# Feasibility Study of Real-Time Monitoring of Pin Connection Wear Using Acoustic Emission
## 4 Discussion  
Influenced by various kinds of factors, such as loads, rotational speed, and lubrication condition, the wear process of low rotational speed pin connections is long and complicated. However, the wear forms of pin connections can be distinguished by an acoustic emission technique in this experiment. Comparing the results of the AE signals and the results of the micrographs, it can be found when only abrasive wear occurs on the surface of pins (such as on the third and the forth specimens), the signal strength value is low (their values are 9.475 mVs and 19.08 mVs respectively). By contrast, when adhesive wear occurs on the surface of pins (such as the first and the second specimens), the signal strength value is much larger than in abrasive wear (in the first and second specimens, their signal strength value is \(3.28\times 10^{3}\) mVs and \(3.674\times 3\) mVs respectively, which is around 190 times larger than that in the third and fourth specimens). This means the value of the signal strength of adhesive wear is much larger than abrasive wear, which is in agreement with the results of [102]. Therefore, adhesive wear is the main mode of wear in low rotational speed pin connections, and it can seriously affect the health condition of low rotational speed pin connections. Furthermore, analyzing the occurring time of abrasive wear and adhesive wear in this experiment, it is obvious that abrasive wear occurs in the early stage of the first and the second specimens, and in the whole process of the third and the fourth specimens, this can be demonstrated by the clear grooves on the surface of the four specimens in the micrographs, and the signal strength value, and the adhesive wear that occurs in the later period of the first and the second specimen. This means that abrasive wear may occur before adhesive wear in this experiment, and the reason for this should be further studied.  
On the other hand, the value of the signal strength slightly increased before the appearance of adhesive wear. This is because at the beginning of experiment, particles of abrasive wear originate from the lubricant [116], and they are small in size, causing only slight wear. With the reduction of lubricant, the minute bumps on the surface of the specimens or the hard particles detached from the surface of specimens become the main reason for abrasive wear, which causes more serious wear. In conclusion, it can be seen that the signal strength is sensitive to the wear degree of the pin connections, and it can identify the adhesive wear and abrasive wear by comparing their signal strength value; hence, the signal strength is suitable for monitoring the wear degree of the pin connections.  
Furthermore, the rotational speed may be different in actual engineering, which may cause different characteristics in AE signals and wear modes. However, when a severe wear occurs, the released energy will increase dramatically, which causes the strength of AE signals to increase dramatically. Therefore, the signal strength of the AE signals can still be used to detect the wear degree of different rotational speed pin connections. In addition, whether wear modes are influenced by the loading level and rotational speed shall be studied in the future.  
Figure 11: Micrographs of the fourth specimen: (**a**) 2D micrographs; (**b**) 3D micrographs.