Understanding Single-Plane Balancing

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Portable balancer & Vibration analyzer Balanset-1A

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Vibration sensor

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Dynamic balancer “Balanset-1A” OEM

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Definition: What is Single-Plane Balancing?

Single-plane balancing is a balancing procedure in which the rotor’s unbalance is corrected by adding or removing mass in only one radial plane perpendicular to the axis of rotation. This method is appropriate when the unbalance is predominantly static in nature—meaning the rotor’s center of mass is offset from the axis of rotation, but there is no significant couple or moment causing the rotor to wobble.

Single-plane balancing is the simplest and most economical balancing method, requiring only a single correction plane and typically just one trial weight run to complete.

When to Use Single-Plane Balancing

Single-plane balancing is suitable for specific types of rotors and operating conditions:

1. Disc-Type Rotors

Rotors where the axial length (thickness) is small compared to the diameter are ideal candidates. These are often called “narrow” or “thin” rotors. Examples include:

• Grinding wheels
• Circular saw blades
• Single-stage fan or blower impellers
• Flywheels
• Disc brake rotors
• Single pulleys

2. Rigid Rotors Operating Below First Critical Speed

For rigid rotors that operate well below their first critical speed, single-plane balancing may be sufficient even if the rotor has some axial length. The key is that the rotor does not undergo significant bending or flexing during operation.

3. When Unbalance is Known to be Static

If the unbalance condition is caused by a single localized source—such as material buildup, a missing fan blade, or an eccentric mounting—and vibration measurements show predominantly in-phase motion at all bearing locations, single-plane balancing is appropriate.

The Single-Plane Balancing Procedure

The procedure follows a straightforward, systematic approach using the influence coefficient method:

Step 1: Initial Measurement

With the rotor operating at its normal speed, measure and record the initial vibration vector (amplitude and phase) at one or more bearing locations. This represents the vibration caused by the original unbalance.

Step 2: Attach Trial Weight

Stop the machine and attach a known trial weight at a convenient angular position (typically 0°) on the chosen correction plane. The trial weight should be of sufficient magnitude to produce a noticeable change in vibration—typically 25-50% of the initial vibration level.

Step 3: Trial Run

Restart the machine and measure the new vibration vector at the same location(s). This measurement represents the combined effect of the original unbalance plus the trial weight.

Step 4: Calculate Correction Weight

The balancing instrument performs vector addition and calculates the influence coefficient. It then computes the exact mass and angular location for the permanent correction weight that will minimize vibration.

Step 5: Install Correction and Verify

Remove the trial weight, install the calculated correction weight permanently (by adding or removing mass at the specified location), and run the machine to verify that vibration has been reduced to an acceptable level. If necessary, a trim balance can be performed to fine-tune the result.

Advantages of Single-Plane Balancing

• Simplicity: Requires only one correction plane, making it easier to implement and understand.
• Speed: The procedure typically requires only two or three runs (initial, trial, and verification), saving time and reducing machine downtime.
• Cost-Effective: Fewer measurements and simpler calculations mean lower labor costs and less expensive balancing equipment.
• Accessibility: Many locations on the rotor may be accessible for adding correction weights, providing flexibility in where weights are placed.

Limitations and When Not to Use Single-Plane Balancing

Single-plane balancing has important limitations that must be understood:

1. Cannot Correct Couple Unbalance

If the rotor has significant couple unbalance—where unbalance forces exist at opposite ends of the rotor but in opposite angular positions—single-plane balancing will not be effective. This condition requires dynamic balancing with corrections in at least two planes.

2. Not Suitable for Long Rotors

Rotors with a length-to-diameter ratio greater than approximately 0.5 to 1.0 typically require two-plane balancing. Examples include motor armatures, pump shafts, and long fan rotors.

3. May Not Reduce Vibration at All Bearings

A single-plane correction optimized for one bearing location may not adequately reduce vibration at other bearing locations, particularly if the rotor is long or operating near a critical speed.

4. Ineffective for Flexible Rotors

Rotors operating above their first critical speed undergo bending and require multi-plane balancing techniques that account for the rotor’s mode shapes.

Relationship to Static Balancing

Single-plane balancing is closely related to static balancing. In fact, single-plane balancing performed on a rotating machine is essentially dynamic measurement of static unbalance. Static balancing can be performed with the rotor at rest (on knife edges or rollers), while single-plane balancing is performed with the rotor spinning, allowing for more accurate measurement under real operating conditions.

Typical Applications and Industries

Single-plane balancing is widely used across many industries for appropriate rotor types:

• Woodworking and Metalworking: Circular saw blades, grinding wheels, cutting discs
• HVAC: Single-stage centrifugal fans and blowers
• Agricultural Equipment: Combine harvester components, single pulleys
• Automotive: Flywheels, brake rotors, single pulleys
• Material Handling: Conveyor pulleys, idler rollers

For these applications, single-plane balancing provides an optimal balance between effectiveness, simplicity, and cost, making it a fundamental technique in the field of rotor balancing.

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