With the rapid development of planetary transmission technology, planetary gear reducers have become widely used in various industries due to their numerous advantages, including small size, light weight, compact structure, high load-bearing capacity, high transmission efficiency, smooth operation, strong impact resistance, large transmission ratio, and ability to synthesize and decompose motion.
In modern planetary transmissions, in order to meet the performance requirements under heavy load conditions (small outer diameter), the weakest link in planetary transmissions is often the gear transmission. To improve the load-bearing capacity of the reducer, it is necessary to improve the performance of involute cylindrical gears under heavy loads to meet the requirements of heavy loads.
In heavy-duty gear transmissions, internal and external meshing gear pairs must meet the requirements for strength, service life, and meshing quality; ordinary designs are no longer sufficient. Based on actual production experience, the following suggestions are offered for reference and learning.
1. Increase tooth width
When the required outer diameter of the transmission remains constant, appropriately increasing the width of the internal gears can effectively increase the load-bearing capacity of the gears and improve the load-bearing torque of the reducer. Simultaneously, it can reduce the unit load under constant torque. It can also reduce tooth deflection, decrease noise excitation, and thus reduce transmission noise. However, an excessively large tooth width coefficient may lead to poor rigidity and large deformation of the pinion, causing uneven loading across the tooth width and compromising contact accuracy. Typically, the tooth width coefficient for cylindrical gears is 0.4 to 0.8.
2. Circular arc tooth cylindrical gear transmission
Circular arc gear transmission is a type of helical cylindrical gear transmission, a form of cylindrical gear transmission. It has advantages such as high load-bearing capacity, low production cost, simple manufacturing process, and long service life. For example, under the same conditions of module 0.8, input speed, operating conditions, service life, and gear width, the load-bearing capacity of a β＝15° helical cylindrical gear is approximately 1.36 times that of a regular cylindrical gear.
3. Increase the gear module and increase the tooth profile angle.
To maintain a constant outer diameter of the reducer while increasing its load-bearing capacity, this can be achieved by appropriately increasing the gear module and reducing the number of gear teeth. A larger module gear improves the gear's bending strength, a crucial indicator of its load-bearing capacity. It also increases the meshing angle α′, which in turn increases the overall radius of curvature, reduces Hertzian stress, and further enhances load-bearing capacity.
4. Increase the allowable contact stress of the gear ring
In planetary gear reducers, the internal gear ring typically has lower hardness than the external gear, uses different materials, and undergoes different processing and heat treatment. The strength of a planetary gear reducer is usually checked by verifying the transmission contact stress between the sun gear and planet gears (ensuring the transmission contact stress is less than the allowable contact stress between the sun gear and planet gears); the bending stress between the sun gear and planet gears (ensuring the bending stress is less than the allowable bending stress); and the transmission contact stress between the planet gear and the internal gear (ensuring the transmission contact stress is less than the allowable bending stress of the internal gear ring). The allowable contact stress of the gear ring is usually the first point of failure. Therefore, to increase the load-bearing capacity, the allowable contact stress of the gear ring must be guaranteed.
5. Gear shaping
Under heavy loads, gear pairs undergo significant bending deformation during gear meshing. This deformation shortens the base pitch of the teeth on the driving gear and lengthens the base pitch of the corresponding teeth on the driven gear, causing interference during engagement and disengagement, specifically at the tooth tips and roots (as shown below).
Entering the bite
Engagement and disengagement
For heavy-duty gears, tooth tip modification can prevent tooth tip overload caused by tooth direction error. Tooth profile modification, root modification, and tooth tip modification are good ways to improve the transmission performance of heavy-duty gears. The amount of tooth tip modification is closely related to the load. Appropriate modification can ensure the smoothness of gear pair meshing (engagement and disengagement), reduce dynamic load and noise, and improve bending strength and contact strength.
6. Lubrication Options
Improper use of lubricating grease is one of the main causes of gear failure. The parameters of lubricating grease are directly related to gear load, operating speed, gear type, and operating temperature. Selecting a suitable and appropriate lubrication method and lubricating grease can effectively improve the service life of equipment.
Planetary gear reducers generally have three lubrication methods: ① Grease lubrication ② Splash lubrication (oil bath lubrication) ③ Forced lubrication (circulating oil station spray lubrication). The lubrication method needs to be appropriately selected based on the gear's operating conditions. The selection criteria are mainly based on the gear's circumferential speed (m/s) and rotational speed (min⁻¹). Generally speaking, if lubrication methods are classified according to circumferential speed, grease lubrication is used for low speeds (v>7m/s), oil bath lubrication is used for medium speeds (2.5m/s <v>15m/s), and forced lubrication is used for high speeds (12m/s <v).
