Analysis of Noise Mechanism in Loader Power Transmission System

Abstract: In view of the specific situation of wheel loaders, the mechanism of noise generation in the power transmission system of loaders is analyzed, and the main calculation methods are given. At the same time, corresponding suggestions for the control principles of various noises are put forward.

【 Key Words 】 Loader, powertrain, noise

Preface

For wheel loaders, their noise consists of two parts: radiated noise and noise beside the driver's ear. The composition of radiated noise is rather complex, but it mainly comes from the exhaust noise of the engine, the operating noise of the cooling fan, and the noise of the vehicle body structure induced by engine vibration. The noise in the driver's cab of a loader is mainly low-frequency sound, which is structural noise induced by the vibration of the engine and powertrain. Components related to low-frequency structural noise include the powertrain system, transmission system, body system, etc. Generally speaking, the powertrain and its related parts are the main sources of vibration, and the optimal combination among them is the primary task of reducing noise.

1. Engine noise

The vibration and noise of the engine are the main sources of vibration and noise for loaders. The excitation force on a diesel engine can be divided into the direct excitation force generated by combustion and the mechanical force during the operation of the diesel engine.

The noise on diesel engines can be classified into three major categories based on their generation mechanisms: aerodynamic noise, combustion noise and mechanical noise. Among them, aerodynamic noise in the exhaust system is usually the main noise source. Generally speaking, if the exhaust noise of diesel engines can be effectively reduced, the total noise level of diesel engines can be significantly lowered.

Under normal circumstances, the noise of a diesel engine increases linearly with the increase of its rotational speed. For A naturally aspirated four-stroke diesel engine, the noise increases by 30dB (A) for every 10 times the rotational speed. For a four-stroke turbocharged diesel engine, the noise increment is 40dB for every 10 times the rotational speed. If noise peak waves occur during the growth process, it indicates a problem in the identification of noise sources. 1/3 octave band spectral analysis can be used to initially identify the main noise components.

● Aerodynamic noise

The mechanism of exhaust noise generation: During the operation of a diesel engine, the gas flow at the exhaust valve is unstable. It is transmitted to the exhaust system outlet in the form of pressure fluctuations. At the tailpipe outlet, the speed fluctuations generate radiated noise. It can be seen that the exhaust noise originates from the unstable flow within the exhaust system. The definition of exhaust noise usually refers to the total noise radiated by the exhaust system, including the radiated noise from the pipe walls and muffler walls as well as the aerodynamic radiated noise from the tailpipe outlet. If the pipe walls and muffler walls of the exhaust system are assumed to be rigid, then exhaust noise refers only to the dynamic noise of the gas. The most effective way to reduce exhaust noise is to design and install a highly efficient and low-resistance exhaust muffler. The main factors affecting exhaust noise include engine speed, the number of cylinders, load, and the size of the exhaust pipe, etc.

When the exhaust of an internal combustion engine begins, the gas temperature is approximately 800-1000℃ and the pressure is about 0.4-0.5Mpa. However, when the exhaust valve opens and a gap appears, the exhaust gas rushes out of the gap in the form of pulses, forming a noise with high energy and complex frequency. According to the mechanism by which noise is generated during the exhaust process, there are the following components.

① Gas pressure pulsation sound;

② The vortex sound generated through the valves, valve seats and other parts;

③ Noise caused by airflow disturbances in the boundary layer

④ Exhaust outlet jet noise.

In the frequency spectrum of exhaust noise from multi-cylinder diesel engines, there is often a distinct noise peak at low frequencies, and this noise is the fundamental frequency noise. Since the exhaust of each cylinder occurs periodically at the specified phase. Therefore, this is a kind of periodic noise. The frequency of fundamental frequency noise and the number of exhausts per second, that is, the burst frequency, are the same. The frequency calculation formula for fundamental frequency noise is:

f = Nn / 60τ

In the formula: N - the number of diesel engine cylinders;

n - Diesel engine speed; (r/min

τ - Stroke coefficient of internal combustion engine; Four-stroke τ = 2, two-stroke τ = 1

