Causes, failure mechanism and diagnosis of shaft cracks
1. turnSignificance of shaft crack diagnosis
1Rotating machinery is widely used in various sectors of industrial production. It is of special significance to find cracks before catastrophic shaft breakage, monitor crack propagation and realize reasonable maintenance for ensuring safe production and developing national economy.
2Rotating machinery in the petrochemical industry generally has a high speed and a large load. After long-term operation, the shaft is prone to transverse fatigue cracks, resulting in serious shaft breakage accidents.
2. turnCauses of Shaft Cracks
TurnThe failure probability of shaft cracks is much lower than other failures, but there are many potential causes of shaft cracks, such:
stress concentration caused by various factors
complex force state
Poor working conditions and environment, etc.
The crack is not sufficiently sensitive to the vibration response (depth1/4diameter of the crack, the shaft stiffness changes only10%left and right, the critical speed changes only5%left and right), may develop into a fracture shaft accident, great harm.
3,failure mechanism of shaft crack
TurnThe response of the shaft crack to vibration is related to the axial position of the crack, the depth of the crack and the force. According to the stress state of the part where the crack is located, the crack will take three different forms.
1.closed crack
When the shaft rotates under compressive stress, the crack is always closed.
This is the case, for example, when the rotor weight is small, the unbalanced centrifugal force is small, or the unbalanced force is on the other side of the crack.
The closed crack has little effect on the vibration of the shaft and is not easy to detect.
2.Cracking
When the crack zone is in a tensile stress state, the shaft crack is always in an open state.
The cracking will cause the asymmetry of the shaft stiffness, make the vibration have nonlinear characteristics, and accompanied2 ×,3 ×,...high frequency components.
As the crack grows,l×,2 ×and other frequencies also increase in amplitude.
3.Opening and closing crack
When the stress in the crack area is generated by self-weight or other radial load,TurnThe cracks will open and close every time the shaft rotates, and the effect on vibration is more complicated.
The theoretical analysis shows that the vibration response of the cracked rotor can be considered according to the two influencing factors of eccentricity and gravity, and then linearly superimposed.
Due to the influence of eccentricity, the vibration peak will appear between the two critical speeds corresponding to the asymmetric stiffness; The result of the gravity factor is that when the speed is near the critical speed of the crack-free shaft, a larger peak will appear. .
The opening or closing of the crack is related to the initial state, eccentricity, gravity and vortex velocity of the crack, as well as the depth of the crack.
If the rotor is synchronous vortex, the crack will only maintain one state, that is, open or closed, which is related to its initial state.
In the case of non-synchronous eddy currents, the crack may remain open or closed under certain conditions, but under normal conditions,TurnThe crack opens or closes with each turn of the shaft.
Rotating shaft
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2024-01-02
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