Identifying Mechanical Loosening with Vibration Spectrum Analysis

Mechanical looseness is generally classified into two categories: structural looseness and rotating component looseness.

The reasons for mechanical looseness can include poor installation, prolonged wear, damage to the foundation or base, damaged parts, excessive clearances in the fit, and so on.

Mechanical looseness can exacerbate vibration problems caused by imbalances and misalignments, and can also lead to further failures due to the progression of the looseness.

There is no definitive standard for defining the types of looseness.

Currently, there are three common types of mechanical looseness, each of which has its own unique vibration frequency spectrum and vibration phase characteristics.

1. Type A: loose structure frame and foundation

1. Loose phenomenon and treatment measures

This type of looseness encompasses the following faults:

• Loose structures or inadequate strength in the equipment footing, base plate, and concrete foundation.

• Deterioration or breakage of the grout.

• Deformation of the frame or base.

• Loose anchor bolts, etc.

These looseness issues can be easily observed on the site, and their destructive impact is usually significant. In severe cases, they can exacerbate the imbalances or misalignments of the equipment.

Treatment measures:

  • Strengthening the foundation and correcting the structure.
  • Tightening loose bolts, etc.

For equipment that is already out of balance or misaligned, it is important to address the imbalance or misalignment simultaneously.

2. Typical loosening frequency spectrum and basic characteristics

The typical frequency spectrum of looseness is depicted in Figure 1, and the fundamental characteristics revealed by the looseness are presented in Table 1.

Fig. 1 Typical loosening spectrum diagram of type A

Table 1 Basic Characteristics Reflected by Type A Looseness

ParameterBasic characteristics
FrequencyThe loosening frequency spectrum is dominated by higher 1×-turn frequency vibration
VibrationGenerally, the radial vibration is large, especially the vertical vibration is large, and the axial vibration is small or normal
PhaseComparing the vibration in the vertical and horizontal directions, it can be found that the vibration has directivity, and the phase difference is 0 ° or 180 °


  • This typical looseness feature is comparable to the vibration caused by an unbalanced or eccentric rotor, and they can be differentiated based on phase.
  • Usually, high vibration is concentrated on one rotor only (such as the driver, driven machine, or gearbox), which distinguishes it from imbalance or misalignment as high vibration caused by these issues is not limited to just one rotor.
  • In some exceptional cases, such as bolts used to secure the pump bearing seat, the force is in the axial direction. If these bolts become loose, the axial 1x-speed vibration will be elevated, resembling a misalignment fault. However, once these bolts are tightened, the vibration will be reduced.

2. Type B: looseness caused by rolling motion or structural fracture and bearing pedestal fracture

1. Loose phenomenon and treatment measures

This type of looseness only occurs when the following faults occur:

  • A damaged structure or bearing base.
  • An instability caused by unequal length of support feet, at times.
  • Loose bearing seat bolts in rare cases.
  • Minor fitting issues with loose bearings or improper parts (with no significant impact).

These looseness issues can also be observed on-site, but the matching problems of internal components can only be detected and confirmed through disassembly and examination.

Treatment measures:

The vibration can be reduced by replacing damaged parts, fixing the fit of incorrect parts, tightening bolts, etc.

2. Typical loosening frequency spectrum and basic characteristics

The typical frequency spectrum of looseness is depicted in Figure 2, and the fundamental characteristics reflected by the looseness are presented in Table 2.

Fig. 2 Typical loosening spectrum diagram of type B

Table 2 Basic Characteristics Reflected by Type B Looseness

ParameterBasic characteristics
FrequencyMultiple turn frequency harmonics, when the radial 2×-turn frequency amplitude exceeds 50% of the 1×-turn frequency amplitude, it indicates that such a fault occurs.
VibrationThe amplitude is somewhat unstable. When the load is high, the vibration increases greatly.
PhaseIf a strobe lamp is used to collect phase readings, two unstable reference points will usually be displayed.


• Under normal conditions, these vibration symptoms will not occur if there are no other excitation forces.

• If the looseness is caused by the loose bearing of the bearing pedestal or the loose parts on the shaft, the vibration will remain mostly at the 1x- and 2x-speed until it worsens into a pulsation or impact.

In this case, the pulsation will result in the non-linearity of the time domain waveform, leading to many harmonics that are more severe than Type C looseness.

• Sometimes, the failure of the coupling is further exacerbated by the fracture and looseness of the equipment foot, causing wear and looseness in the elastic block of the coupling. This spectrum also displays harmonics that surpass Type C looseness.

3. Type C: looseness caused by loose bearings or improper fitting between components

1. Loose phenomenon and treatment measures

This type of looseness encompasses the following faults:

• Loose bearings in the bearing pedestal.

• Excessive internal clearance in the bearings.

• Loose bearing bushes in the bearing seat.

• Loose rotor.

• Loose bearings or running rings, etc.

These issues can be observed by opening the end cover of the bearing pedestal. This type of looseness is directly associated with the bearings and shafts of rotating equipment.

When the looseness is severe, the bearings, shafts, or related mating parts will suffer wear, or in severe cases, the rotating equipment may become completely blocked.

Treatment measures:

It can be solved by replacing the bearing or bushing and adjusting the fit between components.

2. Typical loosening frequency spectrum and basic characteristics

Typical loosening frequency spectrum is shown in Fig. 3, and the basic characteristics reflected by its loosening are shown in Table 3.

Fig. 3 Typical loosening spectrum diagram of type C

Table 3 Basic Characteristics Reflected by Type C Looseness

ParameterBasic characteristics
FrequencyThe harmonics of multiple frequency conversion sometimes reach 10× or even 20×, which are very obvious in the spectrum.If the harmonic amplitude becomes larger, the frequency component with an interval of 1/2 times the frequency will also be generated (i.e. 0.5 ×, 1.5 ×, 2.5 ×). Sometimes there is even 1/3 times of the frequency conversion harmonic.
VibrationThis looseness tends to produce highly directional vibration with relatively high amplitude.
phaseGenerally, the phase measurement of this type of loose fault is somewhat unstable, but if the vibration itself becomes highly directional, the difference between the horizontal and vertical directions will be close to 0 ° or 180 °.


• Looseness may also occur after the component has reached its operating temperature and has undergone thermal expansion.

• The presence of a distinct 1/2x-peak suggests that a more complex loosening issue is present, possibly involving friction.

• When the rotor, such as a pump impeller, is loose, the phase changes after each start.

• The vibration spectrum of this type of looseness, characterized by many 1×-speed harmonics, actually indicates a more severe problem, such as looseness in the bearings and ring running.

This issue can result in shaft clamping and significant equipment failure.

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