Key (engineering)

A key is a machine element in mechanical engineering and is used to implement a shaft-hub connection. The connection is form-fitting and is used to transmit torques (eg drive shaft - gear ).

Construction

The key is a solid, elongated metal part having a rectangular cross -section, is inserted into a correspondingly machined keyway in the shaft and protrudes from this.

The associated hub is provided with a continuous groove, and cleared, and is pushed to the assembly axially on the keyway. The key wears by form-fitting at their flanks. It thus acts as the carrier and transfers the torque of the shaft on the driven member ( in the case of a drive shaft ) or the torque of the driving member to the driven shaft (in the case of an output shaft ). In the axial direction of the wheel must be secured on the shaft to prevent displacement. Common forms of axial fixation are shaft shoulder / backup ring or shaft shoulder / locknut.

Dimensioning

The size of the key results

  • For the cross-section of the shaft diameter (for example 40 mm, the key cross-section ( width x height ) is provided for the 12x8 mm shaft diameter) and
  • For the length of the torque to be transmitted (calculation on surface pressure between the hub and the keyway and between keys and shaft, the latter may be omitted often because usually the material of the shaft is stronger than that of the hub ).

Calculation

The calculation is performed only on surface pressure, since the standard dimensions are chosen for the key so that the allowable shear stress is not exceeded when the permissible surface pressure is observed.

Standard

Keys are standardized according to the shape and dimensions in DIN 6885. This standard specifies, inter alia, also found that the tolerance of the slot width a fit with the tolerance P9, N9 or JS 9 after the fit system unit shaft according to DIN 7155 is used. The key corresponds to a unit shaft with tolerance position h (upper dimension = 0). Design and calculation bases of the key connections to DIN 6892 defines the firm.

Variants

There are rundstirnige Keys, SQUARE- springs, rundstirnige Parallel Keys with holding and jacking screws and washer Keys.

The usable length of a rundstirnigen key is the total length minus the width of which, as the two-sided curves do not wear in the hub. In geradstirnigen featherkeys, the total length should be recognized as carrying length.

A different kind of the keys are the disc springs, formerly referred to as Woodruff key. This is not milled with an end mill, but with a disc cutter a semi-circular groove in the shaft and inserted a semi-circular disc as a wedge. The hub is the same as stated in other keyways. The disadvantage of this variant is that the shaft is weakened by the necessary, deeper groove more, so this embodiment is less frequently used.

Pros and Cons and Alternatives

  • Benefits of keyways: inexpensive
  • Simply montier-/demontierbar
  • Drawbacks of keyways: Notch effect on the wave
  • The keyway weakens the load-bearing cross -section of the shaft and hub
  • Additional protection necessary against axial displacement
  • Conditionally suitable for load change of direction
  • Not suitable for shock load
  • Transmittable torque is relatively low
  • Produces unbalance ( Remedy by several symmetrically arranged featherkeys possible)
  • Alternatives to keyways: Press Add
  • Tooth profile, spline connection
  • Polygonal profile
  • Disc springs
  • Taper bushes (eg pulleys )
Engineering fit
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