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strive
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Homework Statement
Hi
Can someone please tell me how to set equations for the following problem:
A cord of length L is wound on a hollow bobbin (inner radius r and outer radius R). Cord on cord friction (static) coefficient (Ctc) and cord on bobbin (Ctb) friction (static) coefficient is given, cord elasticity modulus (E) is given, cord radius is given and cord tensile strength (sigma) is given. (any other necessary data can be obtained)
What is the max pressure on the bobbin internal radius (reaching cords braking point) and what is the necessary force on the cords ends to hold the winding from unwinding?
(The bobbin is ideally elastic, thus it only serves to give shape to the cord winding and to equally distribute the pressure.)
Homework Equations
The Attempt at a Solution
I presume that first winding thickness (denoted t) must be obtained from cord length and its diameter and from bobbins outer diameter (this is done).
If this winding was a solid wall the max internal pressure would be simple to calculate by:
p=t*sigma/r (presuming bobbin wall thickness is small enough to be ignored.
But this is a cord. Each loop (of the winding) is in contact with several others. Should I presume the friction (static) between these contacts acts like a binder in composite materials?
If so how do I calculate the actual strength of the wall (SIGMAwall) in order to use it in the above equation?
Also as the cord has a circular cross section this leaves some empty space in the windings cross section and this leaves space for elastic deformation of the cord (of its cross-sectional shape), thus I predict some tension during winding of the cord might help to distribute the load uniformly among the cord layers.
Should I presume the force on the cord ends would be equal to this force subtracted by the sum of the friction (static) force on the last layer of the winding?
Thank you for your time
p.s.: regarding my mathematical/physical background – I am a last year student of mechanical/aerospace engineering (undergraduate)