in power transmission why compound shafts(shafts at varying diameter and varying sections) are used?
In a nutshell:
- Ability to assemble
- Need for additional components such as pulleys, gears etc...
- Reduced/increased localized stresses
- Vibrational modes avoidance
The moment increases the closer you get to the base of the tower, therefore more section is required to resist this moment. The moment is generated from tension in the wires, wind on the wires, wind on the pole etc. If no guy wires are used, then the tower is modeled as a cantilevered beam. The deflection of the tower can also be controlled by decreasing/increasing the section and taper of the shafts.
The shafts stack on top of one another, like a telescope. Breaking the entire poles into pieces also makes shipping a lot less dramatic.
I completely missed that definition of "power transmission." I guess the OP should clarify what context they were referring.
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