Friction#

If a cable runs over a contact point then friction can increase the extreme tension in that cable.

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

Generic formulation#

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.

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 results than prescribed by DNVGL. If 55% + 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.

DAVE#

Friction is applied during post-processing only. It is not considered when calculating the equilibrium positions. This means that the system with friction is, in general, not in equilibrium.

This also means that friction does not affect the loads in shackles.