A shaft key is a positive-locking torque transmitter: a small steel or alloy insert machined into a keyseat on both shaft and hub, sized to GB/T 1095-2003 (parallel keys) or GB/T 1096-2003 (Woodruff keys) for general machinery, with ISO 773 and ISO 774 governing the metric parallel-key geometry and tolerance classes.
Selection is driven by four numbers: shaft diameter, design torque, hub material, and required assembly access. Mismatches on any of these produce the same failure mode — keyway crushing, key shear, or shaft fracture at the keyway root — and that mode accounts for a disproportionate share of drivetrain downtime in conveyors, pumps, and gearbox inputs.
Key Types and Where Each Fits
Parallel keys (square or rectangular) are the default for shafts above Ø6 mm where a closed or open keyway is acceptable; square sections share shear and bearing load equally, rectangular sections bias load toward the bearing face to raise torque capacity at the cost of more aggressive hub-side keyseat milling [S1].
Woodruff keys (half-moon, GB/T 1096) are specified for shafts up to roughly Ø100 mm where a deep keyway would otherwise weaken the shaft; the circular bottom follows the milling cutter profile, giving a self-aligning seat and lower cost on small-batch production. They tolerate slight shaft-to-hub misalignment better than parallel keys but carry lower torque per unit length and concentrate stress at the keyway's top edges.
Gib-head keys add a protruding head so the key can be removed without full hub access — common on large shaft ends, turbine couplings, and mine-mill pinions. Taper keys (1:100 slope) provide self-tightening under reversing loads but require accurate axial preload and are rarely used below Ø50 mm.
Selection Criteria by Shaft Diameter and Torque
For parallel keys under GB/T 1095, shaft-to-key cross-section is fixed by shaft diameter: Ø22–30 mm shafts typically use a 8×7 mm key, Ø30–38 mm use 10×8 mm, Ø38–44 mm use 12×8 mm, and Ø58–65 mm use 18×11 mm [S1].
Shear capacity scales with key width × length, while bearing (crush) capacity scales with key height × length × shaft surface hardness. On a Ø50 mm shaft with a 14×9 mm key in 45# steel, typical working shear stress is held below 90 MPa for steady loads and 50–60 MPa for shock or reversing duty; pushing shear above 120 MPa invites key cutting, the most common field failure in shaft couplings feeding gearboxes.
Compare the three dominant options on a Ø50 mm shaft transmitting 800 N·m at 1450 rpm:
- Parallel square key (14×14×100 mm): highest torque capacity, easiest to source, requires through or open keyway on both parts.<br>- Parallel rectangular key (14×9×100 mm): ~30% lower bearing area, used when hub wall thickness is constrained.<br>- Woodruff key (e.g. 14×9 mm): quickest machining, better for taper-fit hubs, ~20% lower torque rating than a full-length parallel key.
Failure Modes and Stress Concentration Reality
Keyways are stress raisers. A sharp-cornered rectangular keyway on a shaft can raise the local fatigue stress concentration factor (Kt) to 3.5–4.5; the value drops to ~2.5–3.0 with a fillet radius of 0.4–0.6 × key height, which is why GB/T 1095 specifies a side-fit clearance band and a bottom-fit allowance rather than zero-tolerance line-to-line contact [S1].
Three failure modes dominate field service: key shear (over-torque, undersized key, or soft material), keyway crushing (insufficient hub-side bearing length, low surface hardness below HB 200), and shaft fracture at the keyway (Kt overshoot, reversed bending, no fillet). Installation discipline — particularly the shaft-side fillet and the hub-side parallelism to the shaft axis within 0.02 mm per 100 mm — is the lever that separates a reliable joint from a recurring repair [S1].
Material, Fit, and Standardization
Standard material is medium-carbon steel such as 45# (AISI 1045) for general use, with 40Cr (AISI 5140) or 35CrMo added for higher torque or impact duty. Surface hardness of HB 220–260 is the typical target for parallel keys in gear and coupling hubs; the shaft keyseat must be at least 10 HB points harder than the key to push wear into the replaceable part rather than the shaft. [S2]
Fit class is governed by ISO 773 (loose) and ISO 774 (close) for the key-to-keyway interface. For reversing or shock-loaded drives, close fit (N9/j9 on key height) is mandatory; for one-direction steady drives, a loose fit (N9/h9) eases assembly and reduces fretting wear at the keyway corners.
When a Shaft Key Is the Wrong Choice
For high-speed, high-torque, or misalignment-tolerant service — turbo-compressors, large fans, marine propulsion shafts — keys lose out to interference fits, splines, or shrink-disk assemblies. [S2]
Tapered bushings and locking-element hubs (e.g. TLK, Ringfeder) replace keys entirely on conveyor pulleys, fan hubs, and small gearbox outputs where the user wants repeatable, greaseable mounting without a keyseat. Keys are also a poor choice where frequent axial repositioning is needed, because each re-mount scores the keyway and accelerates wear.
Installation Discipline and Field Failure Map
Most shaft key failures are not material defects — they are installation errors. Three rules cover the majority of cases: (1) machine the keyseat on the shaft with a 0.4–0.6 mm radius at the bore exit, never a sharp corner; (2) hold hub bore-to-shaft parallelism to within 0.02 mm per 100 mm of key length; (3) check key-to-keyseat side clearance with a feeler gauge — gap above 0.10 mm on a Ø50 mm joint is a future loosening path.
For woodruff keys, the cutter depth must be controlled so the key projects above the shaft by 60–80% of its height; sitting proud by less than 50% under-loads the key and leads to shaft-side keyway elongation, the classic shaft collar and adjacent-bore failure pattern. When field repair follows a key shear, always re-check shaft surface hardness at the keyway: if it has been hot-spotted above HB 280, the shaft must be re-machined or replaced, not just re-keyed.
Sourcing and Standards Reference
GB/T 1095-2003 (keyway cross-section and depth series) and GB/T 1096-2003 (Woodruff keys) remain the baseline metric references; ISO 773/ISO 774 align for export equipment. For non-sparking or hazardous-area service — rare but specified in chemical and explosive-atmosphere plants — keys must be supplied in beryllium-copper or aluminum-bronze alloys matched to ATEX/IEC 60079 material rules for the zone [S1].
Procurement lead time in 2026 is short for standard parallel keys in 45# and 40Cr (stock sizes 6×6 to 32×18, lengths 20–200 mm), with custom sizes at 2–4 weeks. For more on how the key seat is machined and how a parallel shaft coupling interacts with key tolerances in field service, see the shaft key installation tolerance and failure map.
Track for procurement: confirm keyseat fillet radius on the next gearbox rebuild audit, and re-check the hub-side bore parallelism whenever a coupling is realigned. Those two signals will surface a key joint problem before the key itself shears.