The Balancing Act – Rotor Stability

When designing rotating equipment, it is extremely important to take into account the types of unbalance that can occur. Forgetting this step can result in vibrations that lead to damage of the rotating parts, increasing maintenance costs and lowering efficiency. Currently, if a rotating part already vibrates or makes any noises, maintenance engineers rely on OEMs (Original Equipment Manufacturer) or third parties services companies to conduct balancing services.

Types of Unbalances

Figure 1: Static and Couple Forms of Unbalance

The three types of unbalances to consider are static, couple and dynamic. Static unbalance (Figure 1) occurs when a mass at a certain radius from the axis of rotation causes a shift in the inertia axis. Couple unbalance, usually found in cylindrical shapes, occurs when two equal masses positioned at 180 degrees from each other cause a shift in the inertia axis, leading to vibration effects on the bearings. Lastly and most common, dynamic unbalance occurs when you have a combination of both static and couple unbalance.

Balancing Methods used Today

Figure 2 High-Speed Balance

Two of the most popular balancing methods that are performed on existing turbine rotors are low-speed balancing and high-speed balancing methods. A low-speed balance helps determine wear and tear on rotor under minimum speed conditions. This method is generally used by companies who lack access to a high-speed balance cell. A high-speed balance (Figure 2) allows the test be run at or passed operating speed, and even though it is more expensive, it tends to be a more accurate method of testing. The primary reason why  more end users are opting for this type of balance, for both turbine rotors and power generator fields, is that all major components can be tested under rigorous conditions. By doing this, the full operating lives of these components can accurately be determined.

Want to make sure you’re taking into account unbalances when designing rotors in rotating equipment? Look into our rotordynamics software capabilities 


Reciprocating Machinery Dynamics by Abdulla S. Rangwala – Chapter 9

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