Theory of Machine
To connect two parallel and coplanar shafts the following type of gearing is used

Spiral gear
None of the listed here
Spur gear
Bevel gear

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Theory of Machine
The static balancing is satisfactory for low speed rotors but with increasing speeds, dynamic balancing becomes necessary. This is because, the

Effects of unbalances are proportional to the square of the speed
Unbalanced couples are caused only at higher speeds
Unbalanced forces are not dangerous at higher speeds
Effects of unbalances are directly proportional to the speed

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