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Technical data are subject to change without notice.
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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 slight movements.
The forces causing movement are usually cyclic, that
is they operate regularly first in one direction and
then in another. They can act in two main directions,
like the up and down forces associated with piston
engines, or they can rotate with the shaft, like out of
balance forces on a fan. They move the machine back
and forth from its rest position: the machine vibrates.
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 and
type of vibration and compare the two values.
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.
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. Vibration will increase above the
normal level and show that the machine is getting
worse. Usually the deterioration accelerates: heavier
vibration will further weaken the structure which in
turn will raise the vibration level.
