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Natural Frequency - It is the system property, it can be altered by varying the mass, Stiffness and its damping.
Forcing Frequency - is the force applied on the system. it can be running speed or a frequency transmitted from near by member
Natural Frequency is the property of the system which depends on mass and stiffness.
Forcing frequency is the response of the system due to inaccuracies like unbalance, misalignment, etc.
Natural frequency is common for any system and it is not a problem until a forcing frequency matches / getting closer with it.
Well answered by Zafar. The natural frequency is just as he said, think a guitar string that produces a certain note when plucked; Your "plucking" the string injects "broadband" excitation into it (exciting it at all frequencies) but it only vibrates at one frequency to produce its specific note. It vibrates at its natural frequency. In simple terms; the natural frequency is the frequency at which a body "likes" to vibrate.
The forcing frequency is the frequency of vibration generated from a specifc fault in a machine. Consider a fan with4 blades rotating at50Hz. If you insert an obstacle into the path of these blades, that obstacle will be hit by a blade4 times per revolution of the fan;4*50 =200Hz therefore the obstacle will exhibit vibration at200Hz. This forcing frequency is called the "blade pass frequency". Calculation of possible forcing frequencies for any machine is the basis of running vibration analysis on this machine; if you know what frequency vibration each possible fault will generate in this machine then you can tell if this fault is present in the machine by looking for vibration at its forcing frequency.
Hope I've been helpful!
The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed is known as the natural frequency of the object. An object can have a number of natural frequencies for different modes of vibrations. When this body is in contact with an external source of vibration which is vibrating at a frequency which matches the natural frequency of the object, the object starts vibrating with higher amplitude which is called resonance.
Generally, modal analysis is done to find the natural frequencies of an object which is attached to the source of vibration. This is done to ensure that the natural fuequencies don't fall in the range of excitation frequencies to avoid resonance and hence probable failure.
Hope it answers your question.
