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Specifying the right vibration damping levelling element

Specifying the right vibration damping levelling element
Elesa UK managing director Nigel Pritchett looks at selection criteria for vibration-damping levelling elements, and guides us through the product selection process.

Over recent years, there has been a steady improvement in performance of industrial machines leading to great improvements in both working speed and precision. This has stemmed from improved design and better quality of manufacture of the machines themselves. Nonetheless they still operate by means of moving parts and this creates vibration. These moving bodies or non-balanced vibrating masses create vibrations or shocks which can cause malfunction and reduction of the machine lifespan and/or that of adjacent equipment. 

Where contact with personnel is involved then there is also significant concern for the operator - this is covered generally by the European directive about safety and healthy working with reference to noise and vibrations. Specifically mechanical vibration safety regulations for workers, in compliance with European directive 2004/44/CE 25 June 2002, prescribe the evaluation and the location of the health risks of workers exposed to mechanical vibration.

There are two primary classes of isolation from unwanted vibration. Active Isolation isolates and reduces the propagation of the vibration produced by the system, while Passive Isolation isolates and protect the system (for example measurement instruments) from vibrations coming from external sources. Vibration characteristics that need to be considered include frequency, oscillation amplitude, disturbing frequency, natural frequency, rigidness and isolation.

Frequency, measured in revolutions per minute (rpm) or cycles per second (Hz), is the number of oscillations or cycles done in a time unit. The frequency is a function of the mass and the rigidness of the element in subject. With reference to the oscillation axis, oscillation amplitude is the maximum movement of the mass. The oscillation amplitude is a function of the energy received by the oscillating mass, higher frequency means an oscillation amplitude reduction.

Disturbing frequency is the frequency of the disturbing vibration produced by an on-duty machine. In general, it coincides with the number of rotations of the engine (RPM or Hz).

When there are no external disturbances applied on the mass, the natural frequency is the vibration frequency of the system which is supported by the vibration-damping levelling elements (in RPM or (Hz). The system can be classified by analysing the ratio between disturbing frequency and natural frequency. If the ratio is less than 1, the system is rigid, a result opposite to that required. A ratio of between 1 and √2 indicates the resonance area - dangerous and uncontrolled evolution. A ratio greater than √2 indicates isolation of the disturbing frequency.

Rigidness is the load that is applied to the vibration-damping element to a produce a deflection of 1mm (in N/mm). Isolation is the reduction degree of the energy due to the disturbing frequency (%).

Choosing the right vibration damping element starts with defining: the disturbing frequency (the frequency of the disturbing vibration produced by a machine when it is on duty - in general, it coincides with the rotational speed of the engine); the static load applied to every single vibration-damping element (N); the isolation degree required (%); the damping disk deflection value (mm) under a given load; and the stiffness, which is the load that produces a deflection of 1mm (N/mm) when it is applied to the vibration-damping element.

All of this information can be readily obtained and checked on graphs which Elesa, for example, publishes on its datasheets. As an example, suppose an 80% isolation degree is required, with application conditions where the disturbing frequency is 3,000rpm, and the load applied to every levelling element is 4,000N. The graph shows that with a 3,000rpm disturbing frequency and an isolation degree of 80%, the deflection obtained is 0.6mm. Dividing the load applied by this deflection gives a required stiffness of 6,666N/mm. The correct vibration damping element can then be selected from the supplier's product range.
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