What Is a Friction Gear?

Gear noise should be accurately referred to as gear transmission noise, which refers to the noise radiated from the gear body vibration caused by mutual collision or friction when the meshing gear pair or gear set is transmitting.

Gear noise should be accurately referred to as gear transmission noise, which refers to the noise radiated from the gear body vibration caused by mutual collision or friction during transmission when the meshed gear pair or gear set is transmitting ^[1]

In order to reduce the noise of the gear transmission system from the design point of view, we should first analyze the type and mechanism of the noise of the gear system.

In the gear system, according to different mechanisms, noise can be divided into two types: acceleration noise and self-sounding noise. On the one hand, when the gear teeth are meshing, the gear will have a large acceleration due to the impact and will cause the surrounding medium to be disturbed. The acoustic radiation generated by this disturbance is called the gear's acceleration noise. On the other hand, under the action of the dynamic meshing force of the gears, each component of the system will generate vibration. The acoustic radiation generated by these vibrations is called self-sounding noise.

For open gear transmission, acceleration noise is directly radiated from the impact of the gear teeth, and self-sounding noise is radiated from the wheel body, transmission shaft, etc. For closed gear transmission, the acceleration noise is radiated into the air and lubricant inside the gear box, and then radiated through the gear box. The self-sounding noise is caused by the vibration of the gear body through the transmission shaft to cause the support to vibrate, and thus through the vibration of the gear box wall.

Gear design.

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Based on the above analysis, we will discuss in detail from the aspects of gear design, gear processing, and gear train and gear box.

1. Gear design

1.Type and material of gear

(1) Gear type:

From the perspective of stable transmission and low noise, the helical cylindrical gear contacts many pairs of teeth at the same time. The change of the comprehensive stiffness of the meshing is relatively stable. Therefore, the vibration noise may be lower than the same spur gear, sometimes as low as about 12dB. For herringbone gears, due to strict centering requirements, slight errors or uneven wear may affect the load and smoothness of herringbone gears. Therefore, in cylindrical gears, helical gears are the best gear type to reduce noise. From 1969 to 1987, Ain-Shams University Maru AY. Professor Atfia conducted experimental research on involute helical gears, single-arc gears and double-arc gears. He compared the noise of these three toothed gears under different loads and different speeds. Studies have shown that among these three tooth profiles, involute helical gears have the lowest noise and are least affected by the transmitted load and operating speed, followed by single-arc gears and the worst by double-arc gears.

(2) Material of gear

Gear material. Both heat treatment and lubrication will affect the system noise. Generally speaking, manufacturing gears with good attenuation properties can reduce noise. However, the materials with good attenuation performance are not high in strength and cannot be used in any occasion. E.g. Phenolic resins and nylons are often used only to make light-duty gears for light industrial machinery such as sewing machines. In order to reduce the noise, the surface of the steel gear can be sulfurized or copper plated. The purpose of tooth surface sulfur is to reduce the friction coefficient of the tooth surface. Tooth surface copper plating has been used on turbine gears to improve gear contact accuracy. Gear heat treatment also has an impact on noise. For example, after quenching, the attenuation performance of the gear will deteriorate, and the noise will increase by 3-4dB. Therefore, gears that do not require high strength and wear performance do not have to be quenched. As for the influence of lubricating oil and refueling method, it is generally believed that the noise becomes smaller with the increase of the amount of oil and the viscosity of the oil. This is because the lubricating oil has a damping effect and can prevent the direct contact of the tooth surface. When using oil bath lubrication, the gear noise is different due to different oil surface heights, that is, different gear boxes have different optimal oil levels.

2.Gear geometric parameters

(1) Modulus

When a large load is transmitted, the dynamic excitation of the gear teeth is mainly caused by the bending deformation of the gear teeth, and the bending stiffness of the gear teeth is proportional to the modulus, so increasing the modulus can reduce the dynamic excitation of the gear teeth To reduce noise. But when the transmitted load is small or no load, the situation is different. At this time, the effect of gear tooth error will be much greater than the gear tooth deformation. We should consider the effect of the modulus on noise from the perspective of tooth dry machining error. For example, the pitch error P can be obtained as follows: P = C1 + C2M + C3 (1)

Where dogear pitch circle diameter

M-modulus

C1, C2, C3-related constant calculation

The tooth shape error f can be calculated by the following formula:

f = C4M + C5 (2)

In the formula, C4 and C5 are related constants.

It can be seen from the two formulas (1) and (2) that the above two errors are directly related to the modulus, and the modulus is large, and the tooth shape error is large. The noise is also loud. Therefore, when the transmitted load is small or no-load, the module should be taken as small as possible when the strength of the gear allows.

(2) Number of teeth

If the modulus is constant, the gear diameter and gear surface area will change if the number of teeth is changed. In this way, the change in the noise radiation area of the gear causes a change in the noise. Generally speaking, the magnitude of noise is not mainly determined by the energy of the vibration source but by the radiation area of the noise. According to the acoustic principle, if a gear is used as a circular plate, the sound power WR radiated into the air can be calculated by the following formula: WR = O.06pWR / C3 (6 + 0.85RP) xF (3)

In the formula: F-the effective value of the exciting force that changes according to the sine law

Rround plate diameter

o --area density

pair density

-angular frequency

Cconstant

It can be known from equation (3) that as the diameter of the circular plate increases, the noise will increase sharply. Therefore, when designing the gear, reduce the gear diameter as much as possible. In addition, it can be seen from equations (1) and (2) that the pitch error is related to the gear diameter, and the tooth shape error is not related to the diameter, so reducing the diameter will not increase the difficulty in achieving the precision of gear processing.

