Static and dynamic balancing with portable equipment

Static And Dynamic Balancing Using Portable Measuring Equipment

Many people are needlessly apprehensive of performing their own dynamic balancing procedure. To help overcome these fears, this Application article starts by showing how very simple and straightforward such a process can be when using B&K equipment.

Engineering services“STATIC AND DYNAMIC BALANCING PART TWO” - HANDBOOK WITH AUTHOR: JOHN VAUGHAN.

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Introduction


This Application Note will demonstrate with the aid of several worked examples, how easy it is to balance rotating machinery. Straightforward methods will be presented that make use of simple  portable B&K instrumentation to  measure on rotating parts running in their own bearings at normal operating speeds. B&K machines that accept rotating parts and display balancing masses and positions immediately are described in separate publications on the Balancing  Machines Type 3905 and Type 3906.

Standards of balance achieved by the arrangements shown here compare favourably with the results obtained from far more  complicated and expensive balancing machines.

 

Definitions


Primary Balancing describes the process where  primary  forces caused by unbalanced mass components in a rotating object may be re­ solved into one plane and balanced by adding a mass in that plane only. As the object would now be completely balanced in the static condition (but not necessarily in dynamic) this is often known as Static Balancing.

Secondary Balancing describes the process where primary  forces and secondary force couples caused by unbalanced mass components  in a rotating object may be resolved  into two (or more) planes and balanced by adding  mass  increments  in those planes. This balancing process is often known as Dynamic Balancing because the unbalance only becomes apparent when the object is rotating. After  being  balanced dynamically, the object would be completely balanced  in  both static and dynamic conditions.
Static Balance, Dynamic Unbalance
The difference between static balance and dynamic balance is illustrated in Fig.1 . It will be observed that when the rotor is stationary (static) the end masses may balance each other. However, when rotating (dynamic) a  strong  unbalance  will be experienced.

 

Basic Theory

An object that imparts a vibration to its bearings when it rotates is defined as "unbalanced". The bearing vibration is produced by the interaction of any unbalanced mass components present with the radial acceleration due to rotation which together generate a centrifugal force. As the mass components are rotating, the force rotates too and tries to move the object in its bearings along the line of action of the force. Hence any point on the bearing will experience a fluctuating force. In practice the force at a bearing will be made up from a primary force due to unbalanced mass components in or near to the plane of the bearing, and a secondary force due to unbalanced couple components from the other planes.

If an  accelerometer  is  mounted on the bearing housing, the fluctuating vibration force can be detected, and an electrical signal sent to a vibration meter. The indicated vibration level is directly proportional  to the resultant of the unbalanced masses. The direction in which this resultant acts (i.e. the radius containing the centrifugal force) can be determined in an accurate way by comparing the phase of the fluctuating signal leaving the vibration  meter with a standard periodic signal obtained from some datum position on the rotating object.

It is now possible to define the unbalance at the  bearing  by means of a vector, whose length is given  by the magnitude of the unbalanced force (the measured vibration level). and whose angle is given by the direction of action of the force. Further, if the  resultant  unbalanced force at a bearing can be resolved into its primary (first order moments) and secondary (second order moments) components, it will be possible to balance the object.

 

General Measurement Methods

The basic measuring chain

Vibration level can be measured in terms of acceleration, velocity, or displacement. However as most standards for balancing are written in velocity terms, a legacy of the days when vibration was measured by mechanical velocity sensitive transducers, usually velocity will be the chosen parameter. Use of acceleration levels will tend to emphasize higher frequency components, while displacement will emphasize low frequency components.


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