Single plane field balancing

For dynamic field balancing, it is crucial to determine the actual effective point of unbalance in first round so that trial weight can be removed from this point of unbalance or added 180 degrees opposite to that point. Actual point of unbalance is the net effect of uneven mass distribution on the rotating body. Removing weight from this point will reduce the net unbalance of the rotating body. This is also equivalent to adding weight 180 degrees opposite to the actual point of unbalance. This has two advantages:

  1. Removal of trial weight will NOT increase vibrations due to unbalance. This is very useful in case of high speed machines and where field balancing needs to be performed in minimum number of trial rounds. If vibrations increase, then other causes of vibrations needs to be analyzed.
  2.  Problems due to tachometer like dependency on light conditions, mounting requirements, safety of tachometer and setup time are eliminated since automatic balancing uses external stroboscope.

Automatic field balancing combines the real time processing capabilities of digital world with the ease of use of visually locating point of unbalance of classical world.

 

This article discusses and compares single plane balancing techniques. Various techniques have been experimented to reduce the number of trials required before determining the actual point of unbalance. One of the techniques uses a laser guided optical sensor (also called as tachometer) for speed detection, synchronization and marking a reference point (also called as zero point) on the rotating body. Single plane balancing using this technique has 3 steps:

  1. Initial round wherein initial amplitude and phase of vibration unbalance are measured
  2. Trial round wherein a known trial weight is added at an arbitrary location and the corresponding change in amplitude and phase of unbalance is recorded.
  3. Final round wherein correction weight and its location is calculated.

This being one of the most widely implemented technique but with some shortcomings:

  1. Adding trial weight may cause vibrations to amplify in certain situations. This might increase the overall vibrations above tolerance and increase risk of damage in high speed machines, e.g. de-canters, spindles, etc.
  2. Trial weight needs to be removed to add correction weight.
  3. Setup time for tachometer is high. Reflectivity and light conditions create problems for synchronization signal, for example : in case of cooling towers. Also, loose mounting causes additional problems.

 

An traditional field balancing technique which uses stroboscope for locating the point of unbalance determines the absolute point of unbalance in first round. Simple? Then why isn't it widely used these days? Because it was tiresome and required manually tuning stroboscope for speed synchronization and then manually locating maximum amplitude point for field balancing. Also older discrete analog technology had limitation with filter selection and FFT processing. The bright side being convenience : it is easy to visually locate point of unbalance than to measure in degrees since locating exact angle position on an installed rotor is tedious, for example, locating 42 degrees on a rotor without any measuring instrument is tedious.

 

Automatic field balancing combines the best of both world. The concept is simple : "Unbalance occurs at machine speed only and for dynamic field balancing it should be the major cause of vibrations". Figure 1 below shows the setup required for Automatic single plane field balancing. Accelerometer mounting position should be horizontal. Graphical markings are done at equidistant angle on the rotating object. The numbers will not be visible when the object is in rotation.

Single plane field balancing setup
Fig. 1 - Setup for Automatic single plane field balancing

Using advanced digital techniques and our proprietary algorithm, automatic balancing analyzes and extracts machine speed from vibration unbalance signal. Reference point or the zero point is always vibration accelerometer which is used for synchronization. The point of maximum vibration amplitude is the location of effective point of unbalance as seen below. External stroboscope is triggered whenever the point of unbalance is at the zero point of accelerometer location. Figure 2 below shows the concept of locating the point of unbalance using Automatic single plane field balancing with respect to the accelerometer when the external stroboscope is flashing.

Location of unbalance in single plane field balancing
Fig. 2 - Location of unbalance using Automatic single plane field balancing and Stroboscope

 

Watch the automatic 2-round dynamic field balancing demonstration video above to see how it works and how to visually locate point of unbalance. metro B6012 implements automatic 2-round single plane field balancing along with other balancing options and FFT vibration analysis. It is easiest to use vibration analyzer and field balancer suitable for periodic condition monitoring and field balancing of rotors, blowers, impellers, cooling towers, industrial fans, HVAC systems, motors, pumps, grinders, crushers, mixers, spindles and similar rotating machine structures.


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