What Resonance Is and Why It Can Destroy Your Equipment

What resonance is: a simple explanation

Every mechanical structure (frame, foundation, supports) has its own natural frequencies of vibration. These are the frequencies at which the structure "likes" to vibrate.

An analogy: think of a swing. If you push the swing in time with its natural oscillations, the amplitude of the swing grows. Push out of time, and the swing barely moves at all.

Resonance occurs when the rotor's rotational frequency coincides with (or is very close to) one of the structure's natural frequencies of vibration. In that case, even a small exciting force from a minimal imbalance produces enormous vibration.

The danger: the vibration can grow 10-20 times over. This leads to:

• Destruction of fixings and foundations
• Fatigue cracks in the metal
• Catastrophic destruction of the structure

The critical speed

The critical speed is the rotational speed at which the rotor's rotational frequency coincides with the natural frequency of vibration of the "rotor-supports" system.

Rigid and flexible rotors:

• Rigid rotor: the working rotational speed is well below the first critical speed (usually by a factor of 2-3). Such a rotor does not bend under centrifugal forces
• Flexible rotor: it operates at a speed close to, or above, the critical speed. The rotor bends noticeably as it turns

How to detect resonance: diagnostic methods

Fig. 1. Resonance graph: a sharp vibration peak as the critical speed is reached (in this example ~2250 rpm).

Signs of operating in the resonance zone:

• A sharp increase in vibration at a particular rotational speed
• When the speed changes by ±100 rpm, the vibration changes 5-10 times over
• The vibration phase "jumps" from one measurement to the next
• As resonance is passed through, the phase changes by 180°
• The readings are unstable even at constant speed

Detection methods:

1. Coast-down test (run-down):

• The equipment is switched off and slows down
• Vibration is measured as the speed falls
• Vibration peaks at the resonant frequencies are visible on the graph

2. Bump test:

• The stationary equipment is struck with a modal hammer
• The system's response is analysed
• The natural frequencies are determined

Methods for dealing with resonance

1. Changing the working speed

Where possible, change the speed so that the working frequency is well clear of the resonant one (usually by ±15-20%).

2. Changing the stiffness of the structure

• Strengthening the frame and foundations: increasing stiffness raises the natural frequency
• Adding mass: increasing the mass lowers the natural frequency
• Changing the supports: using vibration isolators or stiffer fixings

3. Damping

• Fitting dampers (vibration absorbers)
• Using vibration-isolating mounts

4. Special balancing methods

For working near resonance, there are special balancing methods that disregard the phase (the four-run method). This is, however, a complex procedure that requires experience.