[MBDyn-users] MBDyn and Ropes-Pulley

MBDyn users list mbdyn-users at mbdyn.org
Fri Sep 2 21:16:10 CEST 2022


Dear JM,

You could use three node beam elements for the rope and rigid bodies for 
the pulley. The most difficult part is how to connect the rope to the 
pulley. One option would be to use module-triangular_contact which can 
be found here:

https://public.gitlab.polimi.it/DAER/mbdyn/-/tree/develop/modules/module-triangular_contact

It allows you to model unilateral contact and friction between several 
nodes (e.g. all the nodes of your rope) and a rigid surface (e.g. the 
surface of your pulley which is touching the rope). In your case you 
should define a contact between each node of the rope, using the radius 
of the rope cross section as spherical contact radius, and the pulley it 
is attached to. The surface of the pulley must be meshed by flat 
triangles (e.g. by using Gmsh). When you create the mesh for your rope, 
you have to ensure that a sufficient number of nodes from the rope is 
touching the pulley all the time. Otherwise you may loose the contact 
between the rope and the pulley. So, if the length of your rope is large 
compared to the radius of your pulley, this approach would become quite 
expensive.

Another much simpler and less expensive approach would be to develop a 
new loadable element which imposes an algebraic constraint for the 
distances between the center points of several pulleys. So, you would 
create one structural node at the center point of each pulley and at the 
end points of the rope. Then the algebraic constraint would enforce that 
the total length of the rope is unchanged or you could consider a 
certain expansion of the rope proportional to the tensile load. The 
exact definition of the algebraic constraint will depend on the topology 
of your hoist. I'm attaching a very simple example of such an algebraic 
constraint. Of course the corresponding Lagrange multipliers must be 
added to the equilibrium equations as well.

Best regards,

Reinhard Resch

On 01.09.22 19:25, MBDyn users list wrote:
> Dear all,
>
> I looked at MBDyn documentation and examples and did not identified 
> functions to model and simulate ropes - pulleys assemblies (for 
> exemple for mainsail hoist). Is there something that I could have 
> overlooked?
>
> The use case is not straightforward to model as a "distance" as there 
> are multiple pulleys attached to different locations, different 
> purchase in those, with some angles between the different parts...
>
> Maybe this does not fall in the category of problems to be addressed 
> by a tool like MBDyn.
>
> Best regards,
>
> JM
>
> _______________________________________________
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> MBDyn-users at mbdyn.org
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