Local friction calculation

Local friction is applied during post-processing 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 doubled-doubled sling this results in a more conservative result than prescribed by DNVGL. If 55% x 55% from DNVGL is desired for a doubled-doubled 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.