Balancing of Rotating Machines

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BALANCING OF ROTATING MACHINES

Field balancing principles/Pertti Leskinen/18.3.2014

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Some advice first before starting a balancing exercise for a rotating machine.

• First one must make sure that unbalance is causing the mechanical vibration that one wants to reduce.

• There are other faults having similar vibration behaviour as unbalance.

• Alignment fault is often causing vibration to increase at 1xrot frequency component as well.

• Unbalance force and 1xrot freq. vibration amplitude increases by square of operating speed (ie. frequency).

• At constant speed rotating frequency amplitude and phase must be stable in case of real unbalance problem. If these are varying other faults may be causing the vibration.

• If 1xrot. Freq. amplitude and phase are not stable it may be caused by resonance of machine structure. It is very difficult to perform balancing in this case.

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Unbalance, balancing

Faults that can be mixed to unbalance

• Misalignment • Excessive bearing clearance• Bent shaft or rotor• Cracked shaft• Soft foot • Gyroscopic effects• Load or electrical influence• Locked coupling• Foundation problems

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Mass Unbalance

• A. Force Unbalance (Static Unbalance)

• B. Couple Unbalance (Dynamic Unbalance)

• C. Mixed Unbalance (Static + Dynamic Unbalance)

• D. Overhung Rotor Unbalance

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• Force Unbalance will be in-phase. 1x rpm always present and normally dominates.

– Amplitude increases by square of speed (2x speed increase 4x amplitude increase)

– Can be corrected with only 1 balance weight in one plane at rotor CG.

Centerhang rotor

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• Couple Unbalance (dynamic unbalance) 180° out-of-phase. 1xrpm always present and normally dominates.– Amplitude varies with square of the speed.

– Can also cause high axial vibrations.

– Correction requires balance weights in at least 2 planes.

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Vibration Diagnostic Chart

• Mixed Unbalance most common case.– Phase angle difference depends on distribution of unbalance between bearings

– Can sometimes be corrected with only 1 balance weight in one plane at rotor CG but mostly needs to be balanced as dynamic unbalance

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• Overhung Rotor Unbalance causes high 1xrpm in both axial and radial directions.– Axial phase tends to in-phase while radial often unsteady

– Overhung rotors often have both force and couple unbalance

Overhang rotor

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Unbalance, balancing, some check before starting the balancing exercise

• If it is sure that the machine under study has extra unbalance, check few things first before starting the balancing exercise:

– It is recommended first take some extra time and think over once more why is there unbalance, is it because of dirty in impeller or possible wear or cracks of impeller or shaft. Off course cleaning and repair operations must be performed prior a balancing exercise.

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Field balancing

Before starting a balancing exercise think the following things first:

1. Study the places where to put the masses.

2. Get such important machine information as rotor mass, radius of the balancing mass place in a balancing plane, rotating speed of rotor.

3. Choose the balancing grade where to aim see ISO 1940 standard.

4. Calculate permissible residual unbalance according to ISO standard 1940.

5. Have the balancing weights available.

6. Choose a suitable trial weight (ie. mass), not too heavy, not too light.

7. Trial weight mass = the permissible residual unbalance mass, unless not better information available.

8. (Depending on the machine size and speed the normal trial mass amount is about from 50 g to 300 g when balancing fans, turbines, generators rotating not over 3000 rpm).

• Remember safety before all, stop and think what am I doing!• Don’t use too heavy balancing masses! Use only tens of

grams or hundred of grams, not any kilograms (exception hydroturbine generator rotors).

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Balancing, balancing grades, choose the right balancing grade.

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Balancing, determine permissible residual unbalance

For example if we have a fan with an extra unbalance.And we have choosen balancing grade to be as G 2,5. The rotor mass is 900 kg, rotating speed of rotor is 1500 rpm, radius of balancing weight is 400 mm. Then we can determine the permissible residual unbalance to be about 16 gmm/kg in case of one plane balancing. It can be calculated to be as 16x900/400=36 g (grams).

So this 36 g is the permissible residual unbalance and is suitable as a trial weight use too.

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Field balancing , machine runs on its own bearings at site

Different type of balancing:1. Balancing in one plane2. Balancing in two or more planes

1. Balancing in one plane (disks, narrow fan rotors)– Different methods: graphical solutions and calculations by computer software.

2. Balancing in more than one plane (generator rotors, turbine rotors, in general long rotors)

– Normally performed by using balancing programs.

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Measurement devices needed for balancing

1. Balancing in one plane

– Portable vibration meter or vibration analyser (better).

– Vibration transducer with magnet.

– Tachometer for phase measurement. Not necessary if using four run graphical method.

– In 4 run graphical method you only need a paper with grids, a pen, a sharp , a ruler and a calculator. No balancing program is needed.

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Measurement devices needed for balancing

2. Two (or more) planes used for balancing:Normally a commercial balancing program is used for calculations. The program can be in an analyser itself or in a computer.Your need a portable spectrum analyser with phase measurement feature.Vibration transducer with magnet. Key- Phasor transducer to pick up 1 a pulse/at one revolution of the shaft.When balancing in two planes at least 3 runs must be recorded before the calculations for the balancing weight mass and angle (place).

First a reference run is needed, a trial run 1 where trial mass is put into plane 1, a trial run 2 where the same trial mass is removed from plane 1 and put into plane 2. After that, the needed calculations can be performed. Measure and record 1x amplitude and 1x phase in every run from both planes (plane 1 and plane 2). So at least one 1xamplitude and 1xphase measurement must be performed and recorded in one plane.

