Page 15 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions
VELOCITY

The velocity of a vibrating object is continually changing. At
the upper and lower limits, the object stops and reverses its direc-
tion of travel, thus its velocity at these two points is zero.
While passing through the neutral or position of rest, the
velocity is at its maximum. Since the velocity is continually chang-
ing with respect to time, the peak or maximum velocity is always
measured and commonly expressed in inches-per-second peak.
In the metric system, vibration velocity is measured in meters
per second peak. When expressing the vibration characteristic in
terms of velocity, both the displacement and frequency are consid-
ered. Remember velocity is inches-per-second, and thus the dis-
placement [inches] and the frequency [times per second] are
considered.

ACCELERATION

Again referring to Figure 1-3, the peak or maximum accelera-
tion is shown to be at both the upper and lower limits of the mass
travel. Acceleration is the rate of change of velocity with respect to
time. It can also be expressed as the rate of change in distance with
respect to time, with respect to time. That is it is the change in
displacement with respect to time squared. Normally, vibration
acceleration is measured in terms of G’s or number of times the
normal force due to gravity. Gravitational force has been standard-
ized as 32.1739 feet per second per second or 386.087 inches per
second per second. In the metric system, the standard for gravity
is 980.665 centimeters per second per second. Once again, accel-
eration is measured as G’s peak.
Since the vibrating object must reverse course at the peak
displacements, this is where the maximum acceleration occurs.
Like velocity, acceleration is constantly changing, and the peak
acceleration is usually measured.
The importance of measuring the vibration in terms of accel-
eration is best understood by examining Newton’s first law of
motion: