Page 13 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

It is important to note that an external force is required to
keep the mass in motion. All real systems are damped, that is they
will gradually come to their rest position after several cycles of
motion, unless acted upon by an external force. When this vibra-
tory motion is viewed with respect to time, it is seen as a sine
wave, as illustrated in Figure 1-2. Note that continued motion
would only repeat the sine wave as shown.

Upper limit

Rest position

M Lower limit
Time

Figure 1-2. Generation of a Sine Wave

Some of the characteristics of this vibratory motion are
PERIOD, FREQUENCY, DISPLACEMENT, VELOCITY, AC-
CELERATION, AMPLITUDE and PHASE. The sign wave
shown in Figure 1-2 represents one complete cycle, that is the
weight moved from its rest position to its upper limit back
through its rest position to its lower limit and then returned to
its rest position. Continued vibration of this spring mass sys-
tem would only repeat the characteristics shown in this single
cycle.

PERIOD

The period of the vibration, represented by the letter T, is the
time required to complete one oscillation. That is the total time
required for the mass to move from the rest position to the upper
limit, back through the rest position to the lower limit, and return
to the rest position.