The Main Causes And Countermeasures of Gear Transmission Noise

Traditionally, the two main factors for measuring gear transmission performance are load capacity and fatigue life, and transmission noise and transmission accuracy are often ignored. With the promulgation of ISO14000 and ISO18000, the importance of controlling gear transmission noise has become increasingly apparent, and industrial development and demand have increasingly stringent requirements for transmission errors of high-precision equipment (gear transmission backlash). The currently known factors that cause gear noise can be roughly analyzed from several aspects, including design, manufacturing, installation, use and maintenance.

 

The national standard stipulates 13 accuracy levels for gear pairs. They are represented by Arabic numerals 0, 1, 2...12, among which level 0 has the highest accuracy, and the rest of the levels are descending in sequence, with level 12 having the lowest accuracy. The accuracy levels of the two gears in a gear pair are generally the same, but different levels are also allowed. In this case, the accuracy level of the gear pair should be determined according to the one with the lower accuracy.

Among the 13 accuracy levels, the current processing technology level and measurement methods for levels 0, 1, and 2 are difficult to achieve and need to be developed. Levels 3-5 are high-precision levels, levels 6-8 are medium-precision levels, and levels 9-12 are low-precision levels. Among them, level 6 is the basic level, which is also the commonly used level in design. It is the level that can be achieved by common processing methods such as hobbing and gear shaping under normal conditions, and can be measured with general measuring instruments.

Design reasons and countermeasures

1. Gear accuracy grade

When designing a gear transmission system, designers often consider economic factors and determine the gear accuracy grade as economically as possible. However, they do not know that the accuracy grade is a sign of the noise level and backlash generated by the gear. The American Gear Manufacturing Association has conducted a large number of gear studies and determined that high-precision gears generate much less noise than low-precision gears. Therefore, if conditions permit, the gear accuracy grade should be increased as much as possible to reduce gear noise and transmission errors.

2. Gear width

When the gear transmission system allows, increasing the tooth width can reduce the unit load under constant torque. Reducing gear tooth deflection reduces noise excitation, thereby reducing transmission noise. Research by German H Opats shows that when the torque is constant, the noise curve gradient of a small tooth width is higher than that of a large tooth width. At the same time, increasing the tooth width can increase the load-bearing capacity of the gear.

3. Tooth pitch and pressure angle

Small tooth pitch can ensure that more gear teeth are in contact at the same time, increase gear overlap, reduce the deflection of a single gear, reduce transmission noise, and improve transmission accuracy. Smaller pressure angles have larger gear contact angles and lateral overlap ratios, so they have lower operating noise and higher accuracy.

4. Operating speed

According to the experimental research of H. Opatz of Germany, as the operating speed of the gear increases, the noise level increases. 

5. Gearbox structure

Experimental studies have shown that the use of cylindrical housings is beneficial for vibration reduction. Under the same other conditions, the noise level of ordinary gearboxes is 6dB higher than that of cylindrical housings on average. Resonance testing of the gearbox, finding the resonance position, and adding appropriate ribs (plates) can significantly reduce vibration and noise. In multi-stage gear transmission, the change in instantaneous transmission ratio is required to be as small as possible to ensure smooth transmission, small impact and vibration, and low noise.

6. Gear sound radiation characteristics analysis

When selecting gears with different structural forms, establish a sound radiation model for its specific structure, conduct dynamic analysis, and pre-evaluate the noise of the gear transmission system. This can be done according to the different requirements of users (place of use, whether it is unmanned, whether it is in the urban area, whether there are specific requirements for above-ground and underground buildings, whether there is noise protection, or no other specific requirements).

Manufacturing causes and countermeasures

1. Error influence

Tooth profile error, pitch error, and tooth direction error in the manufacturing process are the main errors that cause transmission noise. It is also a problem that gear transmission accuracy is difficult to guarantee.

Gears with small tooth profile error and small tooth surface roughness have a noise level 10dB lower than ordinary gears under the same test conditions. Gears with small pitch error have a noise level 6-12dB lower than ordinary gears under the same test conditions. However, if there is a pitch error, the impact of load on gear noise will be reduced.

Tooth direction error will cause the transmission power to not be transmitted across the full tooth width, and the end face or end face of the steering tooth in the contact area will increase the local force and cause the gear tooth to bend, resulting in an increase in noise level. However, under high load, tooth deformation can partially compensate for the tooth direction error. The generation of gear noise is directly related to transmission accuracy.

2. Assembly concentricity and dynamic balance

Assembly concentricity will lead to unbalanced operation of the shaft system, and because the gear meshing is half loose and half tight, it will cause increased noise. The imbalance during assembly of high-precision gear transmission will seriously affect the accuracy of the transmission system.