To maintain high-efficiency power transmission of gears, a stable oil film must be formed on the meshing tooth surfaces to prevent metal-to-metal contact. To achieve this, a grease of suitable viscosity must be selected. A high viscosity index indicates that the oil's viscosity changes less with temperature, which is beneficial for maintaining a small viscosity variation within the operating temperature range, thereby improving lubrication performance. In practical selection, lubricating grease is chosen based on load and speed, while also considering factors such as transmitted power, meshing efficiency, bearing efficiency, and the temperature difference between the inlet and outlet ports. Reducing tooth surface roughness is beneficial for lubrication and facilitates the formation of a full oil film. An explanation is given for the point-to-line contact problem in involute gear meshing. The thickness and strength of the oil film are theoretically calculated, and then a suitable lubricating grease is selected. The amount of lubricating grease added is crucial; too little lubricating grease will not achieve the desired lubrication, while too much lubricating grease in a sealed gearbox will cause excessive churning losses.
7. Tooth root shot peening reinforcement
The bending strength of gears is greatly influenced by the surface condition of the tooth root. In particular, the presence of defects such as a decarburized layer on the tooth root surface of carburized and quenched gears makes it difficult to maintain residual compressive stress, thus reducing the bending fatigue strength of the tooth root. In such cases, shot peening the tooth root surface to remove the defective layer can ensure the bending strength of the tooth root and improve fatigue strength.
8. Comprehensive Adjustment of Design Parameters
In planetary gear reducers, the module, center distance, and tooth width are calculated based on the load. The center distance and tooth width are related to contact strength, and are also influenced by factors such as the number of teeth, helix angle, contact ratio, meshing angle, machining accuracy, and displacement coefficient. Among these, the displacement coefficient is a crucial parameter, directly affecting many other parameters. During design, adjusting the displacement coefficient can improve bending strength, enhance meshing quality, and optimize the center distance, thereby extending gear life.
9. Adjustment of displacement coefficient
To improve the load-bearing capacity of gears, it is essential to analyze the causes and failure modes of gear transmissions, identify the main problems, and thus determine the basic principles for selecting the displacement coefficient. Planetary reducers generally use hardened tooth surface gears (HB<350). For hardened tooth surface closed gear transmissions, the main danger is the gradual propagation of fatigue cracks at the tooth root under cyclic stress, leading to tooth root fracture. However, in reality, many hardened tooth surface gear transmissions also lose their working capacity due to pitting and spalling of the tooth surface. Therefore, for this type of gear transmission, the meshing angle α should be maximized (i.e., the total displacement coefficient should be maximized). This not only improves contact strength but also increases the tooth form factor value and improves the bending strength of the tooth root. If necessary, the displacement coefficient can be appropriately distributed to make the displacement coefficients of the meshing gears equal, that is, to achieve approximately equal bending strengths at the tooth roots of the two gears. Correct selection of the displacement system can increase the load-bearing capacity of gears by 20-30%.
10. Gear accuracy and error
Tooth surface strength is related not only to the gear's precision grade but also to the absolute value of the base pitch error. If the gear has the maximum base pitch error, the rolling pressure applied to the teeth is also the maximum, resulting in the most severe pitting damage. Therefore, strict control is necessary.
11. Reduce input speed
The relationship between the output torque and output speed of a planetary gear reducer is as follows: for the same planetary gear reducer, under the premise of ensuring service life, a higher output speed results in a lower torque, while a lower output speed results in a higher torque. At higher speeds, the gears mesh more times per unit time, thus reducing fatigue time; conversely, lower speeds result in longer fatigue time.
12. Selection of Gear Materials
For heavy-duty gears, material selection must consider the working characteristics, requiring a certain surface hardness to ensure contact strength and wear resistance, while the core must possess a certain degree of toughness. Only carburized and quenched gears meet these requirements. For heavy-duty gears, alloy steel with a carbon content of less than 0.2% is generally used, with alloying elements typically including Cr, Ni, Mo, and Ti. These elements have significant effects on improving hardenability, refining grain size, and enhancing wear resistance and toughness. During design, factors such as strength and manufacturing processes must be comprehensively considered.
The methods described above for improving the load-bearing capacity and extending the service life of planetary gearboxes have proven to be effective in production practice and can be used as a reference for general design.
