Reasons for isolating the effects of shock
Shock is created by impact of one mass against another e.g. during operation of power presses, forging machines, drop hammers etc. The shock impulse caused by the impact travels through the machine structure as a deflection wave. If the machine is rigidly connected to its foundation this deflection wave enters the foundation and the surroundings. The shock will generally cause the affected masses to vibrate at their own natural frequencies.
Reduction in shock severity by use of suitable isolators is achieved by the isolators storing the energy of the shock through isolator deflection and subsequent release in a smoother form over a longer period with lower overall amplitude.
A shock pulse may contain frequency components from 0-. It is therefore not possible to avoid resonance with the isolator/mass. If however the duration of the shock pulse is less than one half period of the isolation system resonance may not be serious.
Figure 4.1 shows the output force (into the supporting floor) v time levels from a machine produced shock wave.
Case 1 – the machine under consideration is connected directly to the supporting floor. Case 1 shows a high level of force over a relatively short duration.
Case 2 – typifies a machine installed on spring or elastomeric isolators in conjunction with a foundation block.
It can be seen that the same amount of energy is transmitted in both instances. However in case 2 the energy is transmitted over a much larger time scale resulting in a substantially lower peak force. In reality the force transmitted will present itself as noise and structure borne vibration detectable by humans, it is therefore desirable in most instances to keep the peak value of transmitted force as low as possible by using on spring or elastomeric isolators between the machine and foundation or an elastically supported foundation block.
Shows a machine/structure that is rigidly connected to its foundation. The peak force into the structure is very high and of relatively short duration. Essentially all the force that occurs in the machine is transferred to the structure with the exception of that which is absorbed by the machine.
Illustrates the use of elastomeric or spring isolators between the machine and the supporting foundation. In this scenario with the correct isolator specification the peak force transmitted to the supporting foundation is significantly reduced resulting in reduced structure borne noise and transmitted vibration.
Illustrates the use of spring or elastomeric isolators supporting a foundation block. In this instance the peak force transmitted is reduced to virtually zero. The foundation block increases the system mass and reduces machine vibration and movement through mass damping.