Technical data are subject to change without notice.
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General remarks on machine vibration
In every moving machine, part of the force that makes it work acts on the machine itself. Since no
structure or machine is perfectly rigid, any force acting on it will cause vibration.
Up to a degree, vibration is tolerated because it simply cannot be avoided. Machines are designed
to withstand a normal amount of vibration for a long period of time. To assess the condition of any
particular piece of plant, one has to determine its normal vibration level, then measure the actual
amount of vibration and compare the two values.
What level is normal?
To decide what is normal one has to consider
• the function of the machine and the forces involved
• the rigidity of the machine structure.
A large diesel engine vibrates more than a small electric motor – the forces involved are very
different. More force is needed to vibrate a machine on a stiff concrete foundation than it takes to
shake the same machine on a flexible metal frame. The machine structures are different and so are
their normal vibration levels.
Vibration tends to increase
Due to changes in the operating conditions and the mechanical state of machines, vibration levels
are subject to gradual or sudden changes. Loose fixing bolts or excessive bearing play will make
the structure less rigid – vibration will increase.
A growing soot layer on the impeller blades of an exhaust fan adds to the out of balance forces.
Vibration will increase above the normal level and show that the machine condition is getting
worse. Usually the deterioration accelerates: heavier vibration will further weaken the structure
which in turn will raise the vibration level.
Structural
looseness
Weak
foundation
Stiff
foundation
Small
force
Large
force
42 Measuring vibration
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