Check a parallel-keyed shaft-hub connection to DIN 6892:2025 using approximate Method C. Enter the transmitted torque, shaft and key geometry, application factor and the strengths of shaft, hub and key to determine the equivalent and permissible torque, the utilization and the required key length for predominantly one-direction torque.
Check the load capacity of a parallel-key shaft-hub connection under one-direction torque using the approximate Method C of DIN 6892.
Inputs
Shaft diameter d together with key width b, height h, shaft keyway depth t₁, overall length lPF and key form describe the geometry; the number of keys, nominal torque Mtnom with application factor KA and the strengths of shaft, hub and key describe load and material.
Torque becomes a high tangential force through the small shaft radius d. Pressure falls with the carrying area formed by height h − t₁ and carrying length — key width b does not enter it and with form A even shortens the carrying length. At the same time the keyway weakens the shaft, so a key that passes on its own can still leave the shaft check governing.
Calculation
The application factor scales the nominal torque to an equivalent torque. It acts through the tangential force Ft = 2·Mteq/d on the carrying flank of height h − t₁ and credited length min(ltr; 1.3·d), which yields the permissible torque, the utilization and the required key length.
Example
A 40 mm shaft with a 12 × 8 key to DIN 6885-1 (t₁ = 5 mm), form A, lPF = 63 mm, one key. Mtnom = 500 N·m at KA = 1.25 gives Mteq = 625 N·m. Carrying height is 3 mm and carrying length 51 mm (ltr/d = 1.275, still fully credited). At 300 MPa the hub is the weakest material, so pperm = 270 MPa and Mtperm = 826.2 N·m. Utilization is 0.76 and only 50.6 mm of key length would be required — Method C is satisfied.
Sources and limits: DIN 6892:2025-10 defines the design and calculation principles for parallel-key connections on metallic components; the geometry follows DIN 6885-1/-2/-3 except for key length. This implementation covers Method C under one-direction torque only and models the permissible pressure in simplified form as 0.9 × the lowest strength involved. Verification of shaft and hub, bending, fatigue and Methods A and B are not included.
Technical article
Check a parallel-key joint to DIN 6892 in detail
Check the load capacity of a parallel-key shaft-hub connection under one-direction torque using the approximate Method C of DIN 6892.
What does a parallel key do?
A parallel key is a form-locking shaft-hub connection. A prismatic steel key sits in a keyway machined into the shaft and engages a matching keyway in the hub; torque is transmitted through the flanks of the key, not by friction. Typical hubs are gears, pulleys, sprockets and couplings. The form lock is particularly useful wherever the hub has to be assembled, removed or replaced later, because the joint needs neither heating nor pressing. Standardized dimensions do not by themselves prove sufficient load capacity, though: the size series follows the shaft diameter alone and knows nothing about the transmitted torque or the hub material. The specific connection therefore has to be checked.
What's the difference between DIN 6892 and DIN 6885?
The two standards answer different questions. DIN 6885 defines the geometry: it assigns standardized widths b, heights h and keyway depths t₁ to each shaft diameter. DIN 6892 defines the design and calculation principles, that is the strength assessment of the connection for metallic components.
They interlock: for the geometry of the connection DIN 6892 relies on DIN 6885-1/-2/-3 — with the exception of key length, which is precisely what the calculation determines. In practice that means look the dimensions up first, then calculate the length.
Method C is the approximate procedure of DIN 6892. It assumes constant surface pressure over the carrying area instead of resolving the genuinely uneven distribution along length and height, and it considers the torsional moment only.
That produces clear limits, which this calculator also enforces: carrying length is credited only up to 1.3·d, because beyond that the load does not spread further in practice; at most two keys are permitted, and the second is credited with φ = 0.75 only. Chamfers and transition radii are not represented in the simplified carrying-area model.
Reversing torque falls outside the scope: load reversal requires the more detailed Method B. If the required carrying length exceeds the 1.3·d limit, the calculator says so explicitly instead of reporting an inadmissibly long key.
Method C is therefore suited to preliminary sizing and to predominantly one-direction, quasi-static torque. It replaces neither the shaft nor the hub verification.
