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Shaft Keys Calculator — Shear, Bearing & Key Length (DIN 6885 / DIN 6892)

Governing standard: DIN 6885· Key & keyway DIMENSIONS: DIN 6885:1968 Part 1 · Load capacity (hub surface pressure): DIN 6892 · Shear check: Roloff-Matek · Woodruff key table DIN 6888 · spline screening check (not ISO 4156)

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How DIN 6885 works — the method explained

The MechanixCalc shaft key calculator sizes and verifies parallel keys for rectangular shaft-hub keyway connections. Key sections come from the DIN 6885 Part 1 table, and the load-capacity check follows the DIN 6892 method — surface pressure on the hub keyway flank — alongside the customary Roloff-Matek shear check on the key itself. Enter the shaft diameter, key geometry, torque, application factor and materials, and the tool returns the shear stress, hub bearing pressure, minimum key length and the governing safety factor in a single pass.

Built for drivetrain, gearbox and general power-transmission engineers, the calculator also carries screening checks for Woodruff keys (with automatic selection from the DIN 6888 table) and for splines, plus a multi-key comparison for one, two or three keys — all backed by a PDF engineering report that states the method, the scope limits and the governing verdict.

What this calculator does

  • Parallel key shear and hub bearing-pressure safety factors with an application factor Ka
  • Hub-groove surface pressure by the DIN 6892 method (shaft groove and key flanks not checked), with key sections auto-selected from the DIN 6885 Part 1 table
  • Minimum key length for each failure mode — the length at which the safety factor is exactly 1.00
  • Multi-key comparison for 1, 2 or 3 keys with the fit-tolerance factor k_s, credited up to two keys
  • Woodruff key screening check with automatic selection from the DIN 6888 designation table
  • Spline contact-pressure and root-shear screening check with a 25 % tooth-engagement derate
  • Key and hub material library (C45, 42CrMo4, cast iron, S235, S355) with a custom override
  • Branded PDF engineering report stating the method, the scope limits and the governing verdict

Method & formulas

Shear strength (Roloff-Matek check)

The key is loaded in shear across the plane at the shaft-hub interface. The tangential force is derived from the design torque (nominal torque amplified by the application factor Ka), and the shear stress is distributed over the shear area, which is the key width b multiplied by the effective engaged length L. This shear check is the customary Roloff-Matek / Shigley one — it is not a DIN 6892 criterion, because DIN 6892 evaluates surface pressure only, on the grounds that shear is rarely decisive at standard DIN key sections. For multiple keys, DIN 6892 and Roloff-Matek apply a load-sharing factor k_s (0.75 for two keys) because tolerance stack-up prevents equal distribution; neither credits a third key, so the effective key count is capped at two keys' worth.

Shear stress
τ = (T_d × 2000) / (d × b × L_eff × n_k_eff)

where τ = shear stress (MPa); T_d = T × Ka = design torque (N·m); d = shaft diameter (mm); b = key width (mm); L_eff = effective key length (mm); n_k_eff = min(n_keys × k_s, 1.5) = effective number of keys, capped at two keys' worth

Shear safety factor
SF_shear = τ_allow / τ, τ_allow = min(τ_y / Kt, τ_y)

where τ_y = shear yield of the key material; Kt = fit-tolerance factor (Normal 1.0; Close 0.9; Loose 1.15). The min() clamp means no fit class can license an allowable above the material's own shear yield

Hub bearing (surface) pressure — DIN 6892 method

The bearing failure mode is compressive crushing of the hub keyway flank, and it is the mode DIN 6892 exists to check. The bearing force is the same tangential force, but the contact area is the hub engagement height (h − t1) multiplied by the effective length. The asymmetric DIN 6885 keyway geometry — t1 the shaft slot depth, h the total key height — gives the true hub-flank contact height h − t1 rather than the simplified h/2 approximation. The allowable bearing pressure depends on the hub material and surface hardness.

Scope limit: this calculator evaluates surface pressure on the HUB groove only. DIN 6892 also requires checks on the shaft groove and on the key flanks; with a hub harder than the shaft, the shaft groove can govern, and the tool does not ask for a shaft material. The result states this on every calculation and in the PDF report.

Bearing pressure
p = (2 × T_d × 1000) / (d × (h − t1) × L_eff × n_k_eff)

where p = bearing pressure (MPa); T_d = design torque (N·m); d = shaft diameter (mm); h = key height (mm); t1 = shaft keyway depth (mm); h − t1 = hub engagement height (mm); L_eff = effective key length (mm); n_k_eff = effective number of keys

Bearing safety factor
SF_bearing = p_allow / p

where p_allow = allowable bearing pressure for the hub material (MPa); p = actual bearing pressure (MPa)

Minimum key length and design guidance

The minimum key length for each failure mode is derived by rearranging the shear and bearing formulae. L_min is the length at which the safety factor is exactly 1.00, so clearing it is the collapse boundary, not a pass. The recommended design length adds a 50 % margin (1.5 × L_min) while also respecting the 1.5 × d rule of thumb — neither term is a code requirement, and both are guidance only. The DIN 6885 auto-size lookup selects the standard b × h × t1 × t2 section directly from the shaft diameter, so the designer needs only to confirm the key length against the hub face width.

