Got Resonance? Here Are Remedies

If high vibration in your rotating machines is resonance instead of mass unbalance, these solutions will likely fix the problem.

According to Ron Eshleman, technical director of training at the Vibration Institute, Oak Brook, IL, “Excessive vibration is now more likely to be diagnosed as resonance than mass unbalance, especially in equipment driven by variable-frequency drives (VFD) with a wide speed range.”

Resonance occurs in a system when the frequency of a vibratory force (forcing frequency) is at or near a natural frequency. Vibration levels at resonance are controlled through damping. Vibration amplification is determined by the proximity of the forcing frequency to natural frequency.

While there are several ways to deal with resonance, situations differ. It’s important to select solutions based on specific applications. Dr. Eshleman offered the following advice.

—Jane Alexander, Managing Editor

Analysis

When high vibration is noted in rotating machinery, the analyst should conduct a thorough vibration analysis before applying commonly used resonance fixes. After all, excessive vibration could result from one or a combination of the following non-resonance situations:

• pedestals, foundations, or structures
• loose or missing bolts, broken welds, soft foot, or mismatched parts
• excessive vibratory force
• lack of damping.

To confirm the state of resonance in machinery, the analyst must verify the location of the system’s natural frequencies with a simple impact test, operating deflection shape, or modal test. The modal test will typically yield the most information.

Solutions

• Reducing resonant-equipment vibration levels can involve any of the following actions, or a combination thereof:
• raising or lowering the natural frequency to distance it from the forcing frequency
• adding damping
• attaching an absorber.

Raising the natural frequency is generally the preferred solution—but in variable-speed machines, it may be impossible to raise it adequately. Normally, 15% separation of frequencies is desired.

Raising the natural frequency (fn) involves increasing the stiffness (K) or reducing the mass (M). For a simple system:

f_n = 1/2π [K/M]^½

Note that raising the natural frequency can be a daunting task, given that this frequency is proportional to the square root of the stiffness divided by the mass. Many stiffeners increase mass, which works against the solution.

Lowering the natural frequency involves more system flexibility, or less mass that acts as structural support.

Damping removes energy from the system, but is difficult to implement. It can be added by using elastomeric elements or fluid dampers—when they conveniently fit into the design. Effective damping, however, involves relative motion.

Elastomeric coatings and pads typically don’t have sufficient energy-absorbing ability to reduce low-frequency high vibration.

Absorbers work well with machines in which speed varies only slightly. The ratio of the absorber mass to the machine size determines the speed variability permitted, while remaining within vibration limits. MT

To learn more about dealing with resonance and other vibration issues, contact Dr. Eshleman at reshleman@vi-institute.org, or visit the Vibration Institute at vi-institute.org.