Understanding Correction Weights

1. Definition: What is a Correction Weight?

A Correction Weight, also known as a balance weight or balance bob, is the specific mass that is added to, or removed from, a rotor to correct an unbalance. The entire goal of the balancing process is to calculate the precise amount of weight and the precise angular location to apply this correction.

The correction is made within a pre-defined correction plane. The application of a correction weight creates a new centrifugal force that is equal in magnitude and directly opposite (180 degrees) to the force created by the rotor’s heavy spot, effectively canceling it out.

2. Methods of Correction

There are two primary philosophies for applying a correction weight:

a) Weight Addition

This is the most common method, especially in field balancing. It involves adding mass to the “light spot” of the rotor, which is 180 degrees opposite the measured heavy spot.

• Welding: Steel weights are welded directly onto the rotor. This is a very secure and permanent method.
• Bolting/Screwing: Pre-made weights are attached using bolts or screws in tapped holes. This is common on large industrial fans and rotors designed with specific balance weight attachment points.
• Clip-on Weights: Specially designed clips are used on components like automotive drive shafts and small fans.
• Epoxy: Two-part epoxy putty is a versatile method for adding weight, especially in locations where welding or drilling is not possible.

b) Weight Removal

This method involves removing mass from the measured “heavy spot” of the rotor. It is very common in production balancing environments because it is fast and does not require an inventory of different weights.

• Drilling: The most common removal method. The operator drills a hole of a specific diameter and depth to remove a precise amount of material. Balance tolerance charts often translate a required gram correction into a corresponding hole depth for a given drill bit size.
• Grinding: Mass is removed by grinding material off the surface. This is less precise than drilling but can be effective.
• Milling: A milling machine can be used to remove a very precise amount of material for high-precision balancing.

3. Calculating the Correction Weight

A modern balancing machine or portable vibration analyzer automates the calculation of the correction weight. The general process is as follows:

1. Measure the initial vibration amplitude and phase (this tells the instrument where the heavy spot is and how heavy it is).
2. Attach a known “trial weight” at a known angle.
3. Run the machine again and measure the new amplitude and phase.
4. The instrument now knows how the rotor responded to a known unbalance (this is the influence coefficient). It can then perform a vector calculation to determine the exact amount and location of the final correction weight needed to bring the vibration down to a minimum.

The result is typically presented as: “Add 15.3 grams at 217 degrees” or “Drill a 1/2 inch hole, 8mm deep at 35 degrees.”

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