Several common non-destructive testing methods for bearings

Non-destructive testing (NDT) concept: Non-destructive testing is a testing method to check the surface and internal quality of the tested part without damaging the working state of the workpiece or raw materials, also known as non-destructive testing.

Common nondestructive testing methods include the following:

1) Ultrasonic detection UT: detect the defects inside the parts, using shear wave and surface wave technology can also detect the defects on the surface and near the surface of the parts.

2) Magnetic particle detection MT: detect the surface and near-surface defects of ferromagnetic parts.

3) Color (penetration) detection PT: detect the defects of the surface opening.

4) Eddy current testing ET: detect defects on the surface and near the surface of metal parts.

5) X-ray detection RT: detect the defects inside the parts.

1. one Ultrasonic inspection

Ultrasonic detection principle: It is a method to check the defects of parts by using the ultrasonic wave incident on the inside of the material from one interface to another interface, and the characteristics of reflection at the junction of different interfaces.

When the ultrasonic beam is inside the metal, it encounters the defect and the bottom surface of the part, and the reflected wave is respectively generated, and the pulse waveform is formed on the fluorescent screen, and the position and size of the defect are judged according to the pulse waveform.

The A-type display in ultrasonic graphics is A waveform display, the horizontal coordinate of the screen represents the propagation time (or distance) of the sound wave, and the vertical coordinate represents the sound pressure amplitude of the reflected wave. It can be considered that this method displays echo information at different points along a line in the direction of the probe emitted sound beam.

In the figure, T represents the emitted pulse, F represents the echo from the defect, and B represents the underside echo.

Ultrasonic wave is a kind of sound wave in the mechanical wave, through the transmission of mechanical vibration in the elastic medium, the frequency of the sound wave is higher than 20000Hz, generally between 0.5 and 10MHz, for the inspection of steel and other metal materials, the commonly used frequency is 1 to 5MHz.

Ultrasonic wave is characterized by good directivity: ultrasonic wave is a mechanical wave with a high frequency and a very short wavelength, and the wavelength used in nondestructive testing is on the order of millimeters.

Ultrasonic waves, like light waves, have good directivity and can be emitted in a directional manner. High energy: the ultrasonic detection frequency is much higher than the sound wave, and the energy (sound intensity) is proportional to the square of the frequency. It can generate reflection, refraction and wave mode conversion at the interface. Strong penetration: the ultrasonic propagation energy loss is small, the propagation distance is large, and the penetration ability is strong. Ultrasonic can probe the common defects of bearings: material defects: shrinkage, porosity, inclusion, cracks, etc. Forging defects: folding, white spots, cracks, etc. Heat treatment defects: quenching cracks, etc. Welding defects: porosity, slag inclusion, incomplete penetration, non-fusion, cracks, etc.

2. Magnetic particle detection: also known as magnetic particle detection

Magnetic particle flaw detection principle:

When the surface or near the surface of the part has a defect in the direction of vertical (or close to vertical) magnetic field line propagation, because the permeability of the defect is far less than the permeability of the part, there will be a part of the curved magnetic field line extrusion out of the surface of the part, forming a leakage magnetic field. The leakage magnetic field attracts and adheres magnetic particles to reveal the location, shape and size of the defect. Compared with ultrasonic inspection and X-ray inspection, magnetic particle inspection has high sensitivity, simple operation, reliable results, good repeatability and easy identification of defects. However, this method is only suitable for testing surface and near-surface defects of ferromagnetic materials.

3. Eddy current inspection

Eddy current testing is to put a conductor close to a coil with alternating current, and the coil creates an alternating magnetic field. The alternating magnetic field passes through the conductor and induces electromagnetic induction with it to create eddy currents in the conductor. The eddy current in the conductor also generates its own magnetic field, and the effect of the eddy current magnetic field changes the strength of the original magnetic field, which leads to the change of coil voltage and impedance. When there are defects on or near the surface of the conductor, the intensity and distribution of the eddy current will be affected, and the change of the eddy current will cause the change of the voltage and impedance of the detection coil. According to this change, the existence of the defect in the conductor can be indirectly known.

Eddy current testing can realize 100% on-line inspection of bearing parts. Compared with magnetic particle testing, eddy current testing has the advantages of fast detection speed, high automation and low cost, and is widely used in bearing manufacturing.