● Combustion noise

Combustion noise is generally referred to as the noise that is transmitted from the cylinder pressure to the engine block through the piston, connecting rod, and main shaft during combustion, as well as the noise radiated by the vibration of the internal combustion engine structure surface caused by the cylinder head, etc. When a diesel engine is in operation, the combustion chamber undergoes high-temperature and high-pressure combustion in an extremely short period of time, rapidly releasing energy. This sudden increase in pressure triggers the vibration of the engine structure, thereby radiating noise. Obviously, cylinder pressure is the compelling force of combustion noise. Therefore, there is a functional relationship between combustion noise and cylinder pressure. In addition, it is also related to the stiffness of the engine structure, the acoustic radiation effect on the engine surface, and the transmission characteristics of the surrounding air.

The combustion process of a diesel engine is usually divided into four stages - the ignition delay period, the rapid ignition period, the slow ignition period and the afterburning period. The research on the combustion process of diesel engines generally adopts the pressure curve (P -?) The method of analysis. Figure 1 is a typical cylinder pressure curve.

Both cylinder pressure and combustion noise are periodic phenomena, and the frequency component of cylinder pressure governs the frequency component of combustion noise. Fourier analysis of both cylinder pressure and combustion noise reveals that there is a significant dependence between the sound pressure level and the cylinder pressure level in higher frequency bands. Whether analyzed from the pressure curve graph or the spectrum graph, it is obvious that the key to reducing combustion noise is to control the increase rate of combustion pressure. That is to say, diesel engines should strive to adopt a gentle working process. The rate of pressure increase depends on the ignition delay and the fuel injection pattern. Therefore, there are generally two aspects to reducing combustion noise:

① Increase the compression ratio, appropriately delay the fuel injection advance Angle, and use fuel with a high cetane number. Such measures are used to shorten the fire delay period.

② Reduce the initial fuel injection rate and utilize the intake vortex to decrease the amount of combustible mixture before ignition.

● Mechanical noise

Due to the large number of moving pairs on diesel engines, the mechanical excitation forces generated are also considerable, including noise from the knocking of pistons and cylinders, the sound of timing gears, noise from fuel injection systems, and noise from valve train mechanisms, etc.

In an engine, any one of the rotating components such as the crankshaft, flywheel, and pulley will generate vibration force. Since this vibration force is directly proportional to the unbalance of the components and the square of their rotational speed per minute, when the rotational speed increases, the vibration is also sharply amplified. Therefore, the balance between the rotating components is best to be smaller. Other mechanical noises come from the engine piston, valve mechanism, etc., which constitute part of the engine noise, such as the noise of the piston knocking on the cylinder, the noise of the tappet, the noise generated by the opening and closing of the valves, the noise produced by the vibration of the valves and valve springs, and the noise generated when the timing chain meshes with the sprocket.

Piston knocking is the noise produced when the side of the piston strikes the cylinder wall. When the compressive pressure applied to the piston is converted into combustion pressure, knocking occurs. Piston knocking varies depending on the piston clearance. When the piston clearance is large, knocking sounds are most likely to occur. The characteristic of piston knocking is that it is very loud when the engine is cold, so the piston clearance is large at this time. As the engine temperature rises, the sound also decreases. To reduce piston knocking, it is necessary to decrease the pressure on the main side. Therefore, some engines can reduce the knocking sound by deviating the center of the piston pin from the center line of the piston by a certain distance. Another way to reduce piston knocking is to install a steel frame on the piston skirt to minimize thermal deformation of the piston skirt. This allows for the use of a slightly larger piston, reducing the piston clearance and thus minimizing the knocking sound.

