why do rotating objects are designed to have rotational symmetry?

Question

it about circle

Answer 1

Lots of practical rotating objects don't have any kind of symmetry at all, let alone "rotational symmetry". Take the crankshaft in an engine , for example, which frequently don't have perfect rotational symmetry (when viewed axially)but are perfectly balanced nonetheless. So long the object is rotating about its exact center of mass, it'll spin smoothly. There may be other reasons why the mechanical designer would want an object of rotational symmetry, as for example drill bits or winch pulleys, or even gears, but it's wrong to think that in order for something to spin smoothly, it has to have rotational symmetry. It may just easier to design that way.

Answer 2

Because if they do not, then the rotational imabalance will cause a reciprocating force,