Metric keys follow DIN 6885 and ISO 773 (International Organization for Standardization (ISO)), inch keys follow ASME B17.1 (ASME Codes & Standards), and unit conversions follow NIST Special Publication 811. If the calculator returns an L_min longer than the available hub, the fix is a longer hub, a second key, or a spline, not a shorter key with a higher shock factor: the shock factor K_A is an input of the application, not a tuning knob.

Check the keyseat fit before you trust the crushing result. The hub bearing check assumes that the key carries load over its full height h − t1; a loose key or a worn keyseat reduces that bearing area, and the calculated p_crush then underestimates the real hub pressure.

Next step: Get exact shaft and hub milling depths in the ANSI B17.1 & DIN 6885 keyway calculator, and check arbor press broaching force in the keyway broaching force calculator.