Understanding Residual Unbalance

1. Definition: What is Residual Unbalance?

Residual Unbalance is the amount of unbalance that remains in a rotor after the balancing process has been completed. It is the unbalance that exists within the specified balancing tolerance.

It’s important to understand that achieving perfect balance (zero unbalance) is practically impossible and economically unfeasible. The goal of balancing is not to eliminate unbalance entirely, but to reduce it to a level where the vibration it causes is no longer harmful to the machine during operation. This acceptable remaining amount is the residual unbalance.

2. Balancing Tolerance and Residual Unbalance

The amount of permissible residual unbalance is determined by a balancing tolerance. This tolerance is a specific, calculated value that dictates how much unbalance is acceptable for a particular rotor in a particular application.

The most widely used standard for determining this tolerance is ISO 1940-1 (now part of the ISO 21940 series). This standard uses Balance Quality Grades (G-Grades), such as G6.3, G2.5, and G1.0, to classify the balance requirements for different types of machinery.

• A lower G-Grade number signifies a tighter (smaller) tolerance and therefore a lower permissible residual unbalance.
• The calculation for the specific tolerance (in gram-millimeters or gram-inches) depends on the G-Grade and the service speed of the rotor.

Once the balancing operator has reduced the rotor’s unbalance to a level that falls within this calculated tolerance, the job is considered complete. The unbalance that is “left over” is the residual unbalance.

3. Why Does Residual Unbalance Exist?

Several factors contribute to the presence of residual unbalance:

• Balancing Machine Sensitivity: Every balancing machine has a limit to the smallest amount of unbalance it can accurately measure.
• Tooling Errors: Imperfections in the arbors or tooling used to mount the rotor on the balancing machine can introduce small errors.
• Assembly Shifts: Components can shift slightly when a machine is assembled after balancing (e.g., keys, couplings).
• Operational Changes: Rotors can experience changes during operation due to thermal expansion, wear, or buildup of deposits, which can alter the initial balanced state.
• Practicality: Chasing the last few milligrams of unbalance provides diminishing returns and is often not cost-effective.

4. Measuring and Verifying Residual Unbalance

During the balancing process, the balancing machine measures the initial unbalance. The operator then adds or removes correction weights and re-measures. This iterative process continues until the machine indicates that the measured unbalance is below the target tolerance value.

A good balancing report will always state the initial unbalance and the final residual unbalance, demonstrating that the rotor has met the required quality grade. For example, the report might state: “Final residual unbalance is 0.5 g-mm in the left plane and 0.8 g-mm in the right plane, which is within the specified G2.5 tolerance for this rotor.”

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