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Balancing Rotating Equipment: Static vs Dynamic (Series)

08/05/2021

2021-07-vibration analysis series-part 3

This blog post continues an 8-part series on vibration analysis written by Dr. Sara McCaslin & Nolan Crowley, Business Development Manager at HECO.

Dr. Sara McCaslin: Sara has a Ph.D. in mechanical engineering from the University of Texas at Arlington. Sara has also taught materials science, manufacturing, and mechanical system design at the University of Texas at Tyler.

Nolan Crowley: Nolan is a Business Development Specialist for HECO. Nolan has BS from Miami University along with extensive field experience with powertrains, electric motors, & vibration issues since 2007.

When you don't balance rotating equipment, it causes vibration. That vibration means a strain on bearings and seals followed by accelerated wear and machine failure.

Why Balancing is Necessary

Imbalance occurs when mass isn't evenly distributed around an axis of rotation. And imbalance is particularly problematic for rotating equipment. We usually associate noise and vibration with an out-of-balance piece of rotating equipment, but that isn't the only problem. Imbalance and vibration lead to unintended forces acting on critical parts like bearings and seals. And those forces will contribute to a shorter lifespan and potential for catastrophic failure.

Benefits of Balancing

Balancing your rotating equipment provides several benefits:

  • Increased MTBF
  • Extended service life
  • More efficient operation
  • Smoother running equipment
  • Reduced noise and vibration

Because of benefits such as these, balancing is often included as part of a predictive maintenance plan,.

What Can Be Balanced

Rotating equipment like motors, generators, pumps, and fans can (and really should) be balanced. Specific components within this equipment that need balancing depend on what type of equipment we’re talking about (e.g., lathes may need rotating fixtures balanced). The most commonly balanced components include rotors, axles, rotating shafts, impellers, fan blades, and even flywheels.

Types of Imbalance

There are two types of imbalance: static and dynamic, and both of these are very important. 

And before we dive into the difference between these two, here are three key facts to remember:

  • An object must be statically balanced before it can be dynamically balanced
  • A statically balanced object can still be dynamically balanced
  • A dynamically balanced object is also statically balanced

These provide good reasons why both static and dynamic balancing are important when addressing vibration in rotating equipment.

Static Balancing

Of the two types of balancing, static balancing is the easiest to understand and the easiest to perform. Basically, if the center of gravity of a rotating system lies on its axis of rotation, that system is said to be statically balanced. When that condition is met, it means that the object can remain stationary (static) as long as the axis is horizontal. Nothing is needed to keep it from turning. In short, rotational equipment that has static balance isn’t going to rotate on its own no matter what angle the axis is at.

When an imbalance exists, the axis is going to tend to rotate. It may have to be at a certain angle for that to happen, but that is still considered unbalance. 

Static balancing can take place in the field or it can be performed in a shop using a simple static balancing machine. Field balancing assesses the object’s balance as it rests on its own bearings and support structure. Some basic calculations can be performed to determine how much mass needs to be added and where it needs to be laced in order to achieve static balance.

Dynamic Balancing

Unlike static balance, dynamic balance (often synonymous with rotor balancing) is related to a rotating object in motion. When an object is dynamically balanced, it remains balanced even when it’s turning. Imbalance occurs when mass is not symmetrically distributed about the system’s axis of rotation

Balanced vs Unbalanced

For a balanced system, the only force needed to hold it in place as it rotates is something to support its weight. When the object is rotating it doesn’t generate any forces on the bearing beyond its weight, which is what the bearing is designed for. 

When an unbalanced object rotates, it generates centrifugal forces. Something has to apply forces to keep the object in place, and that responsibility falls on the bearings. While the object is rotating, any point on the bearing will be experiencing non-axial fluctuating forces. And those forces accelerate wear and reduce the useful life of the bearings.

How Dynamic Balancing Works

First, keep in mind that balancing is performed for rotational equipment at normal operating speed and under operating conditions. Now here’s a quick overview of how dynamic balancing works:

  1. Vibration measurements are taken while the system is rotating at a high speed
  2. The unbalanced force resultant is calculated based on the vibration data 
  3. Masses are added or removed from the rotating object to align its mass center with the axis of rotation

Notice that to determine the unbalanced force vibration measurement and analysis are needed. Accelerometers are mounted on the bearing housing and the signals are sent to a vibration meter. The level of vibration is actually proportional to the magnitude of the imbalance forces. 

To find out the direction of the resultant force, a vibration analyst will compare fluctuating phase of the vibration signal with a standard periodic signal from another reference point on the rotating system. Combining the magnitude of the imbalance force with the direction defines the unbalanced force vector. This information allows the rotating system to be balanced by adding strategically placed counterweights.

Vibration is not Always Due to Imbalance

Just because your powertrain or fan is vibrating doesn’t automatically mean the problem is imbalance. For example, there’s about an 80% chance that when we get a call for a vibrating fan that it’s going to be a balance issue and a 20% chance it’s something else. And what could that something else be?

If it’s a motor, then the problem could be ...

  • Loose mechanical connections, such as bolts or welds
  • Problems with couplings
  • Structural issues
  • Bearing defects 
  • Slippage of belts and/or sheaves
  • A cracked rotor
  • Resonance conditions
  • Other motor problems

For fans, vibration can often be traced back to either a build-up of dust on the blades or a shaft that’s bowed because of temperature differentials across the fan.

Vibration analysis is key to determining if the vibration is actually caused by imbalance. More specifically, spectrum analysis via a fast Fourier transform (FFT) and performed by a certified vibration analyst can reveal the most likely source of the vibration.

Conclusion

Both static and dynamic balancing is extremely important if you want to get maximum life out of bearings, seals, and rotating equipment in general. And that includes both static and dynamic balancing: you can’t achieve dynamic balance if your equipment isn’t first statically balanced. 

If you’d like to learn more about balancing rotating equipment, or maybe just finding out if an imbalance is the source of your vibration issues, contact HECO today. Our certified vibration analysts can perform the necessary evaluations and vibration testing to track down the source of vibration, whether it’s due to an imbalance in your system or a simple case of loosened bolts. 

 

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