In the field of high-performance electromechanical actuation systems, the planetary roller screw serves as a critical transmission component, enabling the conversion of rotary motion into linear motion with high precision, load capacity, and reliability. These planetary roller screw assemblies are widely employed in applications such as aerospace, marine steering, and industrial machinery due to their compact design and durability. However, the failure of such components can lead to significant operational disruptions. In this analysis, I will explore a case study involving the fracture failure of a precision planetary roller screw used in an electromechanical actuator. Through comprehensive investigations including fractography, material characterization, and mechanical analysis, I aim to identify the root cause of the failure and provide insights for prevention. The planetary roller screw in question experienced a brittle fracture near the thread run-out region after extensive testing, prompting a detailed examination to understand the underlying mechanisms.
The electromechanical actuator incorporating the planetary roller screw was designed for high-power applications, with a rated load capacity of approximately 41 tons. The planetary roller screw had a nominal diameter of 63 mm and an effective stroke length of 800 mm, contributing to a total assembly weight of around 150 kg. During post-test disassembly, an abnormal radial load was applied, leading to the fracture of the screw shaft at the root near the thread run-out. This incident highlighted the need for a thorough failure analysis to determine whether material defects, manufacturing issues, or operational stresses were responsible. The analysis focuses on the planetary roller screw as the central component, emphasizing its role in the system’s integrity.
To begin the investigation, I conducted a macroscopic examination of the fracture surface. The planetary roller screw exhibited a clean break with minimal plastic deformation, indicating a brittle failure mode. The fracture surface appeared flat and grayish under stereomicroscopy, with a fine-textured region extending about 2.3 mm from the outer surface and a rougher interior area. The crack origin was identified at the thread root surface, with radial patterns suggesting rapid propagation. No obvious corrosion or mechanical damage was observed, pointing towards an overload event rather than gradual degradation. This initial assessment set the stage for further microscopic and material analyses to rule out inherent defects in the planetary roller screw.
Moving to microscopic analysis using scanning electron microscopy (SEM), the fracture surface revealed distinct features. The fine-textured region near the origin showed shallow dimples, indicative of microvoid coalescence, while the rougher area displayed quasi-cleavage morphology, characteristic of brittle fracture. Energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of iron and trace chromium, aligning with typical alloy steel compositions. The absence of significant inclusions or corrosive elements suggested that the planetary roller screw material was sound, focusing attention on mechanical loading conditions. The SEM findings reinforced the hypothesis that the failure resulted from sudden excessive stress rather than material deterioration.
To evaluate the material quality of the planetary roller screw, I performed chemical composition analysis using standard testing methods. The results, summarized in Table 1, compare the elemental content with the specifications for GCr15 bearing steel, which is commonly used in high-strength components like planetary roller screws. The composition falls within acceptable ranges, confirming that the screw was manufactured from GCr15 steel without significant deviations.
| Element | Measured Value | GCr15 Standard Reference (YB9-68) |
|---|---|---|
| C | 0.99 | 0.95–1.05 |
| Mn | 0.28 | 0.20–0.40 |
| Si | 0.24 | 0.15–0.35 |
| Cr | 1.42 | 1.30–1.65 |
| S | 0.007 | ≤0.020 |
| P | 0.025 | ≤0.027 |
| Ni | 0.12 | – |
| Cu | 0.16 | – |
The material properties of GCr15 steel are critical for understanding the behavior of the planetary roller screw under load. Key parameters include an elastic modulus of $$E = 2.19 \times 10^{11} \, \text{N/mm}^2$$, a Poisson’s ratio of $$\mu = 0.3$$, a tensile strength of $$\sigma_b = 861.3 \, \text{MPa}$$, and a yield strength of $$\sigma_s = 518.42 \, \text{MPa}$$. These values indicate that the planetary roller screw is designed to withstand high stresses, but brittle fracture can occur if overloaded beyond its capacity. The hardness profile further elucidates the material’s condition, as discussed next.
