29/05/2013

Balancing - Using Your Mobile Phone

ABSTRACT

Unbalance is one of the most common causes of high vibration in machines.  If the largest vibration component in the spectrum is at 1× (i.e. at the frequency corresponding to rotation speed) then unbalance is the likely cause.   Balancing is therefore a regular maintenance activity in many workplaces. Ray Beebe explains a method that has been successfully used on rotors large and small, with the only instrumentation required for balancing being the stopwatch feature of a mobile phone. 

INTRODUCTION

It is assumed that the severity of vibration has been assessed as unacceptable according to sources such as the relevant part of ISO 10816 [1].  Even if a machine’s vibration meets such criteria, it may be causing annoying vibration of adjacent parts. Looseness is another of the most common causes, and a check of foundation tightness is recommended before proceeding to balancing. If the unbalance is due to deposition on the rotor, such as with fans handling dirty gas, the rotor should be cleaned. In cases where a deposit is difficult or expensive to remove it may be expedient to balance out its effect until a more convenient time for cleaning.

THE TIMED OSCILLATION METHOD

This method has been used successfully for short rotors such as overhung fans up to 2m diameter, and for pump impellers.  It has also been used on smaller fans with two bearings that are nearer to what are considered ‘long’ rotors. The rotor must be in rolling element bearings, or set up on a mandrel and parallel ways, and not be affected by crosswinds.

For field balancing, the usual safety precautions for opening any machine must of course be taken so that the machine cannot be switched on while the work is in progress.

I do not know the source of the method, but I learned it in my first job at a power station in the 1960s.  The procedure is as follows -

• Mark numbers around the rotor, evenly spaced.  For bladed rotors, mark at the blades.

• Choose a swing mass to fasten (by clamp or magnet) on the rotor, to give to-and-fro oscillation time of about 20 seconds. (This gives some allowance for reaction time in operating the stopwatch feature of your mobile phone)

• With the swing mass fixed to the rotor at Position 1, turn the rotor so that the mass is at one horizontal position and stationary.

• Release the rotor and let it swing freely under its own inertia – do not push it in any way.

• Time the to-and-fro oscillation from the horizontal position.  The left part of the sketch shown below illustrates the process.

• Turn the rotor so that the mass is at the horizontal position on the other side, and repeat.

• Calculate the mean time of swing for this position, and plot on a graph of time against  position as shown in the sketch.  With position numbers evenly spaced, one position  appears twice- at the start and the end of the position axis.

•  Move the swing mass to Position 2, etc. and repeat the process.

• Connect the graphed points with a smooth sine wave shape, and read off the maximum and minimum times of swing: T and t.

•  Calculate the size of balance mass required from the following expression (derived from the theory of the compound pendulum [2])

Balance mass = (Swing mass) ×

•  Make a balance mass, and fix it on the rotor at position of maximum swing time.  The fixing must of course be sufficiently strong not to allow the mass to fly off at operating speed. If welded, allow for the mass of the weld. 

  * If the correction is to be made by removing mass from the rotor, remove it from the position of minimum swing.

  * If the correction mass is at a different radius from that of the swing mass, then calculate the mass size to achieve the same moment (i.e. Mass × Radius).

• If required, verify the result by repeating the swing test for three spaced points.

 

 

 

 

 

 

 

APPLICATION TO BALANCE PUMP IMPELLERS

The method was developed and used for each impeller of a high-speed ten-stage pump of the ring-section design (where each stage impeller is assembled in turn with its matching stationary section).  We clamped parallel ways to the bed of a milling machine.  A mandrel was made to suit the impeller and a pot magnet used as the swing magnet.  

The correction was made on the adjacent milling machine using an end cutter.  A table was calculated and drawn up giving the depth of cut required in each case.  The position was marked clearly on the impeller, and it was passed to the machinist who cut the required mass from the back shroud.

The method gave satisfactory results. At the time, the only balancing machine available could only hold the total rotor assembly, and corrections would be made on each of the end impellers.  Arguably, the impeller-by-impeller method gave a more satisfactory result.

BALANCING BY TRIAL AND ERROR

Another method that may be satisfactory does not require any instrumentation.  A story will illustrate the method. 

Among the cars that I have owned  was a  second-hand 1971 Ford Falcon (In-line, six-cylinder, four litre engine, Australian designed and made).  It came with an after-market twin extractor exhaust system. This gave a ‘boy racer’ burble, and the salesman explained that it was the cause of severe vibration of the whole car that was evident at around 90 km/h.

I tolerated this for some time before deciding that with my growing knowledge of vibration problem solving, and fresh from solving a pump problem, I should be able to fix it. With the bonnet up, I revved the engine, expecting to see the exhaust pipes shake about when the critical speed range was reached.   To my surprise, they did not move at all from an equilibrium position.  Then I placed my hand on the alternator and was horrified to feel it shaken off by its rough vibration!  I should have figured this out much earlier, because the alternator support bracket had been repaired by brazing, and I had replaced it with a new one.

I removed the alternator, leaving the rotor in its end casing bearing. When spun by hand and allowed to come to rest, it settled at the same angular position each time. So, I drilled holes in this ‘heavy’ pole face until the spin test showed the position at rest to vary each time.

On reassembling and back in the car, the vibration no longer occurred.  This alternator must have slipped through manufacturing Quality Control unnoticed. (When telling this story, I usually leave out the bit about having to ask our instrument workshop people to repair the screw thread that I had mangled just a little!)

METHODS OF BALANCING

The following table outlines the several methods available.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Some vibration data collector/analyser devices can have capability for field balancing, using a phase angle reference.  I think that the earlier method where a vector diagram is drawn has the advantage of visualising the process, step by step.

REFERENCES

1.    ISO 10816, Mechanical vibration – Evaluation of machine vibration by measurements on non-rotating parts. (Several parts for specific machine types)

2.    Beebe R S,  Machine condition monitoring,  MCM Consultants, 2009 reprint, 
ISBN 0646250884