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.

image-20230530143441270

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:

image-20230530145253355

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.

image-20230530152805971

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.