Global effects of friction#
Friction between a rope (sling, grommet) and a steel surface has an effect on the tension in that rope. Besides that, it may also influence the equilibrium geometry and forces in other elements of the model.
Consider the following two situations. On the left the base-case without friction, on the right the equilibrium position if friction is present at the hook:
The green line is a doubled grommet. The CoG of the lifted object is in the center. In the base-case the load tilts towards the right due to the weight of the shackle.
When friction is present at the hook the module has to be tilted accordingly (right) for the whole system to be in equilibrium.
On the left, the load on the shackle is 57t, on the right the load is 59t. This is obviously not easy to calculate by hand as the friction at the hook is a function of the geometry and the geometry is a function of the friction.
When performing a rigging analysis with DAVE and using “Global” for all friction calculations then effects like this are fully included.
Further reading:
How the loadcases are constructed is further described in [loadcases for global friction](friction global loadcases)
Reduction of amount of loadcases by shifting the friction at small angles to adjacent connections is described [loadcase simplification](Global friction over small angles - simplification)