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What you need to know before starting an assembly ...

Degrees of freedom and mechanical connections (linking)

When two pieces are not in contact, it is assumed that any displacement of one relative to the other can be defined as a combination of 6 independent elementary motions along the axes of an orthonormal coordinate system :

  • - 3 translations parallel to the reference axes, Tx, Ty, Tz,
  • - 3 rotations around axes parallel to the reference axes, Rx, Ry, Rz.

It is said that there are 6 kinematic degrees of freedom (DOF) between the two pieces.


As soon as 2 pieces are in contact, a mechanical connection between the two parts modifies or forbids one or more of the 6 possible elementary movements.

Whatever the constructive solution adopted by the designer, a link (theoretical model associated with this constructive solution) is characterized from the kinematic point of view by :

- the number and nature of his degrees of freedom,

- its name, its orientation in space and the standardized symbols that represent it.




The characteristics of the elementary links are accessible below from the theoretical contacts :
Note: The "theoretical" contact represents the common part of the surfaces in contact of the two parts. We speak of point contact when the common surface is writable in a square of very small dimension, of linear contact when it is a narrow band compared to its length ...)

Click on the icon corresponding to your example to characterize its linking model.