During a routine inspection of a freight elevator, I encountered a case where the traction machine produced significant impact noise during operation. Upon further investigation with maintenance personnel, I discovered that the bevel gears in the reduction gearbox had experienced tooth fracture. The elevator was manufactured in December 2012 and put into service in February 2013, utilizing a bevel gear pair as the speed reduction mechanism. Bevel gears are critical components in power transmission systems, and their failure can lead to severe incidents such as elevator overspeeding or sudden stops. In this analysis, I will delve into the root causes of the bevel gear tooth fracture through microstructural observation, chemical element analysis, and mechanical property testing, emphasizing the importance of proper lubrication and material quality in bevel gears.
Bevel gears are commonly used in elevator traction machines for their ability to transmit motion between intersecting shafts efficiently. However, failures in bevel gears can result from material defects, manufacturing imperfections, fatigue wear, or inadequate lubrication. In this case, the bevel gears were cast from a high-aluminum zinc-based alloy, which is known for its good castability, machinability, and mechanical properties. Despite these advantages, zinc alloys are prone to casting defects like shrinkage porosity and micro-shrinkage due to their wide solidification range. Additionally, bevel gears are sensitive to temperature variations, which can exacerbate these defects under operational stress.
Initially, I hypothesized that the tooth fracture in the bevel gears was primarily due to insufficient lubrication and poor material quality. In bevel gear systems, the high sliding velocities and friction between teeth generate substantial heat. Lubrication is essential to dissipate this heat and prevent wear, pitting, scuffing, or tooth fracture. If the oil level is low or the lubricant is contaminated, the bevel gears may not receive adequate protection, leading to accelerated degradation. Moreover, substandard bevel gear materials with inclusions, chemical imbalances, or severe segregation can weaken the gear teeth, making them susceptible to fracture under load. For instance, sharp notches or pores in critical sections can act as stress concentrators, reducing the gear’s strength and toughness.
To validate these hypotheses, I extracted samples from the fractured bevel gear for detailed analysis. The samples included sections from intact areas and regions near the fracture surface. I employed electrical discharge wire cutting to obtain specimens for microstructural observation, chemical analysis, and mechanical testing. The microstructural samples were thermally mounted, and the mechanical test specimens were precision-machined on a lathe to ensure accuracy.
Microstructural Observation
I conducted microstructural observation using both optical microscopy and scanning electron microscopy (SEM). For optical microscopy, I selected four samples: two from the intact bevel gear material (labeled Sample 1 and Sample 2, which were also used for chemical analysis) and two from areas adjacent to the fracture (labeled Sample A and Sample B).
Under optical microscopy at 50x magnification, Samples 1 and 2 revealed numerous porosity defects and coarse dendritic structures rich in aluminum. These dendritic formations, with their sharp angles and irregular shapes, introduce stress concentrations that can initiate cracks. The porosity, with pore diameters ranging from 100 to 150 μm, likely served as crack initiation sites during service. The presence of such defects undermines the material’s integrity and ductility.
At 100x magnification, the microstructure showed a dendritic white primary α-phase, surrounded by a light gray α+η eutectoid phase, and black β+η eutectic phases distributed along grain boundaries. In zinc-aluminum systems, the α-phase is a solid solution of zinc in aluminum, the η-phase is a solid solution of aluminum in zinc, and the β-phase can be an aluminum-based solid solution or an ordered ZnAl compound. These phases form through eutectic reactions, eutectoid transformations, and precipitation from supersaturated α-phase.
Further examination at 500x magnification under bright and dark fields in Sample 2 revealed bright white particles, identified as ε-phase (CuZn₄ compound), surrounding the β+η eutectic regions. This indicates potential inhomogeneities in the bevel gear material.
For Samples A and B, optical microscopy showed continuous eutectic structures along grain boundaries, which disrupted the matrix continuity and reduced alloy plasticity. Cracks were observed propagating along these grain boundary eutectics near the fracture surface, highlighting the role of microstructural defects in the failure mechanism.
SEM analysis at higher magnifications (1000x, 2000x, and 5000x) provided detailed insights. At 5000x, Samples 1 and 2 exhibited lamellar α+η eutectoid structures and elongated β+η eutectic phases along grain boundaries. Elemental mapping in Sample 2 confirmed zinc and copper enrichment at grain boundaries, corresponding to β+η eutectic and ε-phase precipitates.
In Samples A and B, SEM at 500x magnification revealed river patterns, micro-shrinkage, and microscopic dendritic crystals in pores. Inclusions and shrinkage defects were identified near the crack initiation sites, consistent with casting-related issues in bevel gears.
