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1.2 What Causes Vibration?
The basic cause is already evident in the simple mass-spring system of Figure 1.
it is an UNBALANCED FORCE, or system of forces (in Figure 1 it is the spring
force acting on the weight) acting on or through an ELASTIC OR RESILIENT
MATERIAL (in Figure 1, this is the spring). The unbalanced force may be due to
mass unbalance, such as in an eccentrically mounted rotor, or it may be due to
the variable inertia forces in machinery, which does not move uniformly, e.g.
crank-and-connecting-rod motion, linkages, cam-follower systems. In the latter,
the speeds and directions of motion of machine parts are continuously changing,
e.g. the needle motion in a household sewing machine, bucket motions in
earth-moving machinery, etc. Force unbalance can arise also from electric,
hydraulic and acoustic sources, e.g. transformer hum, water hammer, a
loudspeaker, etc.
1.3 Adverse Effects of
Uncontrolled Vibrations
The objectionable results of machine vibrations, if left uncontrolled, can be
several:
High stresses and force levels may be set up as a result of
vibrations and in extreme cases may lead to part failure. Such failure can be
sudden or gradual, as in fatigue. More frequently, there is increased wear of
parts and unsatisfactory equipment performance. This requires increased
maintenance and may also involve downtime of equipment. For example, in a
machine tool with excessive vibrations, parts may be inaccurately machined and
subsequently rejected. In other cases, an inadequately cushioned machine may
walk away on its foundation. And finally, noise may become excessive,
independent of stress levels, consumer product acceptance maybe jeopardized, and
working conditions may become unacceptable. Usually, the objectionable results
are a combination of these circumstances.
1.4 Principles of Vibration
and Shock Isolation
In discussing vibration isolation, it is useful to identify the three basic
elements of all vibrating systems: the equipment (component, machine, motor,
instrument or part); the vibration mount or isolator (resilient member); and the
base (floor, base plate, concrete foundation, etc.); the vibration mount is a
resilient member (rubber pad, spring or the like), which is interposed between
the equipment and the base. It is usually quite small.
If the equipment is the source of the vibration, the purpose of the
vibration mount is to reduce the force transmitted from equipment to base. The
direction of force transmission is from equipment to base. This is probably the
most common case.
If the base is the source of the vibration, the purpose of the
vibration mount is to reduce the vibrating motion transmitted from the base to
the equipment. The direction of motion transmission is from base to equipment.
This case arises, for instance, in protecting delicate measuring instruments
from vibrating floors, etc.
In either case, the principle of the cushioning action of the
vibration isolator is the same. The isolator is a resilient member. It acts both
as a time delay and a source of temporary energy storage, which evens out the
force or motion disturbance on one side of the vibration mount and transmits or
meters out a lesser, controlled disturbance, at the other end of the mount.
A good vibration mount, therefore, slows equipment response to a
force- or motion disturbance. In engineering terms, the characteristic of a good
vibration mount is that the natural frequency of the equipment with the mount is
substantially lower than the frequency of the vibration source (forcing
frequency). The design of a suitable vibration mount insures that this is the
case. Conversely a poorly designed mount, having an undesirable frequency
characteristic, can be worse than no mount at all.
In addition to its function as a time delay and source of temporary
energy storage, vibration mounts can also function as energy dissipators or
absorbers. This effect is usually produced by the damping characteristics of
materials, viscous fluids, sliding friction, and dashpots,
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