Design Principles and Calculation Examples of Loader Shock Absorbers

Abstract: This paper briefly describes the hazards of vibration and noise in construction machinery, the principle and application of the traditional control method - vibration reduction and noise reduction, and briefly analyzes the design principles of shock absorbers for engineering vehicles. Meanwhile, a calculation example is given based on the design principles of shock absorbers.

【 Key Words 】 Vibration, noise control, shock absorber

1. Introduction

Vibration of construction machinery and equipment is a harmful phenomenon, often causing significant hazards: it leads to vibration and noise pollution and disrupts the normal operation of other related equipment and instruments. Reduce the accuracy of the control and monitoring systems; Vibration will also compromise the riding comfort of vehicle drivers, deteriorate working conditions, reduce work efficiency, and affect the overall performance of the human-machine system.

The noise control of loaders mainly focuses on two aspects: Firstly, it is to reduce the noise of the sound source. That is, by using low-noise and low-vibration engines, cooling fans, gearboxes, hydraulic pumps and other measures, the overall noise of the machine can be fundamentally reduced. Due to cost considerations, it is impossible to completely replace the power source and transmission system of the loader product itself at this stage. Therefore, the main means of noise reduction at this stage is to consider starting from passive noise reduction, that is, through vibration isolation, sound insulation, sound absorption and sealing treatment, to control the path of noise transmission and achieve the purpose of reducing radiated noise. The design and application of shock absorbers for loaders are traditional passive noise reduction measures. The optimized design of shock absorbers has been proven to be the key to achieving satisfactory noise reduction effects.

2 The principle of vibration reduction and noise isolation

Controlling vibration, like controlling noise, should first start from the vibration source and at the same time consider controlling the propagation of vibration. The approaches to vibration control generally include vibration force isolation or applying damping to the structure. Vibration isolation is to reduce the vibration transmission from one structure to another through certain elastic devices. Resonant structures can be reduced by applying damping, which can be achieved in the form of dynamic vibration absorbers or by applying multiple layers of materials on various surfaces of the structure. To sum up, there are roughly the following several approaches.

1) Excitation source, control of vibration source vibration - that is, to control the vibration level to the minimum extent. This is the most thorough and effective method. The main method is to reduce the excitation on the equipment caused by the unbalanced force of the vibration source itself.

2) Avoid resonance - Resonance is a special state of vibration. When the frequency of the disturbance excitation force of the vibrating machinery is consistent with the natural frequency of the equipment, it will make the vibration of the equipment more severe and even have an amplifying effect. This phenomenon is called resonance.

3) Reduce vibration response - Vibration reduction and absorption essentially involve converting the mechanical energy of vibration into other forms of energy such as thermal energy.

4) Control the transmission rate of vibration - Vibration isolation Vibration isolation refers to the installation of a vibration isolation system or device between the vibration source and the vibrating body to reduce or isolate the transmission of vibration.

Vibration isolation and reduction measures are considered from the perspective of isolating or reducing the vibration of the sound source. Their theoretical basis is established on the concept of vibration, with the focus on the noise sound source itself.

The vibration or impact of the sound source directly excites the vibration of the solid structure and propagates in the form of elastic waves within the solid components. This kind of sound wave is called structural sound (or solid sound), and structural sound mainly spreads through solid components. It is different from the airborne sound in which the sound source directly excites the air to radiate sound waves. During the propagation of structural sound, it also radiates airborne sound to the surrounding air medium. In fact, what is ultimately known to the human ear is still airborne sound. For airborne sound, the sound source first radiates sound waves into the air, and we mainly focus on the sound field in the air. For structural sound, the sound source first excites to generate sound waves in solid components. We mainly focus on the sound transmission characteristics of solid components.

For specific machinery and equipment - wheel loaders, their noise consists of two parts: radiated noise and noise near 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. The low-frequency structural noise mainly depends on the isolation level of the low-frequency vibration of the powertrain. Components related to low-frequency structural noise include the powertrain system, transmission system, body system, etc., as these are the main sources of vibration. The optimal combination of each system is the primary task for reducing noise. However, for the noise control of existing products, the optimization of the powertrain system and the installation of vibration pads in the driver's cab is the key to noise control. From the perspective of the entire system: The vibrations generated by the moving parts are well transmitted from the engine, transmission, hydraulic oil pump - rear frame - cab. By optimizing the vibration pads of the powertrain (engine, transmission) and the driver's cab, the purpose of reducing the vibration noise of the loader can be achieved. Therefore, using high-performance shock absorbers at the connection points of power transmission components and the installation connection points of the driver's cab is one of the main means to reduce the noise of loaders.

3 Design principles of shock absorbers

Commonly used shock absorbers include metal spring shock absorbers and rubber shock absorbers. The former features stable performance, large load-bearing capacity, low natural frequency (less than 5Hz), small damping coefficient, low horizontal stiffness, and the ability to transmit high-frequency noise. The latter has a large damping coefficient, is conducive to crossing the resonance zone, has absorption in all three directions, performs well in reducing high-frequency noise, is simple to form, convenient to process, has a low load-bearing capacity, is applicable at temperatures ranging from -40℃ to 70℃, and has a service life of about five years. Rubber shock absorbers are commonly used in loaders.

