Understanding Trial Weights in Rotor Balancing
Definition: What is a Trial Weight?
A trial weight (also called a test weight or calibration weight) is a known mass that is temporarily attached to a rotor at a precisely defined angular location during the balancing process. The purpose of adding this trial weight is to deliberately introduce a known amount of unbalance into the system and measure how the rotor responds. This measured response is then used to calculate the exact correction weight needed to eliminate the original unbalance.
The trial weight is a cornerstone of the influence coefficient method, which is the most widely used technique for field balancing of rotating machinery.
Purpose: Why Use a Trial Weight?
In field balancing, we cannot easily measure the rotor’s physical properties like mass, stiffness, or damping. Instead, we treat the machine as a “black box” and use a trial weight to directly measure its dynamic behavior. The key benefits of this approach include:
- Accurate System Characterization: The trial weight test captures all real-world factors affecting vibration, including bearing stiffness, foundation flexibility, coupling effects, and aerodynamic forces.
- Precision Calculation: By measuring the change in amplitude and phase caused by the known trial weight, the balancing instrument can calculate the exact correction needed with high accuracy.
- No Prior Knowledge Required: The method works without needing drawings, specifications, or theoretical models of the rotor system.
- Accounts for Operating Conditions: The trial run is performed under actual operating conditions (speed, temperature, load), ensuring the balance correction will be effective in real service.
Selection: How to Choose the Right Trial Weight
Selecting an appropriate trial weight is critical for obtaining reliable results. The weight must be large enough to produce a measurable change in vibration, but not so large that it creates unsafe conditions or damages the machine.
Trial Weight Calculator
For precise trial weight calculation based on rotor parameters, use our Trial Weight Mass Calculator. The calculator determines optimal trial weight mass considering rotor mass, installation radius, rotational speed, support stiffness, and vibration levels.
General Guidelines:
- Rule of Thumb: A common guideline is to select a trial weight that will produce a vibration change of approximately 25-50% of the initial vibration level. This ensures a clear, measurable response without excessive vibration.
- Initial Estimate: For an unknown rotor, a trial weight can be estimated as 1-5% of the rotor’s mass, distributed at the balancing radius. Modern balancing instruments often provide a trial weight calculator based on the initial vibration level.
- Calculated Approach: Use the formula Mt = Mr × Ksupp × Kvib / (Rt × (N/100)²), where Mt is trial weight mass, Mr is rotor mass, Ksupp is support stiffness coefficient (1-5), Kvib is vibration level coefficient, Rt is installation radius, and N is rotor speed in RPM. Our online calculator automates this calculation.
- Safety First: Never add a trial weight so large that it would push vibration levels beyond safe limits or trip protection systems.
- Secure Attachment: The trial weight must be firmly attached using bolts, clamps, or magnets to prevent it from flying off during rotation. Putty or clay weights are commonly used for convenience, but must be applied securely.
Procedure: How the Trial Weight is Used
The trial weight method follows a systematic procedure that is at the heart of modern field balancing:
- Initial Run: The machine is operated at its normal speed, and the initial vibration vector (amplitude and phase) is recorded. This represents the vibration caused by the original unbalance.
- Attach Trial Weight: The machine is stopped, and the trial weight is securely attached at a known angular position (typically marked as 0° or referenced to a keyphasor mark) on the correction plane.
- Trial Run: The machine is restarted and run at the same speed. The new vibration vector is measured and recorded. This vibration is the vector sum of the original unbalance plus the effect of the trial weight.
- Calculate Influence Coefficient: The balancing instrument performs a vector subtraction to isolate the effect caused solely by the trial weight. The influence coefficient is then calculated as the ratio of the vibration change to the trial weight.
- Calculate Correction Weight: Using the influence coefficient, the instrument calculates the precise mass and angle for the permanent correction weight that will cancel the original unbalance.
- Install Correction: The trial weight is removed, the calculated correction weight is permanently installed, and a final verification run confirms the vibration has been reduced to acceptable levels.
Practical Considerations and Best Practices
Successful use of trial weights requires attention to several practical details:
- Angular Positioning: The angular location of the trial weight must be recorded accurately. Even a small error in angle can lead to an incorrect correction calculation.
- Radial Placement: If possible, place the trial weight at the same radius where the correction weight will be installed. This simplifies calculations and improves accuracy.
- Repeatable Conditions: All trial runs and the initial run should be performed under identical operating conditions (same speed, temperature, load) for consistent results.
- Multiple Trial Weights: For complex multi-plane balancing, multiple trial weights may be required, placed in different correction planes during separate trial runs.
The trial weight method, while requiring an extra machine run, provides the accuracy and reliability needed for professional balancing work and is the industry standard for in-situ rotor balancing.
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