Page 142 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 133

must be taken because the vibration profile and energy levels generated by a machine
may vary depending on the location and orientation of the measurement.

7.5.1 Mass, Stiffness, and Damping

The three primary factors that determine the normal vibration energy levels and the
resulting vibration profiles are mass, stiffness, and damping. Every machine-train is
designed with a dynamic support system that is based on the following: the mass of
the dynamic component(s), specific support system stiffness, and a specific amount of
damping.

Mass
Mass is the property that describes how much material is present. Dynamically, the
property describes how an unrestricted body resists the application of an external
force. Simply stated, the greater the mass, the greater the force required to accelerate
it. Mass is obtained by dividing the weight of a body (e.g., rotor assembly) by the
local acceleration of gravity, g.

The English system of units is complicated compared to the metric system. In the
English system, the units of mass are pounds-mass (lbm) and the units of weight are
pounds-force (lbf). By definition, a weight (i.e., force) of one lbf equals the force pro-
duced by one lbm under the acceleration of gravity. Therefore, the constant, g c, which
2
has the same numerical value as g (32.17) and units of lbm-ft/lbf-sec , is used in the
definition of weight:

Mass* g
Weight =
g c

Therefore,

Weight * g c
Mass =
g

Therefore,

*
lbf lbm ft
Weight * g c
Mass = = ¥ = lbm
g ft lbf *sec 2
sec 2
Stiffness
Stiffness is a spring-like property that describes the level of resisting force that results
when a body changes in length. Units of stiffness are often given as pounds per inch