The vibration isolation effect can be represented by the vibration isolation system (i.e., the transmission coefficient) η :

η = = Equation (1)

In the formula, N represents the maximum dynamic reaction force transmitted by the elastic component to the foundation

F - The maximum reaction force transmitted to the foundation without vibration isolation

λ -frequency ratio, λ =

ξ - damping ratio, ξ =

The natural frequency f of the vibration isolation system is calculated by the following formula:

f＝＝＝＝ equation (2)

In the formula, Δh represents the compression amount of the vibration isolator under static pressure, and the static displacement Δh = mg/K

K - Dynamic stiffness of shock absorber, kg/cm

● General principles for shock absorber design

① Principles for determining the stiffness of shock absorbers

The vibration of an engine has six degrees of freedom, namely, forward and backward, left and right, up and down, yaw, pitch and roll (displacement along the X, Y and z directions and rotation around the three axes). The arrangement of elastic supports should take into account these six degrees of freedom. When arranging elastic supports (vibration isolation pads), the main considerations should be the direction of the interfering force, the center of gravity of the equipment and the geometric dimensions of the elastic support arrangement. When the interfering force passes through the center of gravity of the equipment and is perpendicular in direction, as long as the elastic support (vibration damping pad) is arranged symmetrically according to the center of gravity, the force on the elastic support arrangement is equal. When the frequency of the interfering force is greater than the natural frequency of the vibration isolation system composed of the equipment and the elastic support arrangement, the surrounding environment of the equipment achieves a good vibration isolation effect. When the mass distribution of the supported object is uneven and the elastic support arrangement cannot be symmetrically distributed according to the center of gravity, vibration pads of the same model but with different stiffness can be used. The vibration pads closer to the center of gravity have greater stiffness, while those farther from the center of gravity have smaller stiffness, so that the combined reaction force generated by each vibration pad is consistent with the center of gravity of the supported object.

The stability of the vibration pads for the engine and transmission assembly is also an important factor in choosing the design scheme of the vibration pads. The basis for ensuring system stability is that the minimum natural frequency of the system's vibration mode is less than a certain frequency value.

To ensure that the vibration isolation pad has sufficient static load-bearing capacity to meet the service life requirements of the loader's power transmission system, while choosing the appropriate stiffness of the vibration isolation pad, it is also necessary to guarantee its rated static load-bearing capacity.

4 Design and calculation examples of shock absorbers

Taking the engine and transmission assembly of the ZL50G wheel loader as an example, the design and calculation of the vibration damping pad:

• Determine the excitation frequency

The ZL50G wheel loader is powered by a straight-six diesel engine. In a six-cylinder engine, each revolution of the crankshaft generates three torque fluctuations. If the idle speed of the engine is 700 r/min, then its excitation frequency is f = 700? 3/60 = 35 (Hz).

• Determine the natural frequency of the system

Generally speaking, in engineering, achieving a vibration isolation rate of 70% to 90% is achievable. Taking the vibration reduction rate I = 0.8, then the vibration transmission rate T = 1-I = 0.2. Ignoring the influence of damping, we have:

T = λ = Equation (3)

In the formula, λ represents the frequency ratio; f represents the excitation frequency; f is the natural frequency of the system.

From Equation (3), it can be obtained that:

λ = 2.45

The natural frequency of the system: f = = 14.3 Hz

From Equation (2) : f =

In the formula, k represents the dynamic stiffness of the shock absorber, in kg/cm

m - The total mass of the engine and transmission unit in kg

m = m+m = 565+463 = 1028kg

Calculate the total dynamic stiffness k of the elastic support (shock absorber) as:

k = m (2πf) = 8290N/mm

The corresponding static deformation is Δh = mg/K = 1.2 mm

Determine the vertical dynamic stiffness K (i = 1,2,3...) of each shock absorber.

First, based on the coordinates of each shock absorber arrangement point relative to the center of gravity of the powertrain, as well as the total mass of the engine and transmission unit, calculate the vertical static load P (i = 1,2,3...) at each point. . According to the formula Δh = p/K, the dynamic stiffness of each shock absorber in the vertical direction can be determined.

By arranging and designing the shock absorbers in this way, satisfactory vibration reduction and noise reduction effects can be achieved on the loader.

References

[01] Chen Xiujuan: Practical Noise and Vibration Control, Chemical Industry Press, May 1996

[02] Zhao Songling: Noise Reduction and Sound Insulation, Tongji University Press, 1985

[03] Liu Haoliang, Yang Yong: "Analysis of Vibration Isolation Performance of Rubber Vibration Isolators for Vibratory Rollers", Road Machinery & Construction Mechanization, November 2004

[04] Zheng Zhongfa, Zheng Guoshi: "Optimizing Engine Vibration Pads to Reduce Vibration and Noise Inside Buses", Automotive Science and Technology, No. 4