How to diagnose industrial gearbox faults?

Industrial gearboxes, also often called reduction gearboxes, are a type of mechanical transmission device widely used in the industrial field. Industrial gearboxes mainly change the speed and torque of the input power through the meshing of a series of gears to achieve the purpose of reducing speed and increasing torque. Many types of gears such as spur gears, helical gears, bevel gears, worm gears, etc. that are well-known in the machinery industry all belong to industrial gearboxes.

The main products of SGR, planetary gear reducers, toroidal worm reducers, combined reducers, cylindrical worm reducers, and small reduction motors, are also industrial gearboxes. The application range of industrial gearboxes is very wide, including papermaking, sugar making, cement, chemical and rubber industries.

(1) The main forms of gear failure in industrial gearboxes

Among industrial gearbox failures, the most common one is gear failure. The manifestations of gear failure include: tooth breakage, tooth surface fatigue, tooth surface wear, tooth surface scratches, plastic deformation, chemical corrosion, foreign matter embedding, etc.

Tooth breakage is the most serious fault in industrial gearboxes. When the gear pair is meshing and transmitting motion, the action force of the driving wheel and the reaction force of the driven wheel act on the other gear teeth through the contact points. When one of the contact points is located at the top of the gear teeth, if it is suddenly overloaded or impact overloaded, it is easy to cause overload breakage at the root of the tooth.

Tooth surface wear or scratches are another common fault of industrial gearboxes. Possible fault forms include: adhesive wear, abrasive wear and scratches, corrosive wear, burns, tooth surface bonding, etc.

Tooth surface fatigue mainly includes pitting and spalling. Tooth surface pitting refers to when the alternating stress on the working surface causes micro fatigue cracks, and after the lubricant enters the cracks, the lubricant in the micro fatigue cracks expands the cracks under high pressure to form pitting. If the fatigue cracks on the surface extend deeper and farther, causing a large area or large pieces of metal to fall off, it is called spalling.

When the soft tooth surface gear transmits too much load (or under large impact load), it is easy to produce plastic deformation of the tooth surface. Under the action of excessive friction between the tooth surfaces, the contact stress of the tooth surface will exceed the shear strength of the material, and the tooth surface material will enter a plastic state, causing plastic flow of the tooth surface metal, forming grooves on the tooth surface near the pitch circle of the driving wheel and convex edges on the tooth surface near the pitch circle of the driven wheel, thus destroying the correct tooth shape. Sometimes "flash" may appear on the driven tooth surface of certain types of gears. In severe cases, the extruded metal fills the top gap, causing severe vibration and even fracture.

（2） Gear failure analysis method

The analysis methods of industrial gearbox gear failure mainly include: power spectrum analysis, sideband analysis and inverse band analysis.

Power spectrum analysis can determine the frequency composition of gear vibration signals and the distribution of vibration energy in each frequency component. It is an important frequency domain analysis method. When applying power spectrum analysis, the horizontal coordinate of the frequency axis can be linear or logarithmic. Logarithmic coordinates (constant percentage bandwidth) are suitable for fault generalization detection and prediction, and the analysis of noise is close to the response of the human ear; but for gear systems with more sideband components, the use of linear coordinates (constant bandwidth) will be more effective.

The sideband analysis method contains a wealth of gear fault information. To extract the sideband information, the frequency resolution must be high enough during spectrum analysis. However, when the interval between the sideband spectral lines is smaller than the frequency resolution, or the interval between the spectral lines is uneven, the sideband analysis is hindered.

The inverse frequency band analysis method is an advanced signal processing technology based on spectrum analysis, which is used for fault diagnosis. This method is to perform a secondary transformation on the spectrum of the original signal, that is, to perform an inverse Fourier transform on the logarithmic power spectrum of the signal to obtain a inverse frequency spectrum in the time domain. This fault analysis method is suitable for the vibration spectrum of a gearbox with several pairs of gears meshing at the same time, where several sidebands are cross-distributed together, and it is not enough to perform frequency refinement analysis to identify the sideband characteristics.

In general, gearbox failure has a great impact on equipment. At the very least, it will cause a decrease in transmission efficiency, reduced accuracy, shortened equipment life, increased noise and vibration, etc. At worst, it will cause downtime and production interruption, and even cause safety hazards such as gearbox rupture. Therefore, it is very necessary to monitor, diagnose and maintain the gearbox in time to prevent the occurrence of gearbox failures.(Author:SGR,Angie Zhang)

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