2. Transmission system noise

The main components of the transmission system of a wheel loader include important parts such as the three-element hydraulic torque converter, power shift gearbox, drive shaft, drive axle and tires. These components all need to rotate at high speed when the loader is in operation. When the various moving pair parts contained therein interact with each other, they will generate vibration or noise. For instance, gears and bearings are common in most components of a transmission system. Due to different manufacturing precision, rigidity and other conditions, gears will generate varying degrees of vibration and noise during operation. Due to the inherent nature of its operation, bearings are always subject to friction and vibration. This is the main source of its noise. Gear noise and bearing noise are the main sources of noise in mechanical transmission systems. For the hydraulic transmission system used in loaders, apart from gear noise and bearing noise, the fluid flow noise of the hydraulic torque converter is also an important aspect. The unbalanced force during the operation of the drive shaft and the loosening and wear of pins can all generate impulse noise. Studying the causes of noise in each component of the transmission system and their solutions is a very effective and important aspect for reducing the radiated noise of loaders.

The mechanism of gear noise generation - During the gear meshing process, the pitch line impulse force and the meshing impulse force are the excitation sources of vibration and noise. Under the excitation of these two forces, on the one hand, they will generate forced vibration with frequencies of the meshing frequency and higher harmonics; on the other hand, they will also generate transient self-excited vibration with frequencies of the natural frequency. Strong resonance may occur when the meshing frequency and the natural frequency are integer multiples of each other. Therefore, gear noise has two forms of manifestation: one is meshing frequency noise, and the other is noise generated by vibration at the natural frequency.

Gear noise is related to load and rotational speed. Tests have proved that at low speeds, when the load doubles, the gear noise increases by approximately 3dB (A), and when the load remains constant, when the rotational speed doubles, the noise increases by approximately 6dB (A). At high speeds, the gear noise is proportional to the square of the load, that is, when the gear load doubles, the noise level increases by 6dB (A). The effective power transmitted by gears is directly proportional to the product of the pitch line velocity and the tangential component force. Therefore, the sound power level emitted by the gear device is directly related to the transmitted power. When the transmitted power doubles, the noise level increases by 6dB (A). Gear noise has a certain linear relationship with load and speed. The influence of lubrication of gear devices on noise cannot be ignored either. Appropriate lubrication can reduce the friction between tooth surfaces, absorb vibrations and play a certain role in noise reduction.

The noise of the loader's transmission system mainly includes several aspects such as the noise from the transmission and drive axle caused by gear meshing and vibration, the noise from the drive shaft resulting from rotation and vibration transmission, and the tire noise formed due to tire rolling when the vehicle is traveling at high speed. In terms of the contribution to the total noise of the loader, the noise energy of the transmission system is relatively small compared to the engine noise. Under the current circumstances, the transmission system is not the main noise source of the loader, but as the noise levels of other assemblies decrease, the proportion of noise energy it occupies will relatively increase.

3 Fan noise

Fan noise is generated by the rotation of the cooling fan and is directly proportional to the rotational speed. Fan noise is mainly caused by the fan blades cutting through the air or the air turbulence generated by the components behind the fan. By changing the diameter, quantity, shape or Angle of the blades, as well as using variable blade fans or improving the shape of the fan cover, fan noise can be reduced.

Fan noise also accounts for a considerable proportion among the noise sources of internal combustion engines. Fan noise is mainly caused by the noise of blade rotation and vortex noise. The former is narrowband noise, while the latter is broadband noise. In addition, the fan's air shield and other structures can also generate mechanical noise due to resonance.

Rotational noise is the noise excited by the pressure pulsation surface caused by the periodic impact of the rotating blades of a fan on air particles. This periodic pressure pulsation is composed of a steady-state fundamental frequency and a series of harmonic components superimposed. These pulsating components can be expressed by the following formula:

f = inz / 60 (Hz)

In the formula: z - the number of fan blades;

n - Fan speed, (r/min);

i -- 1,2,3... .

When the fan rotates, the frequency of the vortex noise depends on the relative velocity between the blades and the gas. The circumferential velocity of the blades varies with the distance from the center of the circle. Therefore, the frequency of the vortex noise is continuous, and the noise spectrum is also continuous. Eddy current noise is generally wideband noise, and its main peak frequencies are:

f = KV/d (Hz)

In the formula: K - constant; 0.15 to 0.22

V - Peripheral linear velocity of the fan, (m/s);

d - The thickness of the blade in the direction of the incident airflow (m).

