Abstract: Based on the gear failure caused by gear shaving in a certain type of drill, this paper analyzes the causes of failure from multiple aspects such as gear material, heat treatment, gear module, dynamic load, and stress analysis. Improvement measures are proposed, and the actual effects after improvement are obvious.
1. Introduction
The drill discussed in this paper belongs to the category of lightweight drills, with a power of 320W and a rotational speed of 3000r/min. Gear transmission is a single-stage transmission, with 47 teeth on the large gear and 5 teeth on the small gear. The material used for the large gear is 40Cr, with a module of 0.6mm, and the tooth surface has undergone high-frequency heat treatment to form a hard tooth surface. The material used for the small gear is 35CrMo, and its tooth surface has also undergone high-frequency heat treatment to form a hard tooth surface.
In combination with actual cases in electric tool manufacturing enterprises, this paper analyzes various factors leading to gear failure in drills and proposes corresponding improvement solutions.
2. Failure Forms
According to customer feedback, the drill experiences power failure during use. After dismantling the failed drill for analysis, it was found that there was local wear on the tooth tips of both the large and small gears. After careful inspection, it was found that the tooth tips of five teeth on the large gear were severely worn, and the tooth profile was distorted. The rest of the teeth were basically normal. The tooth tips of all teeth on the small gear were worn, and the tooth profile shape became smooth and lost its original function after damage.
3. Failure Analysis
3.1 Working Conditions
Customers mainly use the drill for tasks such as tightening screws, drilling holes in wood, and expanding holes. During operation, the power is high, exceeding the rated nominal power. Especially during the process of drilling holes in wood, the increase in torque causes the temperature of the tool body to rise.
3.2 Cause Analysis
The causes of failure include improper material selection, low hardness of gear material, excessive contact stress, unreasonable tooth profile design, and excessive dynamic load.
3.3 Material Analysis
Chemical composition analysis of the large and small gear materials showed that the composition was in compliance with requirements. The specific values are shown in Table 1 and Table 2.
Table 1: Element Content of Standard 40Cr and Failed Gear (%)
| Material | C | Si | Mn | P | S | Cr | Mo |
|---|---|---|---|---|---|---|---|
| Standard 40Cr | 0.37~0.44 | 0.17~0.37 | 0.5~0.8 | ≤0.035 | ≤0.035 | 0.8~1.1 | – |
| Failed Large Gear | 0.4 | 0.2 | 0.51 | 0.02 | 0.009 | 0.82 | – |
Table 2: Element Content of Standard 35CrMo and Failed Gear (%)
| Material | C | Si | Mn | P | S | Cr | Mo |
|---|---|---|---|---|---|---|---|
| Standard 35CrMo | 0.17~0.35 | 0.17~0.37 | 0.4~0.7 | ≤0.025 | ≤0.025 | 0.8~1.1 | 0.15~0.25 |
| Failed Small Gear | 0.19 | 0.25 | 0.58 | 0.012 | 0.003 | 1.0 | 0.17 |
3.4 Metallographic Analysis
The large gear requires a hardness of HRC42∼HRC46 after quenching with high frequency following quenching and tempering. The actual measured hardness was HRC41∼HRC43, slightly below the lower limit. The tooth section consists of fine martensite, and the core is sorbite plus ferrite, meeting the metallurgical structure requirements of heat treatment.
The small gear requires a hardness of HRC48∼HRC52 after quenching with high frequency following quenching and tempering. The actual measured hardness was HRC51∼HRC51.5. The tooth section consists of finer martensite, and the core consists of sorbite plus ferrite, meeting the heat treatment requirements.
3.5 Appearance Analysis
From the failure manifestations, there were no tooth fractures, surface cracks, or tooth surface spalling on the gears. The tooth tops of the large gear showed 3∼5 slight distortions and severe wear. The tooth tops of the small gear also showed wear, with the tooth top edges becoming smooth. This indicates that the failure is unrelated to the contact strength and bending strength of the gears. Instead, it is due to insufficient meshing height between the large and small gears. During hole expansion and screw tightening, due to insufficient rigidity of the casing, the small gear slips over the surface of the large gear, causing meshing separation.
4. Solutions
4.1 Adjust Gear Parameters to Increase Meshing Height
Increasing the gear module can achieve the improvement effect. By increasing the module, the overall performance and strength of the gears can be improved simultaneously. However, it is necessary to satisfy the conditions of the original casing size and constant output speed, which limits the scope of changes. After repeated calculations, the gear module was finally changed from 0.6mm to 0.7mm, the number of teeth on the large gear was changed from 47 to 39, and the number of teeth on the small gear was changed from 5 to 4. This ensured that the transmission ratio of the modified gears remained basically the same as that of the original gears.
Table 3: Comparison of Gear Parameters Before and After Modification
| Parameter | Before Modification | After Modification |
|---|---|---|
| Gear Module (mm) | 0.6 | 0.7 |
| Number of Teeth (Large Gear) | 47 | 39 |
| Number of Teeth (Small Gear) | 5 | 4 |
| Transmission Ratio | – | Approximately the same |
4.2 Increase Hardness
Local distortion was observed on the tooth tops of the large gear in the failed samples. In the improvement plan, the surface hardness of the large gear was appropriately increased from the original HRC42∼HRC46 to HRC45∼HRC50, with a hardened layer depth of not less than 0.9mm.
5. Conclusion
Through the adjustment and improvement of the gear module, the samples received positive feedback after user verification. The actual effects of the improved drill were obvious, effectively extending the product life cycle.
In summary, gear shaving failure in drill gears can be addressed by adjusting gear parameters to increase meshing height and increasing the hardness of the gears. These measures can significantly improve the performance and durability of the drill, ensuring its reliability and satisfaction in practical applications.