Measures to prevent cavitation in relief valves
The four-wheeled agricultural transport vehicle was developed in the mid-1980s and is a kind of modern transportation suitable for the transportation of middle and short-distance rural areas in China. The Ministry of Public Security defined in 1993 the “Safety Standard for Agricultural Transport Vehiclesâ€: “Four-wheeled agricultural use. A transport vehicle is a four-vehicle motor vehicle whose engine is a diesel engine and whose power is no more than 28 kW, whose mass is not more than 1500 kg and whose maximum speed is not more than 50 km/h.". In the "Agricultural Transport Vehicle Safety Technical Requirements", the definition of a four-wheeled agricultural transport vehicle is: "The maximum design speed is not more than 70km/h, the maximum design total mass is not more than 4500kg, the length is less than 4.6m, and the width is not more than 2m and high. Agricultural vehicles with four wheels no larger than 2.5m.". The “Technical Conditions for the Safe Operation of Motor Vehicles†implemented nationwide from October 1, 2004 clearly states that low-speed trucks (formerly “four-wheel agricultural vehiclesâ€) are vehicles with a maximum design speed of less than four wheels. . At this point, the four-wheeled agricultural vehicle was formally incorporated into the automobile category. The changes defined by the above terms can be seen from the fact that the development of the original agricultural vehicle to the low-speed truck today reflects the change of rural road conditions, the increase of purchasing power, and the demand for efficient and safe transportation tools. Low-speed trucks are not a substitute for the simple concept of agricultural vehicles, but rather an overall improvement in vehicle performance.
For example, the braking performance requires the functions of service braking, emergency braking and parking brake. The driving brake requires the use of a dual pipe system. Therefore, the design specifications for low-speed trucks should also be adjusted. Taking brake stability as an example, in the design specifications of a traditional agricultural vehicle, it is generally considered that the vehicle speed of the agricultural vehicle is relatively low, and the probability of the rear axle slipping during braking is less. In order to ensure that the vehicle does not lose steering ability during braking, the synchronous adhesion coefficient is selected to be low (generally around 0.4), so as to ensure that when the ground attachment coefficient is greater than the synchronous attachment coefficient, the rear wheel is first locked and the front wheel is locked. The wheel does not lose steering ability. In fact, with the improvement of rural road traffic conditions, the actual running speed of low-speed trucks has been greatly improved, and the design requirements for braking stability should be consistent with that of ordinary trucks. Brake Stability The directional stability of a car when it is braking includes no sideways slippage, no sideways slippage, and no loss of steering ability. The ability to skid and lose steering is related to whether the wheels are locked and the order of front and rear wheel locking. According to the automobile theory, in the braking process, the occurrence of rear axle skidding caused by the front wheel locking is a more dangerous working condition, and the front axle side slip caused by the front wheel locking is small and relatively stable, but There will be no turning ability in the corners. Ideally, the front and rear wheels should be locked at the same time or an anti-lock braking system must be installed. For low-speed trucks, in order to reduce costs, generally do not install anti-lock braking system or braking force adjustment device, only through reasonable distribution of front and rear brake braking force, to avoid the occurrence of rear axle sliding, in order to obtain satisfactory Braking stability.
The ratio of the braking force of the front and rear brakes of low-speed trucks is usually fixed. This distribution ratio is represented by the braking force distribution coefficient. It is defined as that during braking of the Fμ2 rear brake, the front and rear wheels must be held at the same time. At the time of death, the relationship between the front and rear brake braking force can be seen as the relationship between the braking force of the front and rear brakes and the position of the center of mass, the wheelbase and the overall vehicle quality. Obviously, this proportion relationship is not the same for trucks at full load and no load. As shown in the figure, the I curve represents the proportional relationship between the brake force of the front and rear brakes when the front and rear wheels are locked at the same time, and the β line represents The actual braking force distribution relationship, the ground adhesion coefficient corresponding to the intersection point of the I curve and the β line, is the smaller the synchronous adhesion coefficient (point A) when the synchronous attachment vehicle is unloaded than the full load (point B). . During the braking process of the vehicle, the local surface adhesion coefficient when the vehicle structural parameters are constant, the distribution ratio of the braking force of the front and rear brakes directly affects the braking stability. This article according to the characteristics of low-speed trucks, from the requirements to meet the braking efficiency, including the effectiveness of one of the two pipelines in the failure of the pipeline, and meet the braking efficiency requirements and to ensure that there is a large enough synchronous adhesion coefficient to avoid the rear axle side Starting from the three aspects of sliding, the calculation formula of the braking force distribution coefficient was derived, which is more intuitive, comprehensive, and flexible than the method of optimizing the calculation of the synchronous adhesion coefficient and then determining the braking force distribution coefficient. Since the finally obtained braking force distribution coefficient is a value range that satisfies all the conditions, the designer can be flexibly selected according to the specific structure of the braking system of the designed vehicle model. The change in the shape of the valve body oil chamber causes a change in the fluid pressure field and speed.
Therefore, measures to prevent the relief valve from generating cavitation include the following: cross-flow valve section should be designed to grow into a conical gap to reduce the sudden contraction and sudden diffusion of oil, and some small holes can also be added to the valve seat. By means of them, the downstream fluid that has recovered the pressure is guided back to the contraction part to prevent the negative pressure from being produced; for the pipe-connected overflow valve, the return valve channel of the relief valve can be made into a right-angled bent structure to increase the pressure. The critical back pressure of the relief valve. In addition, there are data that the main spool structure with an annular groove can effectively inhibit cavitation. Cavitation and Control Measures of Hydraulic Cylinder For the vertical (or inclined) installation, the hydraulic cylinder mainly driven by the load and the piston's own weight descends, when designing the down circuit, proper pressure on the upper chamber should be considered to ensure that the upper chamber is always filled with oil to prevent hydraulic pressure Cavitation occurs in the cylinder. For example, hydraulic cylinders of the WYl00 type hydraulic excavator, etc., one of the reasons for adopting the oil return throttle circuit is to prevent the occurrence of cavitation. The hydraulic cylinder abruptly closes its oil inlet during the movement. Due to the inertial load, the inlet side of the hydraulic cylinder may produce an absolute pressure lower than the air separation pressure, thereby generating cavitation. When it is turned on again, the hydraulic cylinder will cause malfunctions such as shaking and crawling. Therefore, various valves should be opened and closed slowly during operation.
The air in the oil mixed with air or the air in the hydraulic cylinder is not completely drained, causing cavitation under the effect of high pressure and causing large noise. At this time, the air must be exhausted in time. Conclusion For the WYl00 hydraulic excavator hydraulic system, the above analysis of the mechanism of cavitation and the preventive measures should be taken from the hydraulic pump, throttle valve, relief valve, hydraulic cylinder and other aspects. In addition, the sealing of pipes, etc., and the pipe diameter and bending shape should also be given high priority.
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