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Four run graphical balancing method

Four Run Balancing Procedure First take a Reference measurement as the following :Take an initial vibration reading and name it as Vel0. Then stop the unit. Pick three (3) positions where a suitable mass can be attached. Put a 120 degrees angle between the points.Name the positions 1, 2 and 3 and mark the places with permanent marker or paint. Choose a trial weight or mass (remember permissible residual unbalance according to ISO 1940 standard). Name it as mT. Attach the weight into position 1. Then start the unit.Take a vibration reading and name it as Vel1. Then stop the unit. Remove the mass m from position 1 and put it to the next position 2. Then restart the unit and name the vibration reading as Vel2. Then stop the unit. Remove the mass m from position 2 and put it to the next position 3. Then restart the unit and name the vibration reading as Vel3. Then stop the unit and remove the trial mass mT.

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Four run graphical balancing method

Using grid paper, choose a scale so that the largest vibration reading can be drawn into the paper. Using Vel0, draw a circle using the recorded units as the radius. Plot the three positions you choose on the circumference. Again, using the recorded vibration as the radius, draw three circles according to their respective positions. The three circles should over lap some where on the page. Draw a line from the center of Vel0 to where the three circles intersect. This is the angle at which the final correction mass mc will be placed. Measure the line and record in the scale units you have chosen. Name it VelR Correction mass can be calculated using the following equation:

mc = mT * Vel0 /VelR

Note! During the test runs the original vibration reading Vel0 will be exceed .

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Four run graphical balancing. Note, that this is just a principal example (ie. not a real measurement).

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240 120

α

Record of Ref run/ Vel0

Vel0

Record of Run1/Vel1

Record of Run3/Vel3

Record of Ref run2/ Vel2

Result:Draw the vector/VelR

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One Plane Balancing, Graphical Solution

Vibration VelT, effect of Trial mass/weight only

Vibration Vel0, original unbalance

Final balance mass/weight

Trial mass/weight

Same angle β, between Vel0 and VelT compared to the angle between Trial weight and Balance weight

Vibration Vel0+T, effect of original unbalance + Trial weight

0

90

180

270

45

135215

315

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Balancing

• Some useful equations:

• Centrifugal force caused by an unbalance

• Fc=m*e*w^2, where w=2*π*f, f =frequency of rotation in Hz (1/s)

• e= eccentricity = distance of mass centre from centre of rotation.

• Example: If we have a rotor with mass of 2165 kg and put a Trial mass of 600 g at radius of 1000 mm and rotor is rotating at 1490 RPM. Trial mass causes a Force of 0.6 kg*1 m*(2* π*1490/60*1/s)^2 = 14607,6kgm/s^2 = 14608 N = 1461 kp.

• This is 1461/2165*100% of the mass of rotor = 67%

• Permissible residual unbalance per unit of rotor would be (if chosen balancing grade G6,3) 40 Uper/m. eper, gmm/kg Now we can calculate how much unbalance mass is allowed at radius of 1000 mm in this rotor of mass 2165 kg.

• Calculation: 40 gmm/kg * 2165kg/1000mm=86,6 g, so the Tial mass 600 g has been quite heavy.

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Field Balancing of an overhang -type FAN

• ’Overhang' –type FAN with two roller bearings having circulation system lubrication

• Motor 110 kW frequency converter driven .• Normal service speed is under 1400 rpm• Rotor mass unknown• Rotating direction CW• Diameter of impeller 1400 mm (1,4m)•

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Field Balancing example of an overhang -type FAN, calculations made using Emerson’s balancing program running in a computer.

• First step:• Decision which

• type of balancing

• is suitable.

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Step 2:Definition of measurement points

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Step 3:1x vibration and 1x phase measurement readings at Reference run. Original situation, no extra masses added.

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• Step 4:• Installing a Trial weight

or Mass on rotor (on balancing plane at radius of 700 mm).

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• Step 5:• Readings with Trial

Mass 67 g

• installed

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• Step 6:• Results of balancing

calculations.

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• Step 7:• Trial mass 67 g was welded in place so it was wanted not to be removed,

it was left in its place.

• So this mass 67 g must first be compensated by adding 67 g to opposite side of rotor (angle 180 deg from the Trial mass).

• Calculation result of balancing mass 43 g must be installed near to the position of Trial mass 67 g (12.4 deg). These two masses 43 g calculated balancing weight and 67 g compensation mass are in opposite side of each other. This is equal to the situation: The summary effect of them is 25 g balancing weight added to opposite side of the Trial weight 67 g.

• So the final balancing solution was performed so that Trial mass 67 g was left in its place and 25 g mass was added to opposite side of rotor (180 deg from Trial weight or mass).

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Field Balancing of an overhang -type FAN

Measurement points of the FAN

Run 1.First run, no masses

added.

Run 2.67 g Trial mass added Run 3.

Trial mass 67 g was left in place and 25 g

balancing mass added in opposite side of rotor

(180 deg from Trial mass). Speed 1400 rpmSpeed 1402 rpm Speed 1402 rpm

FAN DE horizontal 7,64 mm/s /330° 5,3 mm/s /121° 2,2 mm/s /47°

FAN DE vertical 3,8 mm/s /155° 2,4 mm/s /284° 1,3 mm/s /210°

FAN NDE horizontal 12,3 mm/s /315° 7,5 mm/s /105° 3,7 mm/s /35°

FAN NDE vertical 3,1 mm/s /174° 1,9 mm/s /284° 0,6 mm/s /225°

1x amplitude and 1x phase during the balancing exercise.

Balancing of Rotating Machines

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