1 Overview In hydraulic systems, crawling is a relatively common type of failure. It is a motion phenomenon in which the speed of the hydraulic cylinder or the hydraulic motor runs at a low speed and the intermittent speed is uneven. It is represented by the alternating movement of the piston rod or the motor rotor suddenly and stopped, and the pressure gauge pointer is unstable and swings.
The crawling of the hydraulic actuator is a very harmful phenomenon. For example, the crawling of the hydraulic double-cylinder piston rod of the dump truck can cause other problems such as difficulty and inclination. In the metal cutting machine such as the hydraulic system of the grinding machine, the crawling causes the surface of the workpiece to be ground to spiral. Pattern, seriously affecting the surface precision and roughness of the workpiece. Therefore, it is especially necessary to analyze the causes of crawling and propose effective solutions.
2 The cause of hydraulic crawling caused by hydraulic crawling is complicated, but the most fundamental reason is related to the friction characteristics of the friction surface of the system and the fluid compressibility.
Now take the hydraulic system crawling model shown in Figure 1 as an example to analyze the causes of crawling.
2. As shown in Figure 1, the piston rod of the single-piston rod hydraulic cylinder drives the load of mass to move on the guide rail. The oil entering the cylinder chamber (especially the oil mixed with air) actually has a certain compressibility, and under the action of the pressure, its volume is reduced, forming a hydraulic spring (represented by the volume compression coefficient K), accumulating A certain amount of energy.
When the pressure is reduced, its energy will be released.
2. 2 piston and cylinder, piston rod and cylinder head, there is friction between the load and the contact surface, which makes the difference between dynamic and static friction when moving and stationary.
These two factors are the most fundamental causes of hydraulic crawling.
2. 3 Based on these two factors, let's analyze how the hydraulic system produces creep.
As shown in Fig. 2a, when the left chamber of the hydraulic cylinder enters the oil, the piston cannot move immediately due to the static friction j between the piston and the piston rod (excluding the load friction) and the cylinder. As the pressure in the left chamber increases, the oil and the mixed air are compressed (Fig. 2b), accumulating energy until the pressure F F is generated to the piston, and the piston suddenly starts.
In general, the dynamic friction coefficient f is smaller than the static friction coefficient. After the piston is started, the piston is rapidly accelerated due to the difference in dynamic and static frictional forces. The oil and air compressed by the left chamber immediately expand, and the accumulated energy is released, which accelerates the movement of the piston (Fig. 2c). When the piston moves, due to the presence of oil discharge resistance, the right chamber oil and the mixed air cannot be quickly discharged and compressed, resulting in back pressure. Thus, under the combined effect of reduced left chamber pressure and increased right chamber pressure, the piston is rapidly decelerated and braked. The piston suddenly starts up again until the left chamber pressure rises again to overcome the static friction. In this way, the creeping and stopping alternate crawling occurs (Fig. 3, curve 1). Of course, when the piston moves at a higher speed, it may not be able to stop, and it is accelerated again, which will produce a sudden and slow vibration (Fig. 3 curve 2). Once you have found the cause of the crawl, you can take effective measures to prevent or eliminate hydraulic crawling.
3 measures to eliminate the crawling phenomenon As can be seen from the above analysis, the elimination of crawling should minimize the piston pause time t = 0, the sudden jump phenomenon disappears, and the crawling is effectively controlled. We set the effective area of ​​the piston to A 0, and the average moving speed is v 0. The greater the forward acceleration after the piston is started, the larger the amount of oil and air compressed in the left chamber of the cylinder before starting. According to the continuity equation of the fluid and the conservation of mass, during the piston pause time, the oil volume dV entering the left cavity of the system can be expressed as where c is the proportional coefficient, as the case may be. When the piston starts, the frictional force is suddenly reduced from static friction to sliding friction. The pressure difference d is the pressure increase in the left cavity of the oil chamber. The pressure increment dP is mainly used in the system to compress the right cavity oil and air and offset the right cavity. The back pressure formed by oil and air, so dP can be expressed as: L - the effective length of the working chamber of the cylinder before the piston movement - the volumetric elastic modulus of the oil - the back pressure generated by the right chamber of the oil.
It can be seen that to reduce t 0 and eliminate creep, the static friction force j can be reduced, the sliding friction force F d , the piston area A and the moving average speed v 0 can be increased, and the back pressure P and the oil volume elastic modulus U can be increased. Piston area and speed of motion are determined by design needs and generally cannot be changed. We can only weaken or eliminate crawling by starting from the following aspects.
3. 1 reduce the static friction, increase the dynamic friction, reduce or eliminate the uneven friction in the stroke. In general, F d, and intentionally increase the dynamic friction will cause unnecessary energy consumption, so we only reduce The difference between the two.
3. 1. 1 The friction surface is as smooth as possible. When machining the cylinder, the precision and rolling speed are appropriate to avoid the spiral inner surface, causing uneven friction and causing creep or high-precision cold drawing of the cylinder.
3. 1. 2 Use a good seal and a reasonable sealing structure. If the higher requirements of the cylinder can be used to check the grid and the seal, the piston starting friction can be equal to or even lower than the dynamic friction (Figure 4), which can eliminate creep.
