Keyways and keys DIN 6885
Parallel keys DIN 6885, also called keys, are machine elements that transmit torque between a shaft and a hub by form fit. The standard covers keys with rectangular or square cross-section, with straight ends (Form A) or rounded ends (Form B), for shafts from 6 mm to 500 mm (0,236 in to 19,685 in). They are commonly manufactured in carbon steel C45 (1.0503) with tensile strength of 590 MPa (85,6 ksi), or in stainless steel AISI 304 for corrosive environments. The keyway–key system allows fixed or sliding assembly, depending on the selected fit.
Standard dimensions
Section titled “Standard dimensions”The main dimensions of the key and keyways are selected based on the nominal shaft diameter. Standard tolerances for the key width are h9 (sliding fit) or p9 (fixed fit). The following table summarizes the width × height pairs (b × h) and keyway depths in shaft (t₁) and hub (t₂).
| Shaft diameter d (mm / in) | Width b (mm / in) | Height h (mm / in) | Keyway depth shaft t₁ (mm / in) | Keyway depth hub t₂ (mm / in) |
|---|---|---|---|---|
| 6 – 8 / 0,236 – 0,315 | 2 / 0,079 | 2 / 0,079 | 1,0 / 0,039 | 1,0 / 0,039 |
| 8 – 10 / 0,315 – 0,394 | 3 / 0,118 | 3 / 0,118 | 1,5 / 0,059 | 1,5 / 0,059 |
| 10 – 12 / 0,394 – 0,472 | 4 / 0,157 | 4 / 0,157 | 2,0 / 0,079 | 2,0 / 0,079 |
| 12 – 17 / 0,472 – 0,669 | 5 / 0,197 | 5 / 0,197 | 2,5 / 0,098 | 2,5 / 0,098 |
| 17 – 22 / 0,669 – 0,866 | 6 / 0,236 | 6 / 0,236 | 3,0 / 0,118 | 3,0 / 0,118 |
| 22 – 30 / 0,866 – 1,181 | 8 / 0,315 | 7 / 0,276 | 3,5 / 0,138 | 3,5 / 0,138 |
| 30 – 38 / 1,181 – 1,496 | 10 / 0,394 | 8 / 0,315 | 4,0 / 0,157 | 4,0 / 0,157 |
| 38 – 44 / 1,496 – 1,732 | 12 / 0,472 | 8 / 0,315 | 4,0 / 0,157 | 4,0 / 0,157 |
| 44 – 50 / 1,732 – 1,969 | 14 / 0,551 | 9 / 0,354 | 4,5 / 0,177 | 4,5 / 0,177 |
| 50 – 58 / 1,969 – 2,283 | 16 / 0,630 | 10 / 0,394 | 5,0 / 0,197 | 5,0 / 0,197 |
| 58 – 65 / 2,283 – 2,559 | 18 / 0,709 | 11 / 0,433 | 5,5 / 0,217 | 5,5 / 0,217 |
| 65 – 75 / 2,559 – 2,953 | 20 / 0,787 | 12 / 0,472 | 6,0 / 0,236 | 6,0 / 0,236 |
| 75 – 85 / 2,953 – 3,346 | 22 / 0,866 | 14 / 0,551 | 7,0 / 0,276 | 7,0 / 0,276 |
| 85 – 95 / 3,346 – 3,740 | 25 / 0,984 | 14 / 0,551 | 7,0 / 0,276 | 7,0 / 0,276 |
| 95 – 110 / 3,740 – 4,331 | 28 / 1,102 | 16 / 0,630 | 8,0 / 0,315 | 8,0 / 0,315 |
| 110 – 130 / 4,331 – 5,118 | 32 / 1,260 | 18 / 0,709 | 9,0 / 0,354 | 9,0 / 0,354 |
| 130 – 150 / 5,118 – 5,906 | 36 / 1,417 | 20 / 0,787 | 10,0 / 0,394 | 10,0 / 0,394 |
| 150 – 170 / 5,906 – 6,693 | 40 / 1,575 | 22 / 0,866 | 11,0 / 0,433 | 11,0 / 0,433 |
| 170 – 200 / 6,693 – 7,874 | 45 / 1,772 | 25 / 0,984 | 12,5 / 0,492 | 12,5 / 0,492 |
| 200 – 230 / 7,874 – 9,055 | 50 / 1,969 | 28 / 1,102 | 14,0 / 0,551 | 14,0 / 0,551 |
| 230 – 260 / 9,055 – 10,236 | 56 / 2,205 | 32 / 1,260 | 16,0 / 0,630 | 16,0 / 0,630 |
| 260 – 290 / 10,236 – 11,417 | 63 / 2,480 | 32 / 1,260 | 16,0 / 0,630 | 16,0 / 0,630 |
| 290 – 330 / 11,417 – 12,992 | 70 / 2,756 | 36 / 1,417 | 18,0 / 0,709 | 18,0 / 0,709 |
| 330 – 380 / 12,992 – 14,961 | 80 / 3,150 | 40 / 1,575 | 20,0 / 0,787 | 20,0 / 0,787 |
| 380 – 440 / 14,961 – 17,323 | 90 / 3,543 | 45 / 1,772 | 22,5 / 0,886 | 22,5 / 0,886 |
| 440 – 500 / 17,323 – 19,685 | 100 / 3,937 | 50 / 1,969 | 25,0 / 0,984 | 25,0 / 0,984 |
Standard key lengths range from 6 mm (0,236 in) to 400 mm (15,748 in), with increments of 2 mm up to 20 mm, then 5 mm up to 50 mm, and 10 mm for larger sizes.
