Page 55 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Vibration Due to Unbalance
Chapter
Chapter
Chapter
Chapter
Chapter
Vibration
Vibration
Vibration
Vibration
Vibration
Due to Unbalance
Due to Unbalance
Due to Unbalance
Due to Unbalance
Due to Unbalance

INTRODUCTION

nbalance is the most common source of vibration in rotating
equipment. Vibration due to unbalance occurs at a fre-
U U U U U quency of 1 × rpm of the unbalanced element, and its am-
plitude is proportional to the amount of unbalance. Normally, the
vibration signal is measured in both the vertical and horizontal
planes. With a simply supported rotating element, that is a bear-
ing at both ends of the shaft, the vibration due to unbalance will
be in the radial plane with very little vibration sent in the axial
direction. In the case of an over hung rotor, high axial vibrations
may occur, and the amplitude in the axial plane may equal those
measured radially. See Figure 4-9.
The force generated by unbalance can be expressed as:

2
F = 1.774 * (rpm/1000) * in-oz. (4.1)
Where:
F = Force in pounds.
rpm = is the rpm of the unbalanced element.
in-oz. = inch ounces of unbalance.

NOTE: An inch-ounce of unbalance is simply the amount of un-
balance weight in ounces times the distance from the center of
rotation, expressed in inches. Thus an inch-ounce can be a one-
ounce weight at one inch from the center of rotation, or a tenth of
an ounce at ten inches from the rotational center.