Local friction calculation#
Local friction is applied during postprocessing only. It is not included during the calculation of equilibrium positions. As such it only affects the item on which it is applied.
Purpose#
The purpose of local friction is to include friction for rigging calculations if global friction can not be used.
DNVGL prescribes a 55%/45% load distribution over the two legs in case of a grommet or sling running over a round surface (hook, trunnion, shackle, etc.). This corresponds to a friction of 10% of the tension in the cable at the bend.
As a generic formulation, DAVE uses friction of 10% for each bend.
For grommets / loops#
For loops the standard 10% per bend results in a maximum tension in the cable of
\(T_{max,grommet} = T_{0} \cdot 1.1^M \) where \( M = floor(N/2)\)
The floor (N/2) part ensures the continuity of the tension over the length of the cable as illustrated in the following examples:
For a double grommet N=4 and M=2 resulting in \(T_{max} = T_{0} \cdot 1.1^2 \).
\(T_0\) is the total load divided by 4 meaning that, in terms of percentages, the maximum tension in the cable is \(25\% \cdot 1.1 \cdot 1.1 = 30.25\% \) which is the same as the 55% of 55% prescribed by DNVGL.
Slings#
For cables with distinct ends (eg Slings) the formulation is:
\(T_{max, sling} = T_{0} \cdot 1.1^N\)
as the tension on either end does not need to be equal.
Attention
For a doubleddoubled sling this results in a more conservative result than prescribed by DNVGL. If 55% x 55% from DNVGL is desired for a doubleddoubled sling then use “custom” mode and enter 1.21 as factor.
Special situations#
The generic approach is not always applicable. For example when rotating greased sheaves (49%/51% ) or sliding during upending (32.5% / 67.5%) needs to be considered.
In those cases the increase of the tension due to friction can be entered as a factor over \(T_0\). Some examples:
Factor 
Equivalent when \(T_0 = (1/2)\) Load 
Equivalent when \(T_0 = (1/4)\) Load 

1.00 
No friction 
No friction 
1.02 
49% / 51% 

1.35 
67.5% / 32.5% 

1.21 
55% x 55% 
Defining a custom friction factor can be done via the GUI or by setting the property friction_factor
of a node to the desired value. Set a negative friction_factor to trigger the use of the default behavior.