Introduction to Dynamic Balancing

Presented by: Frank VerebInstrumentation Services Inc.

2016 Spring ConferenceAsheville, NC

Basic Training Outline• What is an unbalanced rotor condition, what can

cause it, and how is it identified?• Why should unbalance be corrected?• Describe the different types of unbalance. How

are they handled differently?• What common vibration problems are confused

with unbalance or can complicate a dynamic balance job?

• How much unbalance is too much unbalance?• Different ways to balance: in situ, shop, by

parts, and by assembly.

The Basics: What is unbalance?• unbalance is the uneven distribution of mass

around a shaft’s rotating centerline.• There are different types of unbalance:

1. Static unbalance2. Couple unbalance3. Dynamic unbalance

• The difference between the types of unbalance correlates to the relationship between the geometric centerline and the rotating (inertial) centerline of the shaft.

Static unbalance

Reference 1

Couple unbalance

Reference 1

Dynamic unbalance

Reference 1

Benefits of Correcting

1. Reduced vibration2. Extended machine life3. Reduced noise levels4. Reduced corrective maintenance and

increased reliability5. PEACE OF MIND

Causes

• Manufacturing errors – Tolerance stacking– Voids– Defective casts

• Operating effects– Debris buildup– Rubs or other loss of material

Identifying unbalance of Installed Machinery• Vibration analysis to the rescue!

– Many other vibration problems can often be confused with unbalance

– Advanced diagnostics can provide a positive ID of a true unbalance condition

• Alternative: “Easter Egging”– Trial and error approach as to whether the

vibration is from unbalance or… something else.

Commonly Confused Vibration Problems• unbalance produces a vibration at a

frequency consistent with the speed of the shaft that is out of balance. So does:1. Misalignment2. Looseness3. Resonance4. Eccentric rotors5. Bent shafts

• How do we make the distinction?

Correcting Static Unbalance

Reference 2

Where to Balance

1. Shop Balance: typical for new machines or machines whose balance planes are not accessible in field

a) By part: during manufacturing processb) By assembly: accounts for tolerance stacking

2. Field Balance (in situ): accounts for the surrounding structure and its influence on machine vibration.

How much is too much?• ISO 1940/1: Worldwide standard for SHOP balancing.

Not necessarily appropriate for field balance jobs as every machine is supported differently.

References 3 & 4

How much is too much?• Best Industry Practices (FIELD):

• Other vibration standards for reference:– ISO 10816-3– HI/ANSI STD 9.6.4

Reference 3

References

1. IRD Balancing Technical Paper 1, Revision 2: "Balance Quality Requirements of Rigid Rotors: The Practical Application of ISO 1940/1,“ March 2009

2. Fox, Randall L., “Dynamic Balancing,” Proceedings of the Ninth Turbomachinery Symposium.

3. Wowk, Victor, “Field Balancing Standards: How Good is Good Enough?” www.reliabilityweb.com.

4. ISO 1940-1:2003.