insulation d8
mountings useful
60
40
20
-20
-40
-60
0125
0-25
0.5
1 √2 2
frequency ratio forcing natural
Fig. 5 Effect of mountings on energy reduction.
The efficiency of a mounting system will, in practice, be less than the theoretical value calculated from the static deflection alone because the dynamic stiffnesses of isolating mate- rials are much greater than the static values. Fig 5 illustrates the effect on energy reduction of resilient machine mountings and shows that if the fre- quency ratio is less than √2:1 there is no benefit to be derived from the mountings.
The relationships given in Fig. 1 (FE Builder November 1970) between human sensitivity and amplitudes and frequencies of vibrations can be used in choosing a suitable mounting. If a machine is producing vibrations that constitute some degree of nuisance, the information given in Fig. I would indicate by how much the amplitude should be reduced to bring the vibra- tion down to a tolerable level and from this a suitable mounting could be selected.
For example, if a machine was pro- ducing vertical amplitudes in nearby property of 20μ at a frequency of 30 Hz, the vibration intensity would be deemed annoying (Dieckmann K-value
=
00
8
16
32
pad of some sort under the machine are all too often useless, generally be- cause the mounting is too stiff and the low frequency structural vibrations have been virtually unchanged; they might even make matters worse.
Siting of machinery
The amplitude of vibration will usually diminish with distance from the source. Machinery or plant causing vibrations should, therefore, be kept as far as possible from buildings likely to be affected.
No precise relationship between amplitude and distance can be stated since both the nature of the ground and the type of vibration or ground shock are involved, but in many cases doubling the distance will reduce the amplitude to about one-third.
Protection of apparatus
The vibration problem should be considered at an early stage by the designer in collaboration with the client (or user) to ensure that vibration levels will be acceptable for the type of apparatus to be used.
Sensitive apparatus should, prefer- 3). If the machine was then fitted ably, not be placed on brackets attach- with a mounting having a natural freed to walls. Benches should be robust quency of 10 Hz for which the requir- ed static deflection under load would be about 2.5 mm, the amplitude would, theoretically, be reduced to about 2.5μ and the vibration would then be just perceptible (K-value = 0.4).
Makeshift remedies such as placing a
and kept from direct contact with walls. Excessive vibration of floors due to people walking is a common source of trouble, particularly in a suspended timber floor, or one of light construc- tion. Well-designed floors, of adequate stiffness, reduce the risk. Floor vibra-
tions can sometimes be reduced by fitting dynamic vibration absorbers. The low natural damping can also be improved by the use of 'sandwich layers' a fairly new technique.
Where floor vibration is likely to be important, sensitive equipment should be located away from the middle of a room. In general, very delicate work should not be carried out on upper floors. The disturbing effects of door- slamming should also be considered; swing doors or special doors closers may have to be provided.
After all other precautions have been taken it may still be necessary to protect individual pieces of sensitive equipment. One method is to build an isloated pier upon which the apparatus can stand. The pier, of brick, stone, or concrete passes through the floor into the foundation or basement of the building.
The gap between floor and pier should be left open if possible, but if material is inserted for dust-sealing or waterproofing it should be of a suit- able flexible type. Care should be taken to ensure that the base of the pier does not pick up vibrations from other sources; if there is a possibility of this, suitable anti-vibration material can be inserted either at the base or at the top of the pier.
In many cases, the use of com- mercially available instrument mounts will avoid the need for more compli- cated measures. The mountings are usually of rubber, loaded in shear, and are similar to some of the mountings used for machinery but of smaller size and load capacity.
A very wide range is available, the appropriate type depending on the fre- quency of vibration affecting the ap- paratus, the degree of isolation re- quired and the weight to be supported. Some types of apparatus are affected by amplitudes of 0.1μ or less, or by very low values of acceleration, and need a very high degree of isolation.
Special mountings, developed parti- cularly for use in aircraft, are also available for the protection of racks of electronic and electrical equipment. Sensitive chemical balances may re- quire special mounting systems that in- corporate damping arrangements, and a self-contained mounting unit with viscous damping has been marketed. Where isolating systems of very low natural frequency are required, special design may have to be undertaken, and steel springs, rather than rubber unit mountings, used. Air-bellows mount-
Far East BUILDER, December 1970