3. 1. 3 Improve the oil film strength of the oil. The high-strength oil film can prevent the oil film from being squeezed out from the contact surface when the piston is stopped, and minimize the transition from the dry friction state to the wet friction state at the time of starting, thereby reducing the difference between the dynamic and static frictional forces.
3. 1. 4 When installing, the structure is arranged reasonably, so as to avoid the phenomenon of eccentric load and other strength, resulting in uneven friction.
3. 2 Improve the rigidity of the hydraulic system The hydraulic system is prone to creep. The existence of the stiffness of the hydraulic oil is an important reason: for example, the stiffness of the mechanical transmission is generally much higher than the stiffness of the hydraulic transmission, so there is almost no creep. Therefore, in the hydraulic system, when conditions permit, it is necessary to use oil with a large volume elastic modulus U to prevent air from entering the hydraulic system and timely discharging air entering the hydraulic system. Air mixed into the hydraulic system, it is easy to cause hydraulic shock, vibration, noise and crawling and other faults, because Guo Guoqiang: hydraulic crawling failure and prevention 4 development recommendations In order to adapt to the development of national defense modernization and modernization needs, HY 473 after several rounds Continuous improvement, has formed the basic model of heavy equipment transport car train. In the future development, it will be developed in series, that is, the military, civilian and civilian special large, medium and small series. Large series of transport vehicles are mainly used for military purposes, such as Dongfeng 31 multi-function vehicle train chassis, Dongfeng 21, 21 A, 50t, 60t tank transport vehicles and other medium-sized series mainly consider light armored vehicles, self-propelled artillery and civilian equipment at 30t, 35t, The 40t class of cargo transport and the small size is mainly the off-road performance of 20t or less. In the drive shape × 6, 6 × 4, 4 × 4, 4 × 2, and the like. In the large series, the super-heavy special vehicle with the spliced ​​type, the main vehicle all-wheel drive and the trailer full-wheel steering can be used. The technical configuration is as follows: the engine model is mainly KHD company Deutz air-cooled diesel engine, with FL F, FL 513F series, naturally aspirated, supercharged, medium clutch below 298kW, with double dry type, oil control gas assist 298 kW or more with W SK 400 hydraulic torque converter, with shift clutch and overload protection device, hydraulic Brake downhill.
The transmission is light and self-made. The 6-speed mechanical transmission is medium-sized. The FullerRT 09C nine-speed transmission is selected for large-size steering. The main steering and emergency steering are used. The single-bridge steering is power steering. The double front axle steering is hydraulic assist. The double swing arm steering mechanism has front wheels. Steering, trailer following steering and synchronous following steering wheel steering.
The brake mode uses double pipe and air brake. When the rear axle (double rear axle) is equipped with double cavity spring brake, the WA BCO valve is used, and the brake anti-locking technology is gradually adopted.
The cab is made of all-metal, closed, and flat-head reversible. It has two doors, three seats, one bedroom, two four doors, four four doors, four doors and six seats. It is equipped with a car air conditioner and a warm air defrost device. Fully softened interiors and instrument clusters.
Gradually adopt hydraulic automatic shifting, hydraulic support of trailer, fast automatic lifting and springboard, automatic locking device, etc., to form a high-quality, high-level heavy equipment transportation system with Chinese characteristics, for national defense modernization and national economy. Development contributes.
Air entering the hydraulic system should pay attention to the following aspects: a. The hydraulic components and the pipe joints should be well sealed. The suction and discharge ports of the system should be arranged reasonably, and should not exceed the liquid level c. Design the correct capacity of the fuel tank. The air entering the oil has sufficient time to precipitate. d. Install an exhaust valve at a suitable position in the system to discharge the air mixed in the oil at any time. e. Set the degassing net back to the oil port with appropriate resistance to prevent oil return. Rapidly, a negative pressure is formed in the oil inlet chamber to precipitate air dissolved in the oil.
3. Set the back pressure in the 3 return line. It can be seen from the formula (4) that setting the proper back pressure can effectively control the crawling of the piston. However, if the back pressure is too large, it will easily cause an increase in power consumption. Therefore, the setting of back pressure should be determined according to the specific situation.
In addition, the wear of the hydraulic components or the contamination of the oil can cause the area of ​​the throttle orifice to change, causing fluctuations in pressure and flow, causing creeping failure. Therefore, the hydraulic oil contamination should be reduced, and the worn components should be repaired or replaced in time.
4 Conclusion In conclusion, there are many reasons for the hydraulic system to crawl. The reasons should be analyzed according to the specific conditions of the site, so that effective measures can be taken to control and ensure the normal operation of the hydraulic system.
Publishing house.
2 Shuguang Guobian. Hydraulic system fault diagnosis and troubleshooting. Beijing: Ocean Press.
3 Li Guojun, Zhan Baiqiong, editor. Engineering Fluid Mechanics. Wuhan: Huazhong University of Science and Technology Press.
Zhang Shiming: Performance Analysis and Development Suggestions for Heavy Armored Transport Vehicles
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