Resonance is a prevalent cause of excessive machine vibration. It results from an external force vibrating at the same frequency as the natural frequency of a system. Natural frequency is a characteristic of every machine and structure.

If equipment is operating in a state of resonance, the vibration levels will be amplified, which can cause equipment failure and plant downtime. Thus, the running speed of the equipment must be out of the resonance range.

Identification

Many techniques can be used to identify a high vibration level caused by a resonance frequency. It is a must to confirm a resonance phenomenon by at least two different tests before correcting it. We will look at a few techniques used in the industry.

Impact Test

Without Equipment

One of the most used methods for measuring a system’s natural frequency is to strike it with a mass and measure the response. This method is effective because the impact inputs a small amount of force in the equipment over an extensive frequency range.

When performing this technique, it is necessary to try impacting different locations on the structure since all its resonant frequencies will always be measurable by affecting one area and measuring the exact area. Both drive point and transfer point measurements should be taken to identify machine resonances.

With Equipment

This test uses an instrumented hammer to excite the system. This hammer, equipped with an accelerometer at one end, is used with the sensor to measure the vibration. It is necessary to use a two-channel vibration analyzer, with one channel linked to the instrumented hammer and the other to the vibration sensor. Using this technique, you can measure the force induced to the system by the instrumented hammer and the response at different frequencies.

The advantage of using this method is that it allows you to check phase shifts and coherence. This information will enable you to create operating deflection shapes to visualize the vibrating body.

Coast down peak

Another method is to check the vibration level using a peak hold function while shutting down the equipment. The vibration level should decrease.

This test will also be conducted while the equipment is being shut down. You can check the vibration and its phase by installing a photo tack and a piece of reflective tape on the rotating gear shaft. This will allow you to see the equipment's amplitude and phase shift at all running speeds. The vibration levels should drop if there is no resonance excited by the turning rate.

Once the resonance is confirmed, change the mass or the stiffness of the equipment to change its natural frequency. If this is not possible, consider changing the device's operating speed. If that doesn't work, consider adding a dynamic absorber to counterbalance the initial exciting force.

Correction

The natural frequency of a system is the frequency of free vibration in which the system vibrates to release its energy. The natural frequency (ωn) of equipment, expressed in radians per second, is a function of its stiffness (k) and its mass (m), as shown by the following equation:

If these two parameters are altered, the natural frequency will change. If we want to change the natural frequency of a body, we have to change the stiffness or the mass. Increasing the mass or lowering the stiffness will decrease the natural frequency while reducing or increasing stiffness will increase the natural frequency.

If we cannot change the stiffness or the mass of the equipment, one easy solution is to change the operating speed of the equipment by 20 to 30 percent, but this is not usually an option. Another solution is to install a dynamic absorber on the equipment to reduce the vibration levels of the equipment. The dynamic absorber is a spring-mass system installed in series with the resonant system to create an out-of-phase exciting force to counteract the initial exciting force.

Implementing Maintenance Strategies

Implementing maintenance programs to diagnose incipient machine failure through oil analysis entails cost. As such, thorough planning is a must to ensure that your resources are well-spent. With CRE Philippines, you will be confident to start your Machine Diagnostics Program through oil analysis with our full support every step.

CRE Philippines also offers Vibration Analysis I Training. This course will help you understand the relevance of vibration analysis, the fundamentals of vibration analysis, and learn methods for quality and dependable measurements. Vibration analysis will be used to start the process of diagnosing common issues.

Source:

Pellegrino, A. (n.d.). How to Identify, Correct a Resonance Condition. ReliablePlant. Retrieved from https://www.reliableplant.com/Read/22455/identify-resonance-condition-vibration