Practical Approach to Precision Field Balancing
By Greg Buscarello
This book provides practical understanding of field balancing of rigid rotors. Designed as the textbook for the training course of the same title, it combines theory in straight forward language with practical procedures and helpful techniques. While no previous experience is required, this book will be useful for both beginners and those with years of balancing experience.
After discussing the importance of balancing and its challenges, the basic principles are explained. Measuring imbalance, the types of imbalance, plotting vectors and other concepts are thoroughly explained.
A major problem when balancing in the field occurs when imbalance is not the true source of the vibration, or it a combination of more than one source. Though this book is not primarily on vibration analysis, it does cover the basic analysis for separating imbalance from other sources. It also addresses another problem maker, resonance. Some previous vibration background is helpful but not required.
Pre-balance checks and careful setup are aimed at avoiding difficulties in the job or even becoming stuck and not being able to complete the task. Proceeding with single plane balancing, the book gives a solid understanding of the theory and how to calculate the corrections graphically using Illustrations and exercises. Sensitivity vectors and one-shot balancing are covered as well as the 4 Run, No Phase method.
With a solid understanding of how field balancing should work, every job should efficiently produce precision balanced machines. However, those with any experience with machinery know that it’s not always that simple. Therefore, we look at the typical things that go wrong, which can make field balancing a frustrating task to undertake. Knowing what to be aware of can help the balancer to be effective without having to learn through making mistakes and struggling to achieve precision balancing.
Next, the somewhat controversial subject of balancing tolerances is discussed for shop and field balancing. Even if precision balancing tolerances have been achieved on a balancing machine, the total imbalance on machines in the field is often much higher due to small errors in assembly. Creating awareness of those errors is key to avoiding them.
Dual plane balancing, including when to apply it and how it is done, is covered. There is difficulty in balancing narrow and overhung rotors in two planes, therefore the Static-Couple method is introduced, including applying tolerances.
The final chapters introduce the concepts of balancing flexible rotors and discuss strategies for working with suppliers to get precision balanced parts.
Table of contents
1 Introduction
- Why balancing is so important
- Causes of imbalance
- Field balancing vs. shop balancing
- When to do balancing
- Why is precision balancing not usually done
- Best practices
- Why field balancing can be difficult
- About this course
- A brief history of balancing
2 Balancing Concepts
- “Imbalance” and “Unbalance”
- Center Of Gravity vs. Geometric Center
- Units of measure
- Vectors
- Static imbalance
- Couple imbalance
- Dynamic imbalance
- “Quasi-static” or false couple
- Phase measurement
- Strobe light
- Optical Transducer
- Two Channel Instrument
- Balancing without Instruments
- Knife edges
- The static balancer
- Measuring static imbalance of a large rotor
- Balancing by marking the high spot
- Plotting vectors
- Splitting and Combining Weights
3 Determining Imbalance through Vibration Analysis
- Machine history
- Frequency
- Misalignment
- Eccentricity
- Bent shaft
- Looseness
- Defective rotor bars
- Combinations
- Amplitude
- Phase
- Vectors
- Resonance
- Understanding beats
- Modulation
4 Resonance and Balancing
- Understanding resonance
- Resonance in spectra
- Vibration phase relative to resonance
- Using phase change to determine a resonant condition
- Determine resonant ranges by watching phase change vs RPM
- Simplified procedure for using a tunable instrument with a strobe light
- Using an FFT instrument for finding resonant frequencies
- Precautions with different types of transducers
- Use of variable speed vibration shaker
- Using a shaker to determine a weak structure vs. a large vibration source
- Resonance vs. critical speed
- Rigid vs. flexible rotors
5 Pre-balance Checks and Setup
- Can the machine be balanced?
- Safety
- Machine prep
- Tools and equipment
- Setting up the job
6 Single Plane Balancing
- Single plane procedure
- Understanding the response to the trial weight
- Graphical calculations
- Sensitivity vectors
- One-shot balancing
- The four-run, no phase balancing method
7 What Goes Wrong
- Insufficient analysis
- Changing operating conditions
- Incorrect orientation
- Unstable measurements
- Balancing with a slow beat
- Resonance
- Looseness
- Imprecise angles
- Changing radii
- Arc effectiveness
- Wrong trial weight amount
- Inaccurate correction weights
- Balancing non-symmetrical rotors
- Foreign material inside the rotor
- The resonance “curveball”
8 Balance Tolerances
- ISO standard
- API Standard
- Field Balancing Tolerances
- Negotiating precision balancing tolerances with suppliers
9 Imbalance from Assembly Errors
- Parts not precision balanced
- Improper key length
- Improper set screw length
- Fits
- Cocked rotors
- Burrs
- Bent shaft
- Looseness
- Balancing couplings
- Improper coupling bolts
- Uneven lubrication
- Belt driven machines
- Taper lock and QD hubs
10 Dual Plane Balancing
- When to choose dual plane
- Dual plane procedure
- Graphical vs. software calculations
11 Balancing using the Static-Couple Method
- Narrow and overhung rotors
- Graphical method of separating static and couple
- Procedure for applying static and couple
- Other static – couple methods
- Applying tolerances for static-couple method
12 Balancing Flexible Rotors
- Multi-Plane Balancing
- Balancing Papermill Rolls while Reducing or Eliminating the Possibility of Whip
- Basic Facts to Understand Before Proceeding
- Balancing Procedure
- Suggestions for Driving Rolls While Still Mounted in the Papermachine
- Removing Whip from a Previously Balanced Rotor or Roll
- Whip Correction Procedure for Resonant Whirl at Rotor’s First Critical Speed
- Whip Correction Procedures for Resonant Whirl at Rotor’s Second Critical Speed
13 Strategies for the Best Reliability through Precision Balancing
- Negotiating precision balancing tolerances with suppliers
- Audits and inspections
- When no supplier can precision balance a part