Unbalance is the most common source of vibration in machines with rotating parts. It is a very important factor to be considered in modern machine design, especially where high speed and reliability are significant considerations. Balancing of rotors prevents excessive loading of bearings and avoids fatigue failure, thus increasing the useful life of machinery.
Unbalance in a rotor is the result of an uneven distribution of mass, which causes the rotor to vibrate.
The vibration is produced by the interaction of an unbalanced mass component with the radial acceleration due to rotation, which together generate a centrifugal force. Since the mass component rotates, the force also rotates and tries to move the rotor along the line of action of the force. The vibration will be transmitted to the rotor's bearings, and any point on the bearing will experience this force once per revolution.
Since man began to build machines for industrial use, and especially since motors have been used to power them, problems of vibration reduction and isolation have engaged engineers.
Over the last 15 or 20 years a whole new technology of vibration measurement has been developed which is suitable for investigating modern highly stressed, high-speed machinery. Using piezoelectric accelerometers to convert vibratory motion into an electrical signal, the process of measurement and analysis is ably performed by the versatile abilities of electronics.
This article answers some of the basic questions asked by the newcomer to vibration measurement.
The calculation of the maximum allowable residual specific unbalance assumes that the mass of the rotor is evenly distributed about the centre of gravity. If the mass of the rotor is unevenly distributed, the calculations are a little more complicated.
In a perfectly balanced rotor, equal forces act on both ends of the rotor when it rotates. If the rotor is shaped as in Fig. 8, however, the forces at each end will be equal, but the allowable residual specific unbalance will be different for each bearing. The position of the centre of gravity divides the rotor in the ratio V3 : 2 /3. The sum of the moments about the centre of gravity must be zero. Therefore the residual specific unbalance at bearing A is 2 /3 of the total residual specific unbalance, while at bearing B it is V3 of the total.
The specific unbalance is used to calculate the size of trial masses, which are used during balancing to make temporary alterations to the mass distribution of the rotor, to determine the relationship between the specific unbalance and the bearing vibrations.
To estimate the value ol a suitable trial mass, the mass oi the rotor in kg and the radius m mm at which the corrections are to be made must be determined. The Maximum Residual MMR, in grammes, is given by:
A suitable trial mass is five to ten times the value of the Maximum Residual Mass.
Many people are needlessly apprehensive about performing their dynamic balancing procedure. This application note will demonstrate with the aid of several work examples, how easy it is to balance rotating machinery. Straightforward methods will be presented that make use of simple, HBK instrumentation to measure rotating parts running in their bearings at normal operating speeds.
Read the part two of this article.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
This will bring together HBM, Brüel & Kjær, nCode, ReliaSoft, and Discom brands, helping you innovate faster for a cleaner, healthier, and more productive world.