Chemical Element Analysis
I performed chemical element analysis on Samples 1 and 2 using spectrometry to determine the mass fractions of key elements. The results are summarized in Table 1. The composition aligns with the requirements for ZA27-2 alloy per relevant standards, indicating that the material met chemical specifications despite the observed defects.
| Element | Sample 1 | Sample 2 | Standard Requirement for ZA27-2 |
|---|---|---|---|
| Aluminum (Al) | 22.5 | 24.3 | 25–28 |
| Copper (Cu) | 2.7 | 2.4 | 2–2.5 |
| Magnesium (Mg) | 0.004 | 0.013 | 0.01–0.02 |
| Iron (Fe) | 0.051 | 0.042 | <0.075 |
| Zinc (Zn) | 74.75 | 73.25 | Remainder |
The chemical consistency suggests that the failure was not due to compositional deviations but rather microstructural and mechanical factors inherent in the bevel gears.
Mechanical Performance Testing
I conducted uniaxial tensile tests on six samples (labeled Sample 3 to Sample 8) from intact regions of the bevel gear. These were divided into two groups: one tested at room temperature (25°C) and the other at 85°C, as per elevator traction machine standards that specify a maximum oil temperature of 85°C. The tests measured tensile strength, yield strength, and elongation, with results presented in Table 2 and summarized using average values in a stress-strain curve.
| Sample ID | Test 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 |
The tensile strength values at room temperature met the standard requirements for ZA27-2 alloy (≥310 MPa), but the elongation values were significantly below the required 8%. This reduction in ductility is attributed to casting defects and the continuous distribution of eutectic structures along grain boundaries. At elevated temperatures, the mechanical properties deteriorated further, with tensile strength dropping by approximately 10-15%. The stress-strain relationship can be modeled using the following equation for engineering stress ($\sigma$) and strain ($\epsilon$):
$$\sigma = E \epsilon \quad \text{for linear elastic region}$$
where $E$ is Young’s modulus. However, the actual behavior deviates due to plastic deformation and defects. The average stress-strain curves for room temperature and 85°C are illustrated in Figure 1, showing decreased strength and elongation at higher temperatures.
The temperature sensitivity of high-aluminum zinc-based alloys can be expressed as:
$$\sigma_T = \sigma_0 \exp\left(-\frac{T}{T_c}\right)$$
where $\sigma_T$ is the tensile strength at temperature $T$, $\sigma_0$ is the reference strength, and $T_c$ is a critical temperature parameter. This equation highlights how elevated temperatures exacerbate material weaknesses in bevel gears.
Discussion on Bevel Gear Failure Mechanisms
The analysis confirms that the bevel gear tooth fracture resulted from a combination of factors. Microstructural defects, such as porosity, shrinkage, and inclusions, acted as stress concentrators, initiating cracks under cyclic loading. The continuous eutectic phases along grain boundaries reduced ductility, facilitating crack propagation. Inadequate lubrication likely caused localized overheating, softening the material and accelerating wear. For bevel gears, the contact stress between teeth can be calculated using the Hertzian contact theory:
$$\sigma_c = \sqrt{\frac{F}{\pi b} \cdot \frac{1}{\frac{1-\nu_1^2}{E_1} + \frac{1-\nu_2^2}{E_2}}}$$
where $\sigma_c$ is the contact stress, $F$ is the load, $b$ is the face width, and $E$ and $\nu$ are the modulus and Poisson’s ratio of the materials. In this case, defects magnified these stresses, leading to premature failure.
Moreover, the performance of bevel gears is highly dependent on maintenance practices. Regular inspection of lubrication levels and oil quality is crucial to prevent temperature rise and ensure efficient heat dissipation. The use of high-quality bevel gears with minimized casting defects is essential for longevity.
Conclusion
In this case study, I identified bevel gear tooth fracture in an elevator traction machine through comprehensive analysis. The failure was primarily due to microstructural defects in the high-aluminum zinc-based alloy and insufficient lubrication, which exacerbated temperature-related degradation. The mechanical tests revealed adequate tensile strength but poor elongation, underscoring the impact of material imperfections. To mitigate such risks, maintenance and inspection personnel should prioritize regular checks on bevel gear conditions, including lubrication and signs of wear. Implementing rigorous quality control during manufacturing and adopting advanced casting techniques can enhance the reliability of bevel gears in elevator systems. By addressing these factors, we can reduce the incidence of bevel gear failures and improve overall elevator safety.