What Cause Machine Vibration?
What Cause Machine Vibration?
(a) Repeating forces
Let’s think a boat anchored in a bay. Waves are hitting the sides of the boat, and as long as the waves continue to act the boat will rock. The boat would be rocking because the waves would be exerting a repeating force on the boat, a force of a pattern repeated over and over again.
Most machine vibration is due to repeating forces similar to those that cause the boat to rock. Repeating forces like them act on machine components and cause the machine to vibrate.
Repeating forces in machines are due to rotation of imbalanced, misaligned, worn, or improperly kept machine components. Examples:
This causes a machine to vibrate. If parts become loose, vibration which is normally of tolerable range might become unrestrained and excessive.
Imagine a child swing freely in air, without the child propelling himself or anyone pushing him. If you observe the motion closely you will see the child swinging at a particular rate.
The rate of the child’s free-swing is a physical property of the child-swing system — like the weight of the child is a physical property of the child. It is the rate at which the child will tend to swing when seated on that swing. It is the child’s most natural swinging rate on the swing, and the only way to change it is to interfere with the natural swinging by propelling himself with his feet, changing his posture, rubbing his feet on the ground and so on.
Machines also tend to vibrate at certain oscillation rates. The oscillation rate at which a machine tends to vibrate is called its natural oscillation rate. This is the vibration rate most natural to the machine, that is, the rate at which the machine ‘prefers’ to vibrate.
A machine left to vibrate freely will tend to vibrate at its natural oscillation rate. Most machines have more than one natural oscillation rate. For example, a machine comprising two substructures of different natural oscillation rates will exhibit at least two natural oscillation rates.
In short, the more complex the machine, the more natural oscillation rates it has.
Now imagine the child is on the swing. If you aide the swinging motion by repeatedly pushing, you would expect the child to swing higher and higher over time.
If our pushing rhythm is such that he is sometimes pushed down while he is going high, we would not expect him to swing properly. To make him swing higher and higher, the pushing rhythm must be in harmony with his natural oscillation rate.
What happens if a machine is pushed by a repeating force with a rhythm matching the natural oscillation rate of the machine?
A similar situation will arise as exemplified above and the machine will vibrate more and more strongly due to the repeating force encouraging the machine to vibrate at a rate it is most natural with. The machine will vibrate vigorously and excessively.
A repeating force cause resonance and might be small and might originate from the motion of a good machine component. Such a mild repeating force would not be a problem until it begins to cause resonance.
We conclude now.
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