Articulation stiffness, damping, and friction can be defined for all of the relevant articulation types, including ball/socket, universal, and hinge.
As articulations only allow rotational motion, only the rotational stiffness, damping, and friction need to be specified. These are defined with respect to the local articulation axes.
To define any articulation stiffness the input values should have units of moment per radian. The corresponding restoring moment acting on the j-th structure in the local articulation axes is
 | (8–11) |
where 
(m = 1, 3) are the stiffness
coefficients of relative rotations along the local articulation axes
and 
is a 3×6 matrix.
Denoting 
(m = 1, 3) as the damping coefficients
of relative rotations along the local articulation axes, the moment
due to articulation damping acting on the j-th structure in this local axis system is similarly
expressed as
 | (8–12) |
Coulomb friction can be optionally defined in time domain analyses, and always uses a local right hand axis system Ax'y'z' (where the x'-axis is aligned with the direction of the instantaneous relative rotational velocity of the two articulated structures). For a hinge, this will always be the hinge axis; for universal or ball/socket joints it will vary as the two structures move relative to one another. The frictional moment is given by
 | (8–13) |
where 
if the relative rotational velocity is less than 0.001 rad/s and 
otherwise, while 
(m = 1, 4) are the input friction
coefficients. These are not conventional dimensionless friction coefficients (as used in the
general friction force equation 
), but are factors to be applied to the appropriate reaction forces (Fx’
, Fy’
, Fz’
) and moments (My’
, Mz’
) to give the frictional moment. 
and 
must not be negative, and the maximum permissible value of any element of 
is 0.025 when a [kg, m, s] unit system is used in the analysis. If another
unit system is used this maximum value criterion will be adjusted to 
, where 
is the acceleration due to gravity expressed in the current unit system and 
is the acceleration due to gravity expressed in the [kg, meter, second] unit
system. However, according to Equation 8–13, the factor 
is non-dimensional; hence it is simply required to be less than 0.025 (but may
also be negative).
Once this frictional moment is calculated, it will be transformed into the local articulation axes (LAA), local structure axes (LSA), or fixed reference axes (FRA) as appropriate.
To calculate the motions and reaction forces of the articulation-linked structures, the articulation stiffness, damping, and friction moments expressed in Equation 8–11 through Equation 8–13 should be converted into the fixed reference axes (FRA) and should then be assembled into Equation 8–10.