mm
N·m
mm
Key ends
Number of keys
Weakest part (shaft, key or hub)
Parallel keys to DIN 6885-1 / ISO 773: key size for the shaft, surface pressure on the shaft and hub keyway sides, key shear, the key length needed for a torque and the torque a key can carry. One or two keys, steel or cast iron hubs, shock factor.
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| Shaft Ø over – to | Key b × h | Shaft groove t1 | Hub groove t2 | Length needed | Torque at your length |
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The key transmits the torque as a force on its side faces at the shaft radius. The flank that sticks into the hub is lower than the one in the shaft (h − t1 against t1), so the hub side usually has the higher surface pressure and decides the key length. Shear of the key itself is rarely critical.
Key sizes from DIN 6885-1 (identical to ISO 773 for b × h; groove depths t1 / t2 for the standard height). The pressure check follows the simplified method of DIN 6892 (method C) as used in Roloff/Matek: uniform pressure over the effective flank. Keys longer than about 1.3·d do not carry more, because the shaft twists along the key; use two keys or a spline instead. Fatigue of the shaft at the keyway is checked separately in the shaft calculator (Kf, Kfs).
This calculator sizes a parallel (feather) key to DIN 6885-1 / ISO 773 for a shaft and hub. Enter the shaft diameter, the torque, the key length and a shock factor, and choose the key ends, one or two keys and the weakest material. It picks the standard key for the diameter and returns the surface pressure on the hub and shaft sides of the keyway, the shear in the key, the key length the torque needs and the torque your key can carry, with a table of the neighbouring key sizes.
A 12 × 8 × 50 mm key transmits 300 N·m with shocks at 60 % of the allowable pressure on the hub flank, which governs. A 36 mm key would just do; 50 mm gives margin without exceeding the useful 1.3 × d.
Key width and height are standardised by shaft diameter in DIN 6885-1 and ISO 773, for example 12 × 8 mm for shafts over 38 up to 44 mm. The calculation then decides the length: the torque divided by the shaft radius gives the force on the key flank, and the flank pressure must stay below the allowable value of the weakest part.
The key sits deeper in the shaft (t1) than it sticks into the hub (h − t1), so the flank area in the hub is smaller and the pressure higher. Hubs are also often made of a softer material, such as cast iron, which lowers the allowable pressure further.
Only the straight part of the flank carries load. For round-ended keys (form A) the effective length is the overall length minus the key width; for keys with one round end (form C) it is minus half the width; square-ended keys (form B) use the full length.
No. Beyond about 1.3 times the shaft diameter the shaft twists along the key, so the end farthest from the load carries little. For higher torques use two keys at 180° (counted with a 0.75 load-share factor), a spline or an interference fit.
A common simplified rule (DIN 6892 method C, Roloff/Matek) is about 0.9 times the yield strength of the weakest part for steel, with a shock factor on the torque. Brittle grey cast iron hubs are limited to roughly half their tensile strength. Keyways also lower shaft fatigue strength, which must be checked separately.
Yes. The keyway is a notch: with an end-milled keyway the fatigue stress concentration is about 2 in bending and nearly 3 in torsion. Use the shaft calculator's fatigue panel with these Kf and Kfs values.
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