(3) The reason for the change in noise caused by tooth width and tooth width is the difference in energy attenuation. Therefore, gears with large tooth widths have good attenuation performance and low noise.

(4) Coincidence

Increasing the coincidence can reduce the noise of the gear transmission. First, increasing the coincidence can reduce the load on a single pair of gear teeth. This can reduce the impact of the load and the gear, and reduce gear noise. Secondly, as the number of contact tooth pairs increases, the transmission error of a single pair of gear teeth is equalized, thereby reducing the dynamic excitation of the gear teeth. In addition, almost all gear tooth parameters that have an effect on gear noise are actually affected by their effect on coincidence. For example, for a cylindrical gear with a degree of coincidence of 1-3, reducing the pressure angle of the gear and decreasing the modulus, so that the height of the tooth tip has a small increase, are all due to the increase of the degree of coincidence and reduce gear noise. Of course, the reduction of the pressure angle increases the flexibility of the gear teeth and reduces the dynamic excitation, which is conducive to the reduction of noise.

(5) Helix angle Since the helical gear gradually enters the meshing from one end of the tooth, the meshing impact is small and the noise is low. Generally speaking, as the helix angle increases, the degree of coincidence increases and the noise decreases. However, when the helix angle is large, the noise reduction effect is worse than when the helix angle is small. This is because when the helix angle is large, processing and installation are difficult, which affects the actual coincidence degree.

(6) Modification, reshaping and displacement of teeth In the actual working state of the gear, the deformation of the gear teeth, the transmission shaft and the casing will cause interference and impact when the gear teeth are engaged and engaged, causing strong Vibration and noise. For this reason, the methods of trimming, shaping and displacement can be used to compensate the meshing deformation to reduce the vibration and noise ^[1] .

Gear processing

(1) Gear machining accuracy has an important effect on gear system noise. In general, improving machining accuracy helps reduce noise in gear systems. However, the improvement of processing accuracy is limited by the processing cost, and the higher the initial processing accuracy, the less obvious the noise reduction effect of improving accuracy.

Among the individual gear tooth errors, the tooth shape error has the greatest influence on noise. If the tooth shape error is large, the gear noise is large, but there is not a simple linear relationship between the two. Because the amount of noise depends not only on the size of the tooth shape error, but also on the shape of the tooth shape. Experiments have shown that the slightly drum-shaped tooth shape is conducive to reducing noise.

Regarding the influence of gear wheel tooth backlash on noise, generally speaking, if the backlash is too small, the noise will increase sharply, while the backlash slightly larger has little effect on noise.

(2) Processing method

There are many ways to process gears. Generally speaking, there is not a very fixed relationship between the machining method and gear noise, because it is also affected by the processing technology. Generally, different processing methods will produce different tooth surface roughness, and improving the tooth surface roughness is beneficial to reduce noise.

Third, the gear train and gear box

1. In terms of gear wheel body structure, the structure of the gear wheel body has an important influence on the noise of the gear system.

First of all, under the dynamic excitation force of the gear teeth, the wheel body as an elastic body will generate vibration and radiate noise. Secondly, the dynamic excitation force acting on the gear teeth will be transmitted to the transmission shaft through the wheel body, and then to the bearing and the box body. In addition, the structure of the wheel body will also affect the transmission error of the gear tooth meshing process, which in turn will affect the size of the dynamic excitation force. For this reason, we can reduce the noise from two aspects of reducing the gear body noise radiation and reducing the transmission of gear body vibration.

(1) Reduce gear body noise radiation

Generally speaking, in addition to the energy of the vibration source, the size of the noise is mainly determined by the radiation area. Therefore, reducing the surface area of the gear can reduce the radiation area of the noise, thereby reducing the radiation noise. In addition, the shape of the gear has a certain relationship with the amount of noise. For example, the thicker the tooth blank and the smaller the diameter, the smaller the noise.

(2) Reduce transmission of gear body vibration

In this regard, we can use some composite structures or fill vibration damping materials in the gear body to increase the damping effect of the gear, thereby reducing the transmission of vibration and reducing noise.

2. In terms of shafting structure

It can prevent the deflection of the transmission shaft. Through the design of the shaft system, the phase relationship of the meshing excitations in the multi-stage gear transmission can be controlled to achieve the purpose of reducing noise.

3.Box structure

The gear box is a typical elastic structure system. It generates vibration and radiated noise under the dynamic load of the bearing. Therefore, reasonable design of the structure and vibration characteristics of the box will help reduce the noise of the gear system. For example, in the design, the minimum vibration of the thin wall of the box is used as the purpose, and the frequency constraints, stress constraints, geometric constraints, etc. are used as the constraint conditions to minimize the vibration of the box wall under dynamic excitation to reduce the noise radiation. In addition, in the design of the box structure, care should be taken to ensure that the structural connection between the bearing support and the fulcrum of the box has sufficient rigidity to reduce system vibration. For large-area thin walls, stiffeners should be provided to reduce the amplitude of vibration noise ^[2] .