The main factors affecting fan noise are as follows:

① Fan speed, diameter and static pressure

Studies show that the greater the air volume of a fan, the higher its noise will be. The size of the fan diameter and the speed directly affect the fan noise. The three are represented by the following relationship:

L up DN

In the formula: L - noise sound pressure level;

D - Fan diameter;

N - Fan speed.

The relationship between the size of the fan diameter, the speed of the fan and the fan air volume:

V up DN

In the formula: V - Fan air volume;

D - Fan diameter;

N - Fan speed.

Therefore, in order to ensure the required air volume, it is appropriate to appropriately increase the diameter and reduce the rotational speed. The size of the fan air volume is determined based on the heat dissipation of the internal combustion engine. From the perspective of noise reduction, enhancing the heat dissipation capacity of the internal combustion engine and its cooling system can reduce the fan air volume and lower the noise.

② Fan efficiency

The general rule is that the lower the fan efficiency, the greater the power consumption and the louder the fan noise.

The power consumed by the fan is: N = pV / ηηηv

In the formula: V - Fan air volume;

η - The hydraulic efficiency of the fan;

η - Mechanical efficiency of the fan;

η - Volumetric efficiency of the fan;

From the formula, it can be seen that if the total efficiency of the fan is increased, the power consumed by the fan will be smaller when the air volume is the same, and the noise will also decrease accordingly. Usually, there is not much change. As long as the hydraulic efficiency and volumetric efficiency of the fan are improved, it is actually beneficial to reduce noise.

③ The shape, material and number of fan blades

The shape of the fan blades also has a significant impact on the fan's efficiency. The shape of the fan blades directly affects the intensity of the vortex near the blades, thereby influencing the efficiency of the fan. Therefore, improving the shape of the blades to have a better streamlined shape and an appropriate bending Angle not only helps to reduce vortex noise but also significantly enhances the fan's efficiency.

Tests show that the material of the fan blades also has a certain degree of influence on its noise. For example, cast aluminum blades produce less noise than those made of stamped steel plates. Nylon blades are quieter than metal blades. Generally speaking, the larger the loss coefficient of a material, the smaller its noise.

By increasing the number of fan blades, the air volume of the fan can be increased under the condition that the rotational speed remains unchanged. Or, under the premise of achieving the same air volume, the fan speed can be reduced, thereby lowering the fan noise. However, when the number of blades is more than 6, increasing the number of blades leads to a limited increase in air volume and often has a negative effect on noise reduction characteristics.

Under the same air volume, the low-speed wide-blade fan generates A noise sound pressure level 4dB (A) lower than that of the high-speed narrow-blade fan, and its power consumption is reduced by 27%. Reducing the gap between the fan and the wind shield can prevent airflow disorder and lower fan noise. Tests show that when the gap is zero, the air volume increases by 27%, while the noise decreases by 3dB (A). Reducing the rotational speed to return the air volume to the original level can further decrease the noise by 2dB (A).

④ The relative positions of the fan, radiator and fan cover

There are two types of fans for construction machinery: suction type and blowing type. The main principle for selection is that the direction of air flow formed by the fan must be consistent with the direction of air flow facing the wind when the main unit is moving forward. The loader engine is rear-mounted and usually uses a blowing fan.

To reduce fan noise, one can also consider the structural parameters of the fan cooling system and the mutual positions of its components. Appropriately choosing the distance between the fan and the heat sink as well as the gap between the fan and the wind cover is also meaningful for reducing fan noise. As the distance between the fan and the heat sink increases, the fan's cooling capacity, flow rate and noise all need to increase. And each reaches its maximum value at a certain point, and then gradually decreases. Tests show that if the end face of the fan is too close or too far from the core of the heat sink, there will be no wind area or backflow phenomenon. It is recommended that the distance between the fan end face and the radiator core be 10% to 15% of the fan diameter. This can not only fully utilize the fan's cooling capacity but also minimize noise.

The air deflector at the front and back of the fan is one of the important sources of vortex noise. The inlet of the fan should be streamlined, and the surface of the airflow channel formed by the fan and the air deflector should be smooth to improve the flow state of the cooling air and thereby reduce the noise of the cooling system.