Formula and variables
Mteq = KA · Mtnom
Ft = 2 · Mteq / d
htr = h − t₁
ltr = lPF − b (form A) · lPF (form B) · lPF − b/2 (form AB)
ltr,credited = min(ltr; 1.3 · d)
pperm = 0.9 · min(Re,shaft; hub strength; Re,key)
Mtperm = pperm · htr · ltr,credited · d/2 · i · φ
Utilization = Mteq / Mtperm
ltr,req = 2 · Mteq / (pperm · htr · d · i · φ)
Symbol / input
Meaning
Mtnom
Nominal torque from the service load case, before the application factor.
KA
Application factor for shock and non-uniform running; it scales Mtnom to the equivalent torque.
d
Shaft diameter at the connection; it provides the lever arm d/2 and the length limit 1.3·d.
b, h, t₁
Key width, key height and shaft keyway depth to DIN 6885-1.
htr
Carrying height h − t₁: the part of the key that bears against the hub keyway.
ltr
Carrying length; it depends on key form and is credited only up to 1.3·d.
i, φ
Number of keys and load-sharing factor: φ = 1.0 for one key, φ = 0.75 for two.
pperm
Permissible bearing pressure; this implementation models it as 0.9 × the lowest of the three strengths.
Mtperm
Permissible torque of the connection according to Method C.
Choose the inputs correctly
Shaft diameter d together with key width b, height h, shaft keyway depth t₁, overall length lPF and key form describe the geometry; the number of keys, nominal torque Mtnom with application factor KA and the strengths of shaft, hub and key describe load and material. Torque becomes a high tangential force through the small shaft radius d. Pressure falls with the carrying area formed by height h − t₁ and carrying length — key width b does not enter it and with form A even shortens the carrying length. At the same time the keyway weakens the shaft, so a key that passes on its own can still leave the shaft check governing.
How to use the calculator
Enter the shaft diameter plus key width b, height h, shaft keyway depth t₁ and overall length lPF; the form (A, B or AB) decides which portion of that length carries load. Add the nominal torque with its application factor KA and the strengths of shaft, hub and key. The calculator returns the equivalent and permissible torque, the utilization and the required key length.
Worked example
A 40 mm shaft with a 12 × 8 key to DIN 6885-1 (t₁ = 5 mm), form A, lPF = 63 mm, one key. Mtnom = 500 N·m at KA = 1.25 gives Mteq = 625 N·m. Carrying height is 3 mm and carrying length 51 mm (ltr/d = 1.275, still fully credited). At 300 MPa the hub is the weakest material, so pperm = 270 MPa and Mtperm = 826.2 N·m. Utilization is 0.76 and only 50.6 mm of key length would be required — Method C is satisfied.
How to interpret the result
Utilization is the equivalent torque divided by the permissible torque, which makes it identical to the ratio of actual to permissible surface pressure. Values up to 1.0 satisfy Method C. What matters is which of the three materials sets the permissible pressure — the calculator reports the lowest of the three strengths separately, and it is usually the hub. A passing key check proves the connection only, not the components: the keyway acts as a notch and lowers the fatigue strength of the shaft, which therefore has to be assessed separately under dynamic load — for example with the DIN 743 Shaft Verification calculator, or with the shaft diameter pre-design for a first packaging estimate. A longer key also reduces mean bearing pressure only inside the model — beyond 1.3·d no further length is credited, so additional length buys nothing there. If the connection is permanently insufficient, an interference fit to DIN 7190-1 is an alternative shaft-hub connection — not a required follow-up step, but a different design.
Diameter, width, height, keyway depth and lengths in millimetres, torques in N·m, strengths and permissible bearing pressure in MPa. Application factor and utilization are dimensionless.
Typical applications
Sizing a parallel-key connection for a known drive torque, checking an existing connection against an increased operating torque, and determining the required key length before selecting the next standard length.
Assumptions, limits and common mistakes
DIN 6892:2025-10 defines the design and calculation principles for parallel-key connections on metallic components; the geometry follows DIN 6885-1/-2/-3 except for key length. This implementation covers Method C under one-direction torque only and models the permissible pressure in simplified form as 0.9 × the lowest strength involved. Verification of shaft and hub, bending, fatigue and Methods A and B are not included.