3. Tooth surface hardness

With the development of gear hardening technology, its large load-bearing capacity, small size, light weight, and high transmission accuracy make its application field increasingly wide. However, the carburizing and hardening used to obtain hard tooth surfaces causes the gears to deform, resulting in increased gear transmission noise and shortened service life. In order to reduce noise, the tooth surface needs to be fine-machined. In addition to the traditional gear grinding method, a hard tooth surface scraping method has been developed. By modifying the tooth top and tooth root, or reducing the tooth shape of the active and passive wheels, the impact of gear meshing in and out can be reduced, thereby reducing gear transmission noise.

4. System index verification

The machining accuracy of parts and components before assembly and the selection method of parts and components (complete interchangeability, group selection, single piece selection, etc.) will affect the accuracy level of the system after assembly, and its noise level is also within the scope of influence. Therefore, verification (or calibration) of various system indicators after assembly is critical to control system noise.

 

Installation reasons and solutions

1. Vibration reduction and blocking measures

During the installation of the gear transmission system, the resonance between the machine body and the foundation support and the connecting parts should be avoided as much as possible to avoid noise. In the gear transmission system, one or several gears often resonate within certain speed ranges. In addition to design reasons, this is also directly related to the failure to find the resonance position during installation without an empty test. And take corresponding vibration reduction or blocking measures. For some gear transmission systems (such as testing instruments) that require low transmission noise and vibration, high-toughness, high-damping basic materials (such as epoxy resin mortar basic materials developed by the Metallurgical Design Institute) should be used to reduce the occurrence of noise and vibration.

2. Geometric accuracy adjustment

Since the geometric accuracy during installation does not meet the requirements of the standard, the transmission system will resonate and generate noise. This should be directly related to improving the installation process, adding tooling, and ensuring the overall quality of the assembly personnel.

3. Loose parts

During installation, due to the looseness of individual parts (such as bearing preload mechanism, shaft positioning mechanism, fork limit mechanism, etc.), the system positioning is inaccurate, abnormal position engagement, shaft movement, vibration and noise are generated. This series needs to start from the design structure, try to ensure the connection stability of each mechanism, and adopt multiple connection methods.

4. Damage to transmission components

During installation, improper operation may damage transmission components, resulting in inaccurate or unstable system movement; damage to high-speed moving components may cause oil film vibration; artificially caused dynamic imbalance of moving parts; all of which may generate vibration and noise. These reasons must be paid attention to and avoided as much as possible during installation. Damaged components that cannot be repaired must be replaced to ensure that the system obtains a stable noise level.

Use and maintenance reasons and countermeasures

Although the correct use and maintenance of the gear transmission system cannot reduce the system noise level or ensure the transmission accuracy, it can prevent the deterioration of its indicators and increase its service life. 

1. Cleaning the inside of the transmission system

The cleaning inside the transmission system is the basic condition to ensure the normal operation of the gears. Any impurities and dirt entering will affect and damage the gear transmission system, eventually causing noise and damaging the transmission system. 

2. Normal operating temperature of the system

Ensuring the normal operating temperature of the transmission system can prevent the system from deforming due to excessive temperature rise, resulting in abnormal meshing and preventing the increase of noise.

3. Timely lubrication and correct use of oil products

Improper lubrication and incorrect use of lubricants will cause immeasurable damage to the system. Ensuring that the system is lubricated in a timely and correct manner can keep the system within a certain noise level range and delay the deterioration trend. High-speed gears will generate a lot of heat energy due to tooth surface friction. Improper lubrication will cause damage to the gear teeth, affect accuracy, and increase noise. The design requires that the gear pair have appropriate clearance (the clearance between the non-working surfaces of the meshing gear teeth to compensate for thermal deformation and store lubricants). The correct use and selection of lubricants can ensure the safe and effective operation of the system and stabilize the noise level.

4. Correct use of the gear motion system

Using the system in its normal operating sequence can minimize damage to the system and ensure a stable noise level. Use the system within the normal load range of the system, because the transmission noise of the gear transmission system increases with the increase of load.

5. Regular maintenance and care

Regular maintenance and care (changing oil, replacing worn parts, loosening fasteners, clearing debris inside the system, adjusting the clearances of various parts to the specified values, verifying the geometric accuracy of various items, etc.) can improve the system's ability to resist noise level degradation and maintain the stability of the system.

 

Conclusion

Gear transmission noise control is a systematic project, which involves the whole process of gear transmission design, manufacturing, installation, use and maintenance to update. It not only puts forward many requirements for designers and manufacturers, but also for installers, users and maintainers. If any of the above links is not effectively controlled, gear transmission noise control will become ineffective.

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