Vibration Isolation Theory
Definition of common terms used:
|Amplitude||A||m||The magnitude of the displacement from mean position of a deflection caused by vibration.|
|Interference frequency||f||Hz||Typically the rotational frequency of a machine|
|Frequency||f0||Hz||The number of vibrations in a freely oscillating system per unit of time (/second for Hz) f0 = 1/2π√ (Kdyn/m)|
|Mass||M||Kg||The mass of the oscillating system|
|Spring force||F||N||The force exerted on or from a spring (or AV mounting)|
|Deflection||d||m||The deformation of a spring (AV Mount) from neutral position|
|Static spring stiffness||Kstat||N/m||The force in Newtons to compress the spring or mounting by 1m|
|Dynamic spring stiffness||Kdyn||N/m||Spring stiffness when an alternating force is applied|
|Tuning ratio||Z||-||The ratio between Interference frequency (f) and natural frequency (f0)|
|Interference force||Fs||N||The force transmitted to the base of an isolated machine|
|Impulse force||Fi||N||The force transmitted to the base of a rigidly mounted machine|
|Level of isolation||I||-||That part of the impulse force which is eliminated by the vibration isolation|
|Damping Coefficient||c||Ns/m||The linear viscous damping coefficient|
|Critical damping||ccr||Ns/m||The linear viscous damping at critical damping. i.e. no over oscillation after displacement|
|Damping factor||D||-||The ration between c and ccr|
|Reduction||R||dB||Isolation expressed in decibels R=20log(1/B)|
|Deflection||δstat||Mm||The static deflection for a spring = F/ Kstat|
An essential difference between rubber and a steel spring is that the rubber material has an inherently high damping capacity built in. This is particularly important for vibration isolation and shock absorption. When deformed and released, a wide hysteresis curve is generated showing a loss of energy as illustrated.
The lost energy is converted to heat which, in normal applications is readily dissipated.
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