Status thresholds used by the tool: SF_min ≥ 2.0 → SAFE; 1.2 ≤ SF_min < 2.0 → CHECK; SF_min < 1.2 → FAIL. These are the tool's own design thresholds — DIN 6892 does not specify a minimum safety factor. Every surface, including the downloadable PDF report, bands on exactly these numbers.

Worked example

Check a single DIN 6885 parallel key on a 40 mm diameter steel shaft transmitting 200 N·m nominal torque (Ka = 1.0, no shock). Key: b = 12 mm, h = 8 mm, t1 = 5 mm (standard DIN 6885 row for d = 38–44 mm), L = 50 mm. Key material: C45 (τ_allow = 290 MPa, Normal fit Kt = 1.0). Hub material: S235 (p_allow = 140 MPa).

Given

  • Shaft diameter d40 mm
  • Nominal torque T200 N·m
  • Application factor Ka1.0 (no shock)
  • Key width b12 mm
  • Key height h8 mm
  • Shaft keyway depth t15 mm
  • Key length L50 mm
  • Key material τ_y290 MPa (C45); Normal fit Kt = 1.0 → τ_allow = 290 MPa
  • Hub p_allow140 MPa (S235)

Result

  • Shear stress τ16.7 MPa
  • Bearing pressure p66.7 MPa
  • SF_shear17.4
  • SF_bearing2.10
  • SF_min (governing)2.10 — SAFE
  1. Design torque: T_d = T × Ka = 200 × 1.0 = 200 N·m.
  2. Shear stress: τ = (T_d × 2000) / (d × b × L) = (200 × 2000) / (40 × 12 × 50) = 400 000 / 24 000 = 16.7 MPa.
  3. Shear safety factor: SF_shear = τ_allow / τ = 290 / 16.7 ≈ 17.4 — shear is very generous.
  4. Hub engagement height: h − t1 = 8 − 5 = 3 mm.
  5. Bearing pressure: p = (2 × T_d × 1000) / (d × (h − t1) × L) = (2 × 200 × 1000) / (40 × 3 × 50) = 400 000 / 6 000 = 66.7 MPa.
  6. Bearing safety factor: SF_bearing = p_allow / p = 140 / 66.7 ≈ 2.10 — bearing governs.
  7. Minimum safety factor: SF_min = min(17.4, 2.10) = 2.10 → SAFE (threshold ≥ 2.0).

Illustrative — verify against your actual geometry, material allowables and application factor. The tool computes both failure modes simultaneously and selects the standard key dimensions automatically from the DIN 6885 table.

Frequently asked questions

Which standard does this shaft key calculator use?

Two different documents do two different jobs. Key and keyway DIMENSIONS come from DIN 6885 Part 1, which is a dimensional standard — it defines b, h, t1 and t2 by shaft diameter and the fit tolerances, but it contains no strength method, no allowable pressures and no safety factors. The LOAD CAPACITY check follows the DIN 6892 method: surface pressure on the hub keyway flank. The shear check on the key itself is the customary Roloff-Matek one, because DIN 6892 evaluates surface pressure only. The Woodruff and spline panels are screening checks and are badged as engineering estimates — DIN 6888 is likewise a dimensional standard, and the spline panel does not implement ISO 4156 or ANSI B92.1. The method and its scope limits are printed in the PDF report.

What is the difference between shear and bearing failure for a key?

Shear failure is sliding of the key across the shaft–hub interface plane; it depends on the key width, length and the key material's shear strength. Bearing (surface) failure is compressive crushing of the hub keyway flank; it depends on the hub engagement height (h − t1), key length and the hub material's allowable bearing pressure. Both modes are checked, and the lower safety factor governs. For soft hubs (cast iron, soft steel), bearing typically governs at short key lengths.

How does multi-key load sharing work?

DIN 6892 and Roloff-Matek apply a load-sharing factor k_s because manufacturing tolerances prevent keys from sharing the load equally — this factor is not in DIN 6885, which carries no strength rules at all. For two keys k_s = 0.75, giving an effective 1.5 keys. Neither reference credits a third key, because tolerance stack-up means it cannot be shown to carry load, so the calculator caps the credited effective count at two keys' worth (1.5). A third key still helps with balance and location; it just does not earn extra capacity in the check.

When should I use a Woodruff key instead of a parallel key?

Woodruff keys (DIN 6888) are self-aligning — their circular disc shape seats automatically in the shaft keyway, making them ideal for tapered shaft ends, light machinery and instruments where assembly alignment matters. However, they cut deeply into the shaft, reducing its cross-section more than a parallel key does, and they are not suitable for high torque loads — the rule of thumb is to switch to a parallel key for torques above roughly 200 N·m.

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