A gearbox input shaft key is sized by picking the standard section from DIN 6885-1 (or ASME B17.1) for the shaft diameter, then proving both bearing pressure and shear stay under allowable limits at the worst-case torque including service factor.
The same procedure works for any parallel-key joint on a shaft key drive: standard section, allowable stresses, minimum length, fatigue check on the keyed shaft. Most procurement problems trace back to skipping one of those four steps, not to a bad standard lookup.
Standard Key Sections and Where They Apply
ISO metric parallel-key series per DIN 6885-1 cover shaft diameters from 6 mm to 500 mm with matching key widths from 2 mm to 50 mm and key heights roughly one-quarter of the shaft diameter [S1][S4]. A 40 mm shaft takes a 12×8 mm square section; a 50 mm shaft takes a 14×9 mm; a 100 mm shaft takes a 28×16 mm [S4][S5].
Square keys (b × b) are the default in general machinery from 6 mm to 100 mm shaft diameter. Rectangular / parallel keys (b × h with h ≈ 0.6 to 0.75 × b) are used where vertical clearance is tight, typically low-profile hubs or shaft shoulders. Woodruff keys are self-aligning and easy to replace in the field but produce a deeper keyway stress concentration and are rarely specified above 75 mm shaft diameter [S4].
Allowable Stresses and the Two Governing Checks
Allowable bearing pressure for steel-on-steel, mild shock, and a stationary key typically lands at 90 to 120 MPa; for shock-loaded drives drop to 50 to 70 MPa, and the shear allowable is normally about 0.6 × σ_b for the same material [S4].
The two checks a process engineer runs are bearing (flank crushing) and shear. Bearing stress: σ = 4T / (d × h × L). Shear stress: τ = 2T / (d × w × L). The longer of the required lengths from each check is the minimum safe key length [S3][S5]. On a square key with the usual shear allowable at half the bearing allowable, the two safety factors come out equal, which is why standard square keys are balanced [S5]. Widen the key and shear improves; deepen it and bearing improves.
Quick Sizing Worked Example: 50 mm Input Shaft
Take a 50 mm input shaft, 14×9 mm square key, C45 steel, mild shock service. At 90 MPa nominal bearing pressure, the joint transmits roughly 350 to 450 N·m at a 60 to 80 mm key length [S4]. The same shaft with a 14×9 × 80 mm taper key and gib-head typically reaches 600 to 800 N·m before the same 90 MPa flank limit is hit, because the taper preloads the flanks [S4].
Length-to-diameter ratio is the second quick test: L should fall between 1.0 × d and 1.5 × d for parallel keys, and the hub length is set so the key does not run out of the keyway in operation. A DIN 6885-1 12×8×63 mm key on a Ø40 mm shaft carrying 200 N·m hits SF 2.27 on bearing, which governs; shear lands at SF 3.02 with a 27.8 mm required length for bearing versus 20.8 mm for shear [S5].
Fatigue and Stress Concentration at the Keyway
The fatigue endurance limit of a keyed shaft drops to roughly 25 to 40% of an unkeyed, polished specimen of the same diameter because the keyway corner acts as a stress raiser with Kt values commonly reported between 2.0 and 2.7 (2025-08) [S4]. Cutting a keyway weakens the shaft; a rough approximation is e = 1 - 0.2(w/d) - 1.1(h/d) [S3].
Keeping the shaft-keyway corner radius at the maximum value your key standard allows is the cheapest fatigue gain available: going from r = 0.2 mm to r = 0.4 mm at the shaft keyway corner can lift fatigue life by a factor of 2 to 3 at the same torque amplitude (2025-08) [S4]. For a rotor on a gearbox input shaft, a half-key mock-up is mandatory when balancing to ISO 21940-32:2012 grade G2.5 or finer; the keyway is half-filled with a dummy half-key of the same height as the real part so the rotor is balanced in its assembled state [S4].
Comparison: Square vs Rectangular vs Woodruff vs Taper on Real Decision Criteria
The choice hinges on torque, hub travel, alignment tolerance, and rework cost. Square keys are the cheapest and easiest to cut on a horizontal mill. Rectangular keys are used where vertical clearance is tight. Woodruff keys give up shaft strength for self-alignment and field replaceability. Taper keys (1:36 or 1:100, gib-head on long variants) deliver the highest torque capacity per unit width because the taper preloads the flanks, but the hub position is fixed by the key-in and must be machined to ±0.05 mm to seat without binding [S4].
If torque is the driver and the hub is locked, go taper. If the hub must slide on the shaft for adjustment, go square or rectangular parallel key. If field serviceability matters more than peak torque, go Woodruff. If hub travel is needed and rework cost is low, the parallel key paired with a shaft collar for axial location is the cleanest spec.
Procurement Pitfalls and Practical Guardrails
Keyway flank crushing is the most common failure mode, not key shear, because bearing is usually the governing check on a properly sized key [S3][S4]. Key rotation in the keyway is a fit problem, not a strength problem, and is fixed by tightening the width tolerance from a normal loose fit to a press fit on the key sides per DIN 6885-1 or BS 4235 [S5].
For very long keys, hub width caps the length; the practical escape is to step up the shaft diameter or use two keys 180° apart, which roughly doubles torque capacity without changing the keyway stress concentration. Shaft fastening by keyed joint is also worth comparing against locking assemblies where hub travel is not required: friction drives eliminate the keyway stress raiser entirely.
Trackable Signals for 2026 Spec Work
Watch for revisions to ASME B17.1 (last major refresh in the current cycle) and any updates to ISO 2492 plain-key series that might shift section tables for shafts above 200 mm. DIN 6885-1 remains the workhorse metric standard; BS 4235 is harmonized with it for cross-procurement in European builds. For a shaft coupling half on the same input shaft, the same key table applies; the limit is the lowest torque capacity in the train. [S5]