1 Improper design of cage structure X1
The pocket hole of the bearing cage is a waist-shaped hole. The results of long-term use show that the cage currently designed can meet the design and use requirements. Therefore, X1 factor of improper cage structure design can be eliminated.
2 Improper selection of cage material X2
Since its development, the bearing has been used with more than one main engine for its life. As of 2021, there has never been any quality problem caused by cage material. The material selection of bearing cage conforms to the bearing design specification, and X2 factor can be ruled out if the cage material is improperly selected.
3 Improper design of rivet structure X3
The design of bearing rivet structure conforms to the bearing design specification, and X3 factors can be excluded if the rivet structure is not designed properly.
4 Improper selection of rivet material X4
The rivet material is ML15 steel wire. The mechanical properties of qualified ML15 steel wire and decarburized ML15 steel wire were tested respectively. The tensile strength and yield strength of decarburized ML15 steel wire were 32.5% and 64.3% lower than those of qualified ML15 steel wire. Therefore, X4 factor of improper selection of rivet material cannot be ruled out.
5 Improper selection of clearance between rivet and rivet hole X5
According to statistics, the main reason for the failure of the bevel drive gear bearing of the oblique drive down in the past is the poor fit relationship between the rivet material and the rivet hole. When the fit clearance between the rivet hole and the rivet is large gap or large interference, the safety factor of the rivet is less than 1, resulting in the fatigue fracture of the rivet. Seen from this fault phenomenon, three rivets are reserved on the cage on the side of the nut (the rivet head is in a circular arc), and the remaining six are completely out of the rivet hole. The improper selection of gap between rivet and rivet hole X5 factor cannot be ruled out.
6 Dimension of phase fitting is out of tolerance X6
Check the original records of engine installation, and there are no out-of-tolerance items related to bearing failure. The dimension measurement of the engine accessory transmission gearbox, middle drive/driven bevel gear and other fault related components is qualified. Therefore, the factor X6 that the dimension of phase fitting exceeds the tolerance can be eliminated.
7 Bearing size out of tolerance X7
The fault bearing was rechecked, and no unqualified factors affecting the fault were found, so the bearing dimension out-of-tolerance X7 factor could be eliminated.
8 Bearing material and metallurgical quality are unqualified X8
According to the management analysis, the material and metallurgical quality of the bearing inner, outer ring, steel ball, rivet and other materials of the lower bevel drive gear of the C241 unit are normal. Therefore, X8 factor of unqualified bearing metallurgical quality can be eliminated.
9 Foreign matters in lubricating oil X9
There was no abnormality in the lubricating oil during the regular inspection in the outfield of the failed machine. After the failure, the lubricating oil filter was inspected, and no other foreign matter was found except for the metal chips worn due to the failure. Therefore, the foreign matter X9 in the lubricating oil can be eliminated.
10 Abnormal motor load X10
Check the external power consumption of the faulty machine, the AC motor works normally, and the electrical parameters on the machine are normal. Therefore, X10 factor of abnormal motor load can be eliminated.
11 Large vibration X11
Check the use of the engine in the field, and there is no abnormality in the use of the engine in the field. Therefore, the factor of large vibration X11 can be eliminated.
12 Poor bearing lubrication X12
According to the appearance inspection and metallurgical analysis of the faulty parts, no poor lubrication of the faulty bearing was found. Therefore, X12 factor of poor bearing lubrication can be eliminated.
13 Nut not tightened X13
According to the inspection of assembly records, the tightening torque of the nut meets the design requirements, and no nut looseness is found during disassembly. Therefore, the factor that the nut is not tightened X13 can be ruled out.
To sum up, the factors that cannot be excluded include: improper selection of rivet material X4 and improper selection of gap between rivet and rivet hole X5.