I conducted Vickers hardness tests (HV0.2) on a cross-section of the planetary roller screw, starting from the surface and moving inward. The results, shown in Table 2 and Figure 1, demonstrate a gradient from a hardened surface layer to a softer core. The surface hardness corresponds to approximately 62 HRC, consistent with quenched GCr15 steel, while the core hardness decreases to around 22 HRC. This gradient is typical for heat-treated components like planetary roller screws, where surface hardening enhances wear resistance without compromising toughness.
| Distance (mm) | HV0.2 | HRC (Converted) |
|---|---|---|
| 0.00 | 750 | 62.3 |
| 0.25 | 737 | 61.8 |
| 0.50 | 696 | 60.1 |
| 0.75 | 658 | 58.3 |
| 1.00 | 702 | 60.3 |
| 1.25 | 690 | 59.8 |
| 1.50 | 658 | 58.3 |
| 1.75 | 663 | 58.5 |
| 2.00 | 653 | 58.0 |
| 2.25 | 540 | 51.7 |
| 2.50 | 449 | 45.1 |
| 2.75 | 384 | 39.3 |
| 3.00 | 307 | 30.7 |
| 3.25 | 253 | 22.8 |
| 3.50 | 219 | – |
| 3.75 | 207 | – |
| 4.00 | 215 | – |
| 4.25 | 210 | – |
The hardness data can be represented by a decay function, illustrating the transition from martensite at the surface to ferrite-pearlite in the core. A simplified model for hardness (H) as a function of distance (x) from the surface can be expressed as: $$H(x) = H_0 e^{-kx} + H_c$$, where $$H_0$$ is the surface hardness, $$k$$ is a decay constant, and $$H_c$$ is the core hardness. For this planetary roller screw, the hardened layer depth is approximately 3.0 mm from the tooth top, aligning with the fracture’s fine-textured region and confirming proper heat treatment.
Metallographic examination of the planetary roller screw sample revealed a microstructure consisting of martensite at the surface and ferrite with spheroidized pearlite in the core. The hardened layer depth, measured from the thread root, was about 2.3 mm, correlating with the fracture initiation zone. This microstructure is typical for GCr15 steel after quenching and tempering, providing high surface strength while maintaining some ductility in the core. Non-metallic inclusions were assessed according to GB/T 10561-2005, with plastic inclusions (sulfides) rated at 1.5 and brittle inclusions below 0.5, summing to less than 2.0—a normal level for quality steel. These findings indicate that the planetary roller screw material was free from significant defects, directing the analysis toward mechanical overload as the failure cause.
To quantify the stress conditions leading to fracture, I applied fracture mechanics theory. The planetary roller screw fracture originated at the thread root, where stress concentrations are inherent due to geometric discontinuities. Treating the thread root as a surface crack, the stress intensity factor $$K_I$$ can be calculated using the formula: $$K_I = Y(\alpha, \beta, \gamma, K_t) \cdot \sigma \sqrt{\pi a}$$, where $$\sigma$$ is the nominal stress at the thread cross-section, $$a$$ is the crack depth, and $$Y$$ is a dimensionless correction factor dependent on parameters such as crack aspect ratio $$\alpha = a/d$$ (with $$d$$ as diameter), $$\beta = a/c$$ (crack shape), $$\gamma = s_1/s$$ (position along crack front), and theoretical stress concentration factor $$K_t$$. For the planetary roller screw, $$K_t$$ is approximately 2.79 based on thread geometry.
Assuming an axial overload of 50% beyond the rated load (i.e., $$6 \times 10^5 \, \text{N}$$), the nominal stress $$\sigma$$ at the thread root diameter of 60 mm is: $$\sigma = \frac{F}{A} = \frac{6 \times 10^5}{\pi (0.03)^2} \approx 212 \, \text{MPa}$$. However, considering bending effects from radial loading, the actual stress is higher. Using conservative values, with crack depth $$a = 2.3 \, \text{mm}$$ and $$Y = 1.5987$$ for the crack edge (where $$\gamma = 1$$), the stress intensity factor becomes: $$K_I = 1.5987 \times 212 \times \sqrt{\pi \times 0.0023} \approx 468.66 \, \text{N/m}^{3/2}$$. Comparing this to the fracture toughness $$K_{IC}$$ of GCr15 steel, which is typically around 2305 N/m^{3/2}, we find $$K_I < K_{IC}$$, suggesting that under pure axial loading, the planetary roller screw would not fail. This implies that additional radial stresses played a key role in the fracture.
