State detection and fault diagnosis of rolling bearings

Rolling bearings are important parts of rotating machinery. Statistics show that in rotating machinery using rolling bearings, about 30% of mechanical failures are caused by rolling bearings. With state detection and fault diagnosis technology, the accident rate can be reduced by 75% and maintenance costs can be reduced by 25% to 50%.

First, the failure form of rolling bearings

(1) Fatigue spalling

The inner and outer raceways and rolling elements of the rolling bearing alternately enter and exit the bearing area. These components are subjected to the alternating load for a long time, firstly generating fatigue cracks from the maximum alternating shear stress below the contact surface, and then expanding to the contact surface to produce on the surface layer. Peeling off, gradually develop into large pieces of peeling, called fatigue spalling.

(two) wear

Due to the relative motion of the raceway and the rolling elements and the surface wear caused by dust foreign matter, poor lubrication will aggravate the wear, resulting in increased bearing clearance, increased surface roughness, reduced bearing running accuracy, and thus reduced machine motion accuracy. It is manifested by an increase in vibration level and noise.

(three) bruises

Due to the microscopic bumps or hard particles on the contact surfaces of the inner and outer races of the bearing and the rolling elements, the contact surface is unevenly stressed. Under the poor lubrication and high-speed heavy-load conditions, the contact surface is locally deformed due to the heat generated by the local friction. Friction welding, the surface metal may be partially melted in severe cases, and the force on the contact surface tears the local friction welding point from the substrate.

(4) Breaking

When the load and vibration of the bearing are too large, the defect position of the inner and outer rings is gradually expanded and fractured under the repeated impact of the rolling elements.

(5) Corrosion

Moisture or direct intrusion of acid or alkaline substances can cause bearing corrosion. When a shaft current passes inside the bearing, an electric spark is generated at the contact point of the raceway and the rolling element to cause electric corrosion, and a seesaw-like unevenness is formed on the surface.

Second, the failure process of rolling bearings

Bearing failure is usually divided into four phases:

(1) The first stage: the ultrasonic frequency vibration phase of the bearing

The earliest fault of the bearing is the vibration anomaly of the ultrasonic frequency in the range of 250 kHz to 350 kHz. As the fault develops, the abnormal frequency gradually decreases to 20 kHz to 60 kHz. At this time, the minor fault of the bearing can be impacted by the envelope and the acoustic emission. The method detects that the impact envelope value can be up to 0.5gE (acceleration envelope, an acceleration index indicating the amplitude in the vibration analysis).

(2) The second stage: the natural frequency vibration phase of the bearing

As the bearing operates, the bearing rolling surface will produce slight defects. The vibration caused by these slight defects will arouse the natural frequency (fn) vibration of the bearing component or the resonance of the bearing support structure. The vibration frequency is generally 500 Hz to 2 kHz. At the same time, the frequency is also used as the carrier frequency to modulate the fault frequency of the bearing. At first, this frequency can only be observed, and the latter appears as a side frequency near the natural frequency. If the acceleration envelope method is used, it will find that the envelope value will rise to about 0.5 to 1.OgE. At this point, the bearings are still safe to operate.

(III) The third stage: bearing defect frequency and its frequency doubling vibration stage

With the further expansion of the bearing minor defects, the bearing defect frequency and its frequency multiplication begin to appear. As the bearing wear progresses further, the frequency multiplication of more defect frequencies begins to appear, and the number of sidebands surrounding these frequency doublings and the natural frequency of the bearing components It is also gradually rising. At this time, the vibration of the bearing is already obvious, and it should be considered to replace the bearing as soon as possible.

(four) the fourth stage: the random broadband vibration phase of the bearing

The bearing is almost completely ineffective, the life of the bearing is nearing the end, and even the power frequency is affected by it and rises and produces many power frequency multiples. The original discrete bearing defect frequency and natural frequency begin to "disappear" and are replaced by random Broadband high-frequency "noise vibration", high-frequency noise vibration and envelope value have decreased, but the envelope impact value will increase significantly before the final failure of the bearing.