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Technical data are subject to change without notice.
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SPM Instrument AB • Box 4 • S-645 21 Strängnäs • Sweden
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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 sim-
ply 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 particu-
lar 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 elec-
tric 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 struc-
tures 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 sub-
ject 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. Vibra-
tion will increase above the normal level and show
that the machine condition is getting worse.
Usually the deterioration accelerates: heavier vibra-
tion will further weaken the structure which in turn will
raise the vibration level.
Fig. 3
Fig. 1
Fig. 2
Small
force
Weak
foundation
Stiff
foundation
Large
force
Structural
looseness
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