The planetary gear reducer is a highly utilized device in the transmission system. In industrial and mechanical applications, the presence of planetary gear reducers is quite common. For a device with such high efficiency, issues of gear damage are inevitable. Generally, gear damage can be categorized into three types: tooth breakage, gear pitting and spalling, and gear wear.
So, what are the underlying causes behind these three types of damage? And how can we address them?
Tooth Breakage
In general, tooth breakage in planetary gear reducers can be classified into fatigue fracture and overload fracture.
During operation, the gear teeth of a planetary gear reducer are subjected to alternating loads, resulting in bending fatigue stress on the critical tooth profile. This leads to the initiation of fatigue cracks at the tooth root. Under the influence of alternating bending fatigue stress, these cracks gradually propagate, ultimately causing fatigue fracture of the gear teeth.
Additionally, in the operational scenario of a planetary gear reducer, gears can experience short-term overloads, impact loads, or severe wear and thinning of tooth profiles. Any of these conditions can result in overload fracture.
Solution:
Increasing the root fillet radius and minimizing the surface roughness values during machining can reduce the impact of stress concentration. This, in turn, enhances the stiffness of the shaft and bearings, alleviating localized loading on the tooth surfaces. Providing sufficient toughness to the core of the gear teeth is essential. Additionally, implementing appropriate strengthening treatments at the tooth roots can improve the gear teeth’s resistance to fracture.
Certainly, if gear breakage has already occurred during operation, it is necessary to replace the damaged parts with new ones of the same model.
Gear Pitting and Spalling
In the prolonged operation of a planetary gear reducer, surface pitting and spalling may occur on the gear teeth. This is primarily due to insufficient contact fatigue strength of the gears.
Unlike wear, where metal is worn away in particle form, pitting and spalling involve the detachment of metal in chunks, causing depressions on the tooth surface and severely compromising the accuracy of the tooth profile.
The process of this damage unfolds as follows: tiny cracks first appear on the tooth surface, with lubricating oil entering these fatigue cracks. Through repeated engagement and disengagement, the cracks progressively extend and propagate. The lubricating oil fills the cracks as they expand, until a small piece of metal detaches, leaving the tooth surface. This phenomenon disrupts the normal meshing performance of the gears.
The main causes of surface pitting on the gear teeth are:
(1) Material, Hardness, and Defects: The material of the gears does not meet the requirements. A major factor affecting the contact fatigue strength of the gears is the lower hardness after heat treatment, which fails to ensure the necessary contact fatigue strength. Additionally, surface or internal defects on the gears contribute to insufficient contact fatigue strength.
(2) Poor Gear Accuracy: Inadequate gear processing and assembly precision, such as poor meshing accuracy and motion precision. Discrepancies in the center distance of the housing for helical gears can also contribute to this issue.
(3) Inappropriate Lubricating Oil: The lubricating oil used does not meet the requirements. Incorrect oil grade, low viscosity, and poor lubricating performance are factors contributing to this problem.
(4) Excessive Oil Level: An elevated oil level leads to higher oil temperatures, reducing the viscosity of the lubricating oil, impairing lubrication performance, and diminishing the working thickness of the oil film.
Solution:
To address surface pitting, it is recommended to increase the hardness of the gear teeth, minimize the surface roughness values, utilize a larger modification factor whenever possible, increase the viscosity of the lubricating oil, and reduce dynamic loads. These measures can help prevent fatigue pitting on the gear teeth.
Gear Wear
Mechanical components are susceptible to varying degrees of wear during prolonged use, and planetary gear reducers are no exception.
Gear wear is the most common form of damage in the operation of planetary gear reducers. Possible causes of this wear include: insufficient lubrication, the presence of metal particles in the lubricating oil from wear, leading to surface wear on the gears;
Gears with materials that do not meet the requirements resulting in abnormal wear; presence of defects such as sand holes, pores, looseness, insufficient nodularization, etc.; inadequate or lack of heat treatment hardness;
Inaccuracies in gear meshing and motion precision; high sensitivity of helical gears to center distance errors, especially positive errors in center distance, not only reducing the bending strength of the gear teeth but also increasing sliding wear.
Solution:
To address gear wear in planetary gear reducers, it is recommended to:
- Increase the surface hardness of the gears.
- Reduce surface roughness values.
- Keep the transmission components and lubricating oil clean.
- Ensure thorough lubrication and add appropriate anti-wear additives to the lubricating oil.
- Introduce several magnetic bodies into the oil tank to utilize magnetic forces in adsorbing metal particles in the lubricating fluid, reducing the metal particle content.
In planetary gear reducers, gears are among the core components, and the key to successful planetary gear design lies in the uniformity and high precision of gear meshing.
Regardless of the extent of gear damage, prompt repair or replacement of the gears is essential to prevent severe impacts on mechanical transmission.
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