What is Field Balancing?
Field balancing is a method that aircraft mechanics use to balance a rotating part without removing the part from the machine it belongs to. Some of the advantages of field balancing are obvious: saved time by not removing the part that rotates and ensuring that the rotating part is balanced as it is installed are just a couple.
Certain conditions must be met in order to perform a proper field balancing procedure. For example, access to the rotating shaft is essential to a proper field balancing procedure. This is because field balancing requires an area to place trial weights and a correction weight.
When can a field balancing procedure be used?
There are many diverse applications of a field balancing procedure. Helicopter vibration analysis, helicopter rotor track and balance, fan balance, and other types of aviation equipment are just a few examples. In fact, 2011, had about 224,475 total active general aviation craft in the United States on record. Aircraft maintenance might be one of the top categories for using a field balancing procedure.
Field Balancing Basics
This article is by no means a comprehensive analysis of field balancing, nor is it a complete guide to give you every step to proper field balancing procedure. However, here are some of the basics you should know before attempting to field balance anything.
Identifying a Rigid Rotor: Motors that operate at a rotational speed below 70% of their critical speed are considered to be rigid rotors. Critical speed is the speed at which a rotors resonance occurs, by exciting the natural frequency o the rotor. It is estimated that 80-90% of rotors are rigid and can benefit from proper field balancing.
Detecting Unbalance: Rotor unbalance is a condition that manifests when the center of mass (aka center of gravity) and the geometric center of do not coincide. While these two points will never coincide exactly, the goal of rotor balancing is to reduce unbalance so that the machine life will not be hampered negatively by residual unbalance. A technician performing a field balance will attempt to bring these two points as close together as possible, typically attempting to meet the industry standard of balance.
Proper Balancing Standards: There are a multitude of different balancing standards used throughout different industries and different machines. Field balancing procedure will also have a different standard that you need to determine based on your situation. ISO and API are a couple of the most common standards, though some organizations will determine their own balancing standard. Typically you should choose a standard that will help maximize the life required of the machines you will be using and that will help preserve essential equipment functions required by your processes.
What Causes Rotor Imbalance?
There are a number of things that can cause rotor unbalance. When performing a field balancing procedure, it is important to identify what is causing the imbalance in the first place, to try and eliminate that as something causing an imbalance in the future. Here are some of the most common things that cause rotor imbalance:
- Blowholes in cast parts
- Improper key length
- Stress relief distortion
- Thermal distortion
- Corrosion or uneven wear
- Deposit buildup and component shift
- Asymmetrical design and assembly errors
- Rotor damage
- Repair work errors such as use of wrong coupling bolts or washers