Vibration Spectrum Analysis of Three Common Mechanical Loosening

Mechanical looseness is generally divided into structural looseness and rotating component looseness.

The causes of mechanical looseness include poor installation, long-term wear, foundation or base damage, parts damage, excessive fit clearance, etc.

Mechanical looseness can make the vibration problem caused by the existing imbalance and misalignment more serious, and can also cause other failures due to the further deterioration of mechanical looseness.

There is no strict standard to define the type of looseness.

At present, there are three common and typical mechanical looseness, all of which have their unique vibration frequency spectrum and vibration phase performance.

1. Type A: loose structure frame and foundation

1. Loose phenomenon and treatment measures

This type of loosening mainly includes the following faults:

• Loose structures or insufficient strength of equipment footing, base plate and concrete foundation;

• Deterioration or breakage of grout;

• Deformation of frame or bottom;

• Loose anchor bolts, etc.

These looseness phenomena can be obviously observed on site, and their destructive force is generally large, which will aggravate the imbalance or misalignment of equipment in serious cases.

Treatment measures:

Consolidate the foundation, rectify the structure, tighten the loose bolts, etc.

For equipment that is already unbalanced or misaligned, it is necessary to adjust the imbalance or misalignment at the same time.

2. Typical loosening frequency spectrum and basic characteristics

The typical loosening frequency spectrum is shown in Fig. 1, and the basic characteristics reflected by its loosening are shown in Table 1.

Vibration Spectrum Analysis of Three Common Mechanical Loosening 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 °

Notes:

• This typical loosening feature is consistent with the unbalanced or eccentric rotor fault feature, which can be distinguished by phase.

• Generally, high vibration is almost limited to one rotor (separate driver, driven machine or gearbox), which is different from imbalance or misalignment, because high vibration caused by imbalance or misalignment is not limited to one rotor.

• For some special cases, such as the bolts used to fasten the pump bearing seat, the force is in the axial direction.

If these bolts become loose, the axial 1×-speed vibration will be high, which is similar to the misalignment fault.

As long as these bolts are tightened, 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 only occurs when the following loosening faults occur:

• Broken structure or bearing base;

• Sometimes the swing caused by unequal length of support feet;

• Occasionally on some loose bearing seat bolts;

• When loose bearings or improper parts have minor matching problems (no impact).

These looseness phenomena can also be observed on site, but the matching problems of internal components can only be found and verified through disassembly and inspection.

Treatment measures:

The vibration can be reduced by replacing the broken parts, adjusting the fit of inappropriate parts, fastening bolts, etc.

2. Typical loosening frequency spectrum and basic characteristics

Typical loosening frequency spectrum is shown in Fig. 2, and the basic characteristics reflected by loosening are shown in Table 2.

Vibration Spectrum Analysis of Three Common Mechanical Loosening 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.

Notes:

• Under normal circumstances, if there is no other exciting force, these vibration symptoms will not occur.

• If the loose fault is caused by the loose bearing of the bearing pedestal or the loose parts on the shaft, the vibration will almost remain at 1× – and 2×-speed until it deteriorates to pulse or impact.

When this happens, the pulse will cause the nonlinearity of the time domain waveform, leading to many harmonics that are looser than type C.

• Sometimes the failure of the coupling is further caused by the fracture and looseness of the equipment foot, resulting in the wear and looseness of the elastic block in the coupling.

This spectrum also has harmonics that precede Type C looseness.

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

1. Loose phenomenon and treatment measures

Such looseness includes the following faults:

• The bearing of the bearing pedestal is loose;

• Excessive bearing internal clearance;

• The bearing bush in the bearing seat is loose;

• Loose rotor;

• Loose bearings or running rings, etc.

These phenomena can be observed by opening the bearing pedestal end cover. This type of looseness is directly related to the bearings and shafts of rotating equipment.

When the looseness is serious, the bearings, shafts or related mating parts will be worn, or even the rotating equipment will be directly 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.

Vibration Spectrum Analysis of Three Common Mechanical Loosening 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 °.

Notes:

• This looseness may also occur after the operating temperature has been reached and the component has been thermally expanded.

• If there is an obvious 1/2x-peak, it indicates that a more complex loosening fault exists (there may be friction).

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

• It seems that the vibration spectrum of this type of looseness (many 1×-speed harmonics) actually indicates that a more serious fault exists, that is, bearing looseness and ring running.

This fault will lead to shaft clamping and serious equipment failure.

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