Hydraulic creep failure and prevention

☆Special device? Parts ☆ Hydraulic crawling faults and prevention Yu Guoqiang (Ministry of Railways Xiangfan Diesel Locomotive Factory Xiangfan 445, Hubei, China) [Abstract] In view of the crawling faults in the hydraulic system, the hazards are described, and the causes are analyzed, and the effective elimination of hydraulic crawling is proposed. Measures.

1 Overview Crawling is a relatively common fault in hydraulic systems. It occurs when the hydraulic cylinder or the hydraulic motor runs at a low speed. The phenomenon of uneven velocity occurs intermittently. It is manifested by the alternating motion of the piston rod or motor rotor snapping and stopping, 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 problems such as rigorousness, tilting, etc. In the metal cutting machine tools such as the grinder hydraulic system, crawling causes the surface of the workpiece to be spiraled. The pattern seriously affects the surface accuracy and roughness of the workpiece. Therefore, it is particularly necessary to analyze the causes of crawling and propose effective solutions.

2 The causes of hydraulic crawling The causes of hydraulic crawling are more complicated, but the most fundamental reason is related to the friction characteristics of the friction surface of the system and the oil compressibility.

Now take the hydraulic system crawl model shown in Fig. 1 as an example to analyze the cause of crawling.

2.1 As shown in Fig. 1, the piston rod of a single rod hydraulic cylinder drives a mass load to move on the guide rail. The fluid entering the cylinder chamber (especially the fluid mixed with air) actually has a certain degree of compressibility. Under the effect of pressure, its volume is reduced, forming a hydraulic spring (which can be represented by a volume compression coefficient K), and accumulates. A certain amount of energy.

When the pressure is reduced, its energy will be released.

2.2 The piston and cylinder, piston rod and cylinder head, there are friction between the load and the contact surface, which makes it move and static there is the difference between dynamic and static friction.

These two factors are the most fundamental cause of hydraulic crawling.

2.3 Based on these two factors, we will analyze how the hydraulic system is crawling.

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 frictional force j between the piston and the piston rod (excluding the load friction) and the cylinder. As the pressure in the left chamber rises, the oil and the mixed air are compressed (as in Figure 2b), and the energy is accumulated until the pressure FF on the piston is generated. The piston suddenly starts.

Under normal circumstances, 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 presence of the difference between the dynamic and static friction forces. The oil and air compressed in the left chamber are immediately expanded and the accumulated energy is released, accelerating the movement of the piston (see Figure 2c). When the piston moves, due to the existence of the oil discharge resistance, the right-cavity oil and the mixed air cannot be quickly discharged and are compressed to generate back pressure. In this way, the piston is rapidly decelerated and braked under the combined effect of a decrease in the pressure in the left chamber and a rise in the pressure in the right chamber. The piston suddenly started again until the pressure in the left chamber rose again to overcome the static friction. As a result of this recurrence, there is a sudden jump and stoppage (Figure 1 Curve 1). Of course, when the piston is moving at a higher speed, it may not be able to stop and it will be accelerated again. This will result in a sudden and fluctuating vibration (curve 2 in Figure 3). Having found the cause of crawling, effective measures can be taken to prevent or eliminate hydraulic crawling.

3 Measures to Eliminate Crawling From the above analysis, it can be seen that eliminating the crawling should minimize the piston pause time t = 0, the sudden jump phenomenon disappears, and crawling is effectively controlled. We set the piston effective area to A 0, and its average moving speed is v 0. Because the forward acceleration is greater after the piston starts, it indicates that the left cylinder oil and air in the left cylinder before the start are compressed. According to the fluid continuity equation and mass conservation, the fluid volume dV of the system entering the left chamber during the piston pause time can be expressed as where c is the proportional coefficient, as the case may be. At the instant of piston starting, the friction force suddenly drops from the static friction to sliding friction, the pressure difference d, then the pressure increment of the left cavity of the oil cavity is the pressure increment dP. In the system, it mainly compresses the right cavity oil and air and offsets the right cavity. Back pressure formed by oil and air, so dP can be expressed as: L - effective length of cylinder working chamber before piston movement U - volumetric elastic modulus of oil - back pressure generated by right oil chamber.