Common mistake: Don't confuse overall length lPF with carrying length: with form A the rounded ends do not carry, so ltr = lPF − b. Nor is key height h the carrying height — what counts is the portion standing proud of the shaft keyway, h − t₁, which for a 40 mm shaft is only 3 mm out of 8 mm. And permissible pressure follows the weakest of the three strengths; a high-strength key inside a cast-iron hub gains nothing.
Frequently asked questions
What is a parallel key?
A prismatic steel key that sits in a shaft keyway and engages a matching hub keyway. It transmits torque through its flanks by form lock, not by friction.
What is the difference between a key and a keyway?
The key is the inserted component; the keyway is the slot machined for it in shaft and hub. Both matter for the calculation: keyway depth t₁ decides, via h − t₁, how much key height actually carries load.
How do I calculate the required parallel-key length?
The required carrying length follows from the equivalent torque, carrying height, shaft diameter and permissible pressure; the calculator outputs it directly and adds b or b/2 back depending on key form. Then pick the next standard length above it.
What is the difference between DIN 6885 and DIN 6892?
DIN 6885 standardizes the dimensions of key and keyway, DIN 6892 the strength assessment of the connection. DIN 6892 takes the geometry from DIN 6885-1/-2/-3, except for key length.
What does DIN 6892 Method C calculate?
The approximate permissible torque of a parallel-key connection assuming constant surface pressure, plus the utilization and the required key length. Load types other than torsion are not included.
When should I use Method B instead of Method C?
As soon as the torque reverses direction, for load spectra, and when the required carrying length exceeds the 1.3·d limit. Method B resolves the load distribution more accurately and handles load reversal.
Can I use two parallel keys?
Yes, Method C permits one or two; more is outside its scope and the calculator rejects it. Two keys offset by 180° are demanding to manufacture, because both have to bear simultaneously.
Does using two keys double the torque capacity?
No. Because of unequal load sharing the second key is credited with φ = 0.75 only, giving a factor of 1.5 rather than 2.0. In the example below the permissible torque rises from 826 N·m to 1,239 N·m.
Why is the effective key length shorter than the total length?
With form A both ends are rounded and do not carry, so ltr = lPF − b. Form AB is rounded at one end (lPF − b/2); only the straight form B carries over its full length.
What limits the load capacity of a parallel-key connection?
Surface pressure against the weakest of the three strengths involved — shaft, hub or key — over the area htr · ltr. Since hubs are often made of a softer material, the hub usually governs.
Does the calculator also verify the shaft?
No. It verifies the parallel-key connection only. The shaft itself, with the notch effect of the keyway, has to be assessed separately, for example to DIN 743 under dynamic loading.
Does the calculator compute shear stress in the key?
No. Method C and this implementation run the check via surface pressure. A classical key shear check is not part of this calculation and is not reported here.
Can Method C be used for reversing torque?
No. The calculator is explicitly valid for one-direction torque only. Under reversal the load migrates between the flanks, backlash becomes relevant, and the assessment belongs in Method B.
Can I simply make the key longer to transmit more torque?
Only up to 1.3·d. In the example below, extending from 63 mm to 80 mm yields just 842 instead of 826 N·m, because the credited carrying length stops at 52 mm. Beyond that only a larger diameter, a second key or a different connection type helps.
C
Approximate Method C
What this calculator does
Torque, application factor, key geometry, and material strengths give the permissible torque, the utilization, and the required key length.
Parallel keys are useful when assembly, removal, or repair matters. The simplified model suits predominantly one-direction, quasi-static torque.
Reversing or impact torque requires a more detailed assessment than Method C.
02
Shaft and hub
Bearing pressure is only part of the verification; keyways weaken both components.
03
Product geometry
Select width, height, keyway form, and tolerances from DIN 6885.
Useful follow-up standards
01
DIN 6892 Method A/B
More detailed load distribution and component stresses for demanding cases.
02
DIN 743
Strength assessment of the keyed shaft under combined and alternating load.
03
DIN 6885
Standard key and keyway geometry for design and production.
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