Zhang Shunxin, Liu Shuying, Liu Wenrun, School of Mechanical Engineering, Hebei University of Technology, Tianjin 3,13, used this principle for theoretical analysis, and gave an example calculation for the stiffness of the gas spring and the natural frequency of the system. This method provides useful information for the development of other types of vibration damping devices.
1 Introduction to the structure of the device A connecting device is installed between the stretcher and the floor of the vehicle. The control system is mounted thereon, and the two cylinders are respectively connected to the air bag through the orifice.
When the car body is not vibrating, the gravity of the patient's 4 stretcher is balanced with the air pressure.
When the vehicle body vibrates up and down, the gas in the two cylinders is simultaneously compressed, so that the pressure in the cylinder and the pressure in the airbag generate a pressure difference, which promotes the flow of the throttle hole between the cylinder and the airbag, and generates a damping effect. Together with the elasticity of the gas in the cylinder, the amplitude is attenuated.
When the vehicle body is subjected to pitching vibration, the left-hand two-way control system has different functions. The road cylinder is subject to pressure as the left road is pressed, and the other road is opposite. When the left-hand road is pressurized, the cylinder pressure is greater than the air pressure, and the air flow is from the cylinder to the airbag. Flow until the pressure on both sides is equal; while on the right, the airbag pressure is greater than the cylinder pressure, and the flow is caused by the airflow to the cylinder. Similarly, the solenoid valve is adjusted to adapt the system to two different cross-sectional area orifices. Two different damping effects are generated to accommodate different road conditions.
The larger the area of ​​the orifice, the larger the flow rate, and the smaller the gas resistance, the smaller the damping effect. Appropriate, control the area of ​​the orifice, so as to adjust the damping force to achieve the best damping effect.
2 gas spring vibration analysis ambulance stretcher damping device must have a vibration damping effect, must ensure a basic constant natural frequency, so that the theoretical analysis results can be used in a targeted and effective way, but to maintain a basic The constant natural frequency characteristics depend on maintaining a constant volume of the pneumatic system, thus ensuring the normal working height of the ambulance stretcher. 5. Here we use the simultaneous action of the cylinder and the airbag.
Proper design and use of the damping device must clarify the characteristics of the applied load, which can be periodic or aperiodic, can be continuous or instantaneous, or it can be harmonic or random. The damping device is placed between the base of the vibration and the device to be protected. It acts as a mechanical filter and is equivalent to a damped spring in a single degree of freedom. If the basis is sinusoidal vibration, then the protected sinusoidal response is also sinusoidal, but the amplitude is reduced; if the basis is random vibration, and the protected device is a rigid body, the response is based on the resonant frequency of the entire system as the main frequency. Harmonic vibration of random components, if the protection device is an elastic body, then its response to the foot vibration is narrower than the excitation vibration.
The device adopts a gas damping damper, and the vibration damping principle is to use the gas to dampen the viscous friction energy between the pipe wall and the gas molecules. Moreover, the damping coefficient is fully adjusted by the characteristics of small orifice throttling to achieve the best damping effect. The biggest characteristic of the gas damper is that the air has low rigidity and a small elastic modulus, so that a resonance frequency lower than 81 can be obtained, and the load condition and the condition of the air load are determined, so the resonance frequency of the device can only be reduced. Start, that is, increase the quality or reduce the stiffness, but in actual engineering, the quality is usually not allowed to change, so under the premise of ensuring the function of each part of the device, the stiffness of the device should be reduced as much as possible.
3 Theoretical calculation The elastic stiffness and natural frequency of the gas damper are calculated according to the piston-type air spring of the cylinder. The internal pressure at the time of re-positioning of the piston is taken as the standard state. The elastic stiffness can be calculated by the following formula. The adiabatic index of the air is determined by the following formula. Take 1.2; 4 cylinder piston area.
When an external force is applied to the piston, the gas reacts and the gas in the cylinder is alternately compressed and expanded, resulting in a pressure increase proportional to the external force. If the sealed air is compressed as a whole, there is no energy loss, but if If there is a small orifice throttling, the pressure increment will cause a phase difference due to the small hole. The air flow through the small hole may be laminar or turbulent, which depends mainly on the external force, the size of the small hole and the cylinder. The volume responds to the vibration mode and so on.
According to this feature, the elastic stiffness of the gas spring in the damping device can be obtained, wherein the static pressure of the gas spring; the cross-sectional area of ​​the piston of the A cylinder; the volume of the Vr airbag; and the volume of the Vco cylinder when the expansion amount is 岑.
The degree of freedom stretcher damping device can be regarded as symmetrically arranged at both ends, two springs with constant rigidity, and symmetrically arranged small orifice throttling to generate damping.
For example, depending on the specific conditions of the ambulance, the unit has a diameter of 63! The cylinder, then its cross-sectional area, if the patient's mass is calculated according to the average human weight of 68kg, assuming that the truss is 25kg, the mass of the stretcher and the patient W is 93, and each cylinder bears half of the world. At this time, considering the vehicle running condition and the space of the vehicle body, take=1559931; =1.1233 according to formula 3, the stiffness of the gas spring is ok, the stiffness and resonant frequency of the system are relatively low, and the design and practicability of the device is very large. 7.
4 Example calculation of the stiffness of the knot spring and the natural frequency of the system. This method provides useful information for the development of other types of vibration damping devices.
Liu Shuying, Zhang Shunxin, Liu Wenrun. Medical ambulance stretcher damping device program design. Journal of Hebei University of Technology, 2000, 29 Zhang Shunxin. Medical ambulance stretcher damping device. Chinese patent 2199215780, 19990307.
Yan Jikuan. Mechanical Vibration Isolation Technology Shanghai Science and Technology Literature Publishing House, 1985.
Liu Yuhua. Viscoelastic Damping Vibration Reduction and Noise Reduction Application Technology M. Beijing Aerospace Press, 1990.
Liu Shuying. Research on ambulance stretcher damping system, 0. Hebei Hebei University of Technology, 2000.
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