The case of worm gear broken teeth in a freight elevator traction machine is analyzed. The worm gear was found to have broken teeth during inspection, and the possible causes were initially judged to be insufficient lubrication and poor quality of the worm gear teeth. The following is a detailed analysis of the case:
1. Initial Judgment of the Causes of Worm Gear Broken Teeth
Cause
Description
Insufficient lubrication
The worm gear pair has a high transmission speed, which leads to increased friction and heat generation. Insufficient lubrication can cause wear, pitting, gluing, and broken teeth. Low oil level and dirty lubricant can also contribute to this problem.
Poor quality of worm gear teeth
Some worm gear teeth may have quality issues, such as large inclusions, unqualified chemical composition, and severe segregation, which can reduce the strength and toughness of the material. Sharp inclusions near the dangerous section can also cause stress concentration and reduce the strength of the material.
2. Microstructure Observation
2.1 Optical Microscope Observation
Sample
Microstructure
Defects
1#, 2#
Many loose pores and large primary Al-rich dendrites. In addition to the dendritic white primary α phase, there are also light gray α + η eutectoid phases and black β + η eutectic phases distributed along the grain boundaries.
The dendrites introduce stress concentration and form cracks, and the pores reduce the strength and toughness of the material. The needle holes in the worm gear can become the source of cracks.
A#, B#
The eutectic structure is continuously distributed along the grain boundary, destroying the continuity of the matrix and reducing the plasticity of the alloy. There are obvious cracks along the eutectic structure on the surface near the fracture.
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2.2 Scanning Electron Microscope Observation
Sample
Microstructure
Defects
1#, 2#
Lamellar α + η eutectoid and β + η eutectic phases distributed along the grain boundaries.
–
A#, B#
Obvious river patterns and shrinkage porosity. Microscopic dendritic crystals are observed in the pores, and shrinkage porosity and inclusions are found near the fracture initiation source.
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3. Chemical Element Analysis
Sample
Chemical Element Mass Fraction /%
Al
Cu
Mg
Fe
Zn
1#
22.5
2.7
0.004
0.051
74.75
2#
24.3
2.4
0.013
0.042
73.25
GB/T 1175 – 1997 Requirements for ZZnAl27Cu2Mg Alloy
25 – 28
2 – 2.5
0.01 – 0.02
< 0.075
Remaining
The chemical element mass fraction of the samples meets the requirements of the standard.
4. Mechanical Property Testing
Sample
Temperature
Tensile Strength / MPa
Yield Strength / MPa
Elongation /%
3#
Room temperature
320
290
1.7
4#
Room temperature
325
278
2.4
5#
Room temperature
315
269
2.6
6#
85 °C
278
254
1.6
7#
85 °C
266
247
2.1
8#
85 °C
284
260
1.4
GB/T 1175 – 1997 Requirements for ZZnAl27Cu2Mg Alloy
Room temperature
310
N/A
8.0
The tensile strength of the samples meets the requirements of the standard, but the elongation does not. This may be due to the casting defects and the continuous distribution of the eutectic structure along the grain boundaries, as well as the influence of temperature.
5. Conclusion
In this case, the worm gear of the elevator traction machine was found to have broken teeth, accompanied by abnormal vibration and noise. Through physical and chemical analysis, it was found that the worm gear had many casting defects, such as loose pores, shrinkage porosity, and inclusions. The tensile strength of the worm gear met the national standard requirements, but the elongation did not. Insufficient lubrication of the worm gear pair can also cause temperature rise and affect the mechanical properties of the worm gear pair. Maintenance personnel and inspection personnel should pay more attention to the use of the worm gear pair and take measures to reduce the risk of worm gear broken teeth.
