White paper: Selection criteria and guidance
notes for vibration-damping levelling elements

By Nigel Pritchett, M.D., Elesa UK Ltd

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, prescribes the evaluation and the location of the health risks of workers exposed to mechanical vibration.

At Elesa we recognise two classes of isolation from unwanted vibration;

Active Isolation
To isolate and reduce the propagation of the vibration produced by the system.

Passive Isolation
To isolate and protect the system (for example measurement instruments) by vibrations coming from external sources.

Vibration characteristics:
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.

Oscillation amplitude
With reference to the oscillation axis this 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
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].

Natural frequency
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 [RPM] or [Hz].

The system can be classified by analyzing the ratio between disturbing frequency and natural frequency: 1) Ratio < 1, the system is rigid, a result opposite to that required. 2) Ratio between 1 e √2, resonance area, dangerous and uncontrolled evolution. 3) Ratio > √2, isolation of the disturbing frequency.

Rigidness
That is to say the load that is applied to the vibration-damping element produces a deflection of 1mm [N/mm].

Isolation
The reduction degree of the energy due to the disturbing frequency [%].

System Vibration Characteristics

1) Choosing the vibration damping element
Basic data required for correct choice of a vibration-damping element are:
- disturbing frequency, which is the frequency of the disturbing vibration produced by a machine when it is on duty. In general, it coincides with the number of rotations of the engine [rpm].
- the static load applied to every single vibration-damping element [N].
- the isolation degree required [%].
- damping disk deflection value [mm] under a given load.
- the stiffness, which is the load that produces a deflection of 1mm [N/mm] when it is applied to the vibration-damping element.

The data sheets of the ELESA catalogue refer to LW.A - Vibration-damping levelling elements and DVA.-Vibration-damping elements, contain all the information needed for the choice of the elements matching the specific working conditions.

2) How to choose the vibration-damping element
- with reference to the nomograph (graphic 1), intersect the disturbing frequency value with the isolation degree required (each isolation degree corresponds to a line on the nomograph) and define the deflection (static deflection mm).
- to obtain the required stiffness of the vibration-damping element, divide the load applied onto the vibration-damping element by the deflection value.
- compare the stiffness obtained with the stiffness shown in the table and choose the vibration-damping element which presents the nearest value (lower) to the calculated one.

3) How to choose the vibration-damping element - Nomograph
Check the values obtained:
- The deflection of the vibration-damping element chosen can be obtained in (graphic 2) on the basis of the static load.
- Intersect the disturbing frequency value with the vibration-damping element deflection value in the nomograph (graphic 1) to obtain the isolation degree offered by the vibration-damping element chosen.
- Compare the obtained value with the isolation degree required.

4) For example:
An 80% isolation degree is required.
Application conditions:
- disturbing frequency = 3,000 rpm;
- load applied to every levelling element = 4,000 N.
- Graphic 1 shows that with a 3,000 rpm disturbing frequency and an isolation degree of 80%, the deflection obtained is 0.6 mm.
- Divide the load applied by the deflection obtained to define the stiffness required, which is 4,000/0.6= 6,666 N/mm.
- Compare the stiffness value obtained (6,666 N/mm) with the values reported in the table. This value is within the stiffness value reported in the table for LW.A-120 (4,000 N/mm) and LW.A-160 (9,000 N/mm). Choose the vibration-damping element with the lower value that is LW.A-120.

5) For a further check
- graphic 2 shows that LW.A-120 deflection is 1mm.
- by intersecting the deflection value with the disturbing frequency of 3,000 rpm in the nomograph, the isolation degree obtained is 90%. This value is even greater than the required one; your choice has proved correct.

For further information on Elesa (UK) Ltd products, e-mail: sales@elesa.co.uk or view website: www.elesa.co.uk   

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