unbalance & balancing
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ROTORUNBALANCE
&BALANCING
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Why Unbalance in the Rotor or impeller occurs?
Unbalance occurs due to
Uneven wear of the rotor/impellers Thermal/ mechanical distortion. Casting defects Improper fabrication Depositions Rotor sag Eccentricity Tolerance accumulation
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Contd
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Contd
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How to detect Unbalance?
Unbalance is detected by
1. High 1X peak in radial direction
2. Phase shift of 90 deg from (H) to (V)
3. Increase in vibration with increase in speed
4. Constant vibration & phase
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How Balancing is done
Selection of trial weight
Normally the trial weight should produce an unbalance
force equal to 10% of Total Rotor weight.
Unbalance force(Kg)= 0.01 x W x R x (rpm/1000)2W in gram, R in cm
R
W
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Trial weight calculation
Rotor w = 10 T = 10,000 Kg ie 5000 kg per bearing
10% of Rotor Wt = 10/100 X 5000 = 500 kg
Let R = 50 cm
speed = 500rpm
Then
Wt =( 500/ 0.001 X 50 cm) X (1000/500rpm)2
= 4 kg
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Steps involved in Balancing
1. Take vibration signature and phase and identify that it is a
unbalance problem
2. Take 1X phase angle
3. Calculate trial weight and put the trial weight
4. Run the m/c and take 1X phase angle
5. Draw Vector diagram
6. Calculate the balance weight and its angle and put the
weight7. Run the m/c again and confirm that vibration has reduced.
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Vector Diagram Wc = Wt X ( O/ T)
O+T is effect of both The original unbalance and trial wt
T i.e [( O+T)T)] is
The trial wt effect
90270
180
0
O + T
O
T
Our aim is to shift the T
opposite to O and increase
the trial wt by ( O/T)
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Example of ID fan Balancing
Initial unbalance = 70 mic / 65 deg 509 rpm
T Wt of 6.5 kg has been put at 10 deg
Trial vibn
40 mic / 175 deg 503 rpmBW = 5 kg / 30 deg
Check run 14 mic 343 deg
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90270
180
0
O+T
O
T
T Wt
T