Dynamic Unbalance: What it Is, What Causes it, and How to Fix It

Dynamic Unbalance: What it Is, What Causes it, and How to Fix It

Dynamic unbalance, also called dynamic imbalance, occurs when rotating machinery is out of balance at its center of gravity and the principal inertia axis is not balanced with the rotational axis. Dynamic unbalance is a combination of static unbalance and couple unbalance. 

Dynamic unbalance causes excess vibration and noise, increased friction, efficiency reductions, and decreases the lifetime of bearings, seals, and other components. This condition worsens over time and, when left unchecked, can increase maintenance costs, worsen safety hazards, and even lead to catastrophic failure.

What Causes a Dynamic Imbalance?

Dynamic imbalance can have several root causes. Here are a few common ones:

Thermal growth: As equipment operates, its temperature increases. This causes the metals to expand in a process called thermal growth or heat distortion. Higher temperatures lead to greater growth, and all areas of the machine may not expand at the same rate, leading to dynamic unbalance. 

Uneven wear: Over time, machinery components like bearings erode and wear down. This can lead to uneven weight distribution around the center of rotation due to loss of mass. Erosion can force the rotor to adjust and lead to dynamic unbalance.

Missing balance weights: Balance weights are sometimes used to keep equipment rotating correctly. If these have been removed, imbalance can happen as a result.

Bent shafts: Even small bends in shafts can cause unbalance. They change the distribution of the weight, causing rotational unbalance.

Manufacturing defects: Uneven material density in the rotor components because of manufacturing defects can cause imbalance while the asset is in use.

Assembly or replacement part issues: Improper initial assembly can cause dynamic imbalance. Additionally, replacement parts that were improperly machined or installed incorrectly can also lead to uneven weight distribution.

What is the Difference Between Static, Couple, and Dynamic Unbalance?

There are three types of unbalances. While they may have similar symptoms and results, it’s important to know the difference since they require different methods of correction. 

Static Unbalance

Static unbalance happens when the center mass of a rotor is displaced from its rotational axis. Due to this offset, static unbalance can cause vertical vibrations and lead to other problems when left uncorrected. 

One way to identify static unbalance is by noting if the machine has a preferred placement when at rest. If rotating parts can move freely, the side with the heavier mass will naturally move to the bottom. Adding or removing a weight in a single plane of the shaft opposite of the heavier side can often counteract the imbalance and realign the center mass to the rotational axis. 

Couple Unbalance

Couple unbalance happens when the principal inertia axis, which passes through an object’s center of gravity, is out of balance with the rotational axis.

Couple unbalance often leads to a wobbling or twisting motion. Correcting it requires two equal weights to be placed 180 degrees apart in two correction planes. 

Dynamic Unbalance

Dynamic unbalance is a combination of static unbalance and couple imbalance. The center mass of the rotor is displaced from the rotational axis, and the principal inertia axis is misaligned from the rotational axis. 

Even though this is the most common type of unbalance, correcting this problem is more complicated than correcting static or couple unbalance alone.

How is Dynamic Unbalance Corrected?

The most effective way to correct dynamic unbalance is by using specialized tools such as a smart balancer. A smart balancer is a handheld device used to identify and correct unbalance issues without having to disassemble the machine. 

Sensors from the smart balancer are attached to various points on the machine. As the machine runs, sensors capture crucial data such as vibration levels, Fast Fourier Transform (FFT) spectrum, and phase information. Tools like the Prüftechnik VibXpert 3 Balancer provide comprehensive analysis in real-time, displaying all information right on the screen to help the user identify unbalance. 

After you’ve identified the unbalance, the next step is correcting it. The VibXpert 3 Balancer assists in calculating the correct position and amount of weight to be added. After you’ve added the weight, you can run the VibXpert 3 Balancer test again to see if the unbalance has been fully corrected. Rechecking is vital to ensure the machine is running smoothly. Once complete, you can also save or download your analysis or export it in formats that best suit your needs. 

Using a smart balancer is the quickest, simplest method of correcting dynamic unbalance and prolonging the lifespan of rotating equipment.

Contact Prüuftechnik to learn more about our best-in-class balancing tools for correcting unbalance.