During disassembly, a radial load was applied due to misalignment, inducing a bending moment at the fracture site. The bending moment $$M$$ can be estimated as: $$M = F \times L$$, where $$F = 1.62 \times 10^5 \, \text{N}$$ (from load cell data) and $$L = 0.15 \, \text{m}$$ (lever arm). Thus, $$M = 2.43 \times 10^4 \, \text{Nm}$$. The section modulus $$W$$ for the screw shaft at the fracture location (diameter 63 mm) is: $$W = \frac{\pi d^3}{32} = \frac{\pi (0.063)^3}{32} \approx 2.45 \times 10^{-5} \, \text{m}^3$$. The combined stress due to axial force and bending is given by: $$\sigma = \frac{P}{A} + \frac{M}{W}$$, where $$P$$ is the axial force and $$A$$ is the cross-sectional area. Substituting values: $$\sigma = \frac{1.62 \times 10^5}{\pi (0.0315)^2} + \frac{2.43 \times 10^4}{2.45 \times 10^{-5}} \approx 52 \, \text{MPa} + 992 \, \text{MPa} = 1044 \, \text{MPa}$$. This exceeds the yield strength $$\sigma_s = 518.42 \, \text{MPa}$$ and even approaches the tensile strength, confirming that the planetary roller screw experienced severe overstress leading to brittle fracture.
To validate these theoretical calculations, I performed finite element analysis (FEA) using ANSYS software. A 3D model of the planetary roller screw was created, with boundary conditions simulating fixed support at the bearing location and applied loads corresponding to the disassembly scenario. The mesh was refined near the thread run-out region to capture stress concentrations accurately. The material properties input included $$E = 2.19 \times 10^{11} \, \text{Pa}$$ and $$\mu = 0.3$$. The FEA results, illustrated in the stress contour plot, show maximum von Mises stress exceeding 900 MPa at the thread root, consistent with the theoretical prediction. This high stress concentration, combined with the brittle nature of the hardened surface layer, explains the cleavage fracture observed in the planetary roller screw. The FEA model also highlights the sensitivity of planetary roller screws to radial loads, underscoring the importance of proper alignment during assembly and disassembly.
The failure mechanism can be summarized as follows: the planetary roller screw, made of GCr15 steel with adequate material properties, fractured due to an abnormal radial load during disassembly. This load induced a bending moment that generated stresses surpassing the material’s yield strength at the thread root, a region already prone to stress concentration. The brittle fracture propagated rapidly from the surface, where the martensitic layer had lower toughness, resulting in the observed quasi-cleavage morphology. This analysis emphasizes that while planetary roller screws are robust under axial loads, they are vulnerable to radial forces, which must be minimized in practice.
Based on these findings, I recommend several measures to prevent similar failures in planetary roller screw applications. First, ensure precise alignment during installation and disassembly to avoid radial loading. Second, consider design modifications such as increasing fillet radii at thread roots to reduce stress concentrations. Third, implement non-destructive testing (e.g., ultrasonic or magnetic particle inspection) for critical planetary roller screws to detect surface cracks early. Finally, educate personnel on proper handling procedures for heavy components like planetary roller screws to prevent accidental overloading. These steps can enhance the reliability and longevity of planetary roller screw systems in electromechanical actuators.
In conclusion, the fracture failure of the precision planetary roller screw was primarily driven by excessive radial stress during disassembly, rather than material defects or inherent weaknesses. Through a combination of fractography, material analysis, and mechanical modeling, this study demonstrates the importance of load management in high-performance transmission systems. The planetary roller screw, as a key component, requires careful attention to loading conditions to maintain its structural integrity. Future work could explore advanced materials or coatings to improve toughness, further optimizing the performance of planetary roller screws in demanding applications.