Load capacity
Section titled “Load capacity”The parallel key transmits torque primarily by shear on its cross-section and by contact pressure on the sides. The shear load capacity is calculated with Tadm = (τadm · b · L · d) / 2, where τadm is the allowable shear stress of the material. For C45 steel with yield strength of 320 MPa (46,4 ksi), τadm = 64 MPa (9,28 ksi) is adopted with a safety factor of 2,5. The surface pressure on the hub must not exceed padm = 100 MPa (14,5 ksi) for steel hubs and 40 MPa (5,8 ksi) for gray cast iron hubs.
| Key material | Tensile strength Rm (MPa / ksi) | Yield strength Re (MPa / ksi) | Recommended τadm (MPa / ksi) |
|---|---|---|---|
| C45 (1.0503) | 590 / 85,6 | 320 / 46,4 | 64 / 9,28 |
| AISI 304 Stainless Steel | 520 / 75,4 | 210 / 30,5 | 42 / 6,09 |
| AISI 316 | 550 / 79,8 | 220 / 31,9 | 44 / 6,38 |
Selection criteria
Section titled “Selection criteria”Selection of the DIN 6885 key is based on three main parameters: shaft diameter, load type, and hub material. Width and height are determined exclusively by shaft diameter according to the standard table. The key length must be at least equal to the hub width, and at most 1,5 times the shaft diameter to avoid uneven pressure distribution. In drives with reversing rotation or pulsating loads, it is recommended to increase the effective length by 30 % or use a double keyway at 180°. For hubs made of soft materials (cast iron, aluminum), the surface pressure must be verified in greater detail and it may be necessary to increase the effective contact height.
Assembly considerations
Section titled “Assembly considerations”Correct assembly requires that the key slides with an H9/h9 fit in the shaft keyway and with a Js9/h9 fit in the hub, unless a fixed fit is specified. Keyways must have rounded edges (R≈0,2 mm / 0,008 in) to reduce stress concentration. In applications where the key transmits axial load, it must be secured with a set screw located at the center of the key. Never strike the key directly on the shaft keyway to avoid deformations; it is recommended to use a nylon hammer and a copper punch. The parallelism tolerance between the keyway flanks must be less than 0,02 mm (0,0008 in) per 100 mm (3,937 in) of length.
Selection tables by application
Section titled “Selection tables by application”The following table proposes typical configurations for common mechanical couplings based on DIN 6885, assuming key and shaft material in C45 steel and steel hub.
| Application | Shaft dia. (mm / in) | Key b×h (mm / in) | Recommended length (mm / in) | Approximate allowable torque (Nm / lb·ft) |
|---|---|---|---|---|
| Electric motor coupled to gearbox | 28 / 1,102 | 8×7 / 0,315×0,276 | 32 / 1,260 | 120 / 88,5 |
| V-belt pulley | 40 / 1,575 | 12×8 / 0,472×0,315 | 45 / 1,772 | 350 / 258 |
| Centrifugal pump coupling | 55 / 2,165 | 16×10 / 0,630×0,394 | 63 / 2,480 | 900 / 664 |
| Gearbox gear | 70 / 2,756 | 20×12 / 0,787×0,472 | 80 / 3,150 | 2100 / 1549 |
| Flywheel | 90 / 3,543 | 25×14 / 0,984×0,551 | 100 / 3,937 | 4500 / 3319 |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the keyway for a 30 mm shaft? The recommended key has a width of 10 mm (0,394 in) and a height of 8 mm (0,315 in), with a keyway depth in the shaft of 4 mm (0,157 in) according to DIN 6885.
What tolerances apply to the keyway width in the shaft? The keyway width in the shaft is machined with P9 tolerance for a fixed fit, which for an 8 mm (0,315 in) key results in a dimension of 8 mm -0,015/-0,051 (0,315 in -0,0006/-0,0020).
How much torque can a DIN 6885 14x9 mm key transmit on a 50 mm shaft? With a length of 56 mm (2,205 in) and C45 steel, the allowable torque is 540 Nm (398 lb·ft), limited by contact pressure in a steel hub of 100 MPa (14,5 ksi).
How is the key length dimensioned? The minimum length must be equal to the hub width (Lmin = Lhub) and the maximum must not exceed 1,5 times the shaft diameter to avoid localized overpressure; thus, for a 40 mm (1,575 in) shaft the maximum length is 60 mm (2,362 in).
What hardness difference should exist between the key and the shaft? It is recommended that the key have a surface hardness between 50 HV and 100 HV lower than that of the shaft, so that the key acts as a mechanical fuse; for example, a shaft with 350 HV is combined with a key of 280 HV to 300 HV.
What is the minimum surface roughness on the keyway flanks? The surface roughness Ra must be 3,2 µm (126 µin) or less on the contact flanks, thus improving pressure distribution and reducing the risk of galling.