It can be seen that to reduce t 0 and eliminate crawling, the static friction force j can be reduced, the sliding friction force F d can be increased, the piston area A and the moving average velocity v 0, and the back pressure P and the volumetric elastic modulus U can be increased. However, the piston area and movement speed are determined by design requirements and generally cannot be changed. We can only weaken or eliminate crawling from the following aspects.

3.1 Reduce static friction, increase dynamic friction, weaken or eliminate uneven friction during travel In general, F d, and intentionally increase the dynamic friction will cause unnecessary energy consumption, so we only reduce the two The difference between the two.

3.1.1 The friction surface is as smooth as possible. When machining cylinders, the fine boring and rolling speeds are appropriate so as not to cause a spiral inner surface, causing uneven friction and causing crawling or using high-precision cold-drawn tube machining cylinders.

3.1.2 Use good seals and reasonable seal structure. If the higher requirements of the fuel tank can be used to grid ring and seal, you can make the piston starting friction equal to or lower than the dynamic friction (Figure 4), can be a good way to eliminate the crawl.

3.1.3 increase the oil film strength. The high-strength oil film prevents the oil film from escaping from the contact surface when the piston is stopped, minimizes the transition from the dry friction state to the wet friction state during start-up, and reduces the difference in dynamic and static friction.

3.1.4 During installation, the structural arrangement is reasonable so as not to cause uneven loading and inconvenience, resulting in non-uniform friction.

3.2 Improve Hydraulic System Stiffness The hydraulic system is prone to creep. The existence of hydraulic oil stiffness is a very important reason: For example, the mechanical transmission is generally much higher in rigidity than hydraulic transmission, so there is almost no crawling phenomenon. Therefore, in the hydraulic system, when the conditions permit, it is necessary to use a fluid with a large bulk modulus U as much as possible, to prevent air from entering the hydraulic system, and to discharge the air entering the hydraulic system in time. The air is mixed into the hydraulic system, which can easily cause hydraulic shock, vibration, noise and crawling and other failures. Because of Guo Qiang: hydraulic crawling faults and prevention 4 development proposals In order to adapt to the modernization of national defense and the needs of modernization, the HY 473 undergoes several rounds of Continuous improvement has formed a basic model for heavy-duty equipment transport car trains. In the future development, the serial development will be the military-civilian dual-purpose large-scale, medium-sized and small series. Large-scale series transport vehicles are mainly used for military purposes. For example, Dongfeng No. 31 multi-function vehicle train chassis, Dongfeng 21, 21A, 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 level of cargo transportation and the small size are mainly the off-road performance of 20t or less. In the drive shape × 6, 6 × 4, 4 × 4, 4 × 2, etc. In the large series, it can adopt twisted type, super-heavy special vehicle with all-wheel drive of the main vehicle and all-wheel steering of the trailer. The technical configuration is: The engine model is mainly based on KHD Deutz air-cooled diesel engine, with FL F, FL 513F series, naturally aspirated, supercharged and clutched up to 298kW, with dual-plate dry type, oil-controlled air assisted 298 kW or more loaded W SK 400 hydraulic torque converter with shift clutch and overload protection device, hydraulic Downhill braking.

Transmission light selection of self-made 6-speed mechanical transmission Medium-sized selection of Fu llerRT 09C Nine-gear transmission Large-scale selection of steering using main steering and emergency steering, single-axle steering for power steering, double front axle steering for hydraulic assistance, double swing arm steering mechanism and front wheels Steering, follow-up steering of the trailer, and follow-up steering of the steering wheel.

The brake type adopts double-pipe and air brake. When the rear axle (double rear axle) is equipped with double-chamber spring brake, WA BCO valve parts are used, and brake anti-lock braking technology is gradually adopted.

The cab adopts all-metal, enclosed, flat-top reversible, two-door three-bedroom, two-door, four-door, four-door, four-door, and four-door and six-door. It is equipped with a vehicle air conditioner, warm air defrosting equipment, Fully softened upholstery and instrument clusters.

At the level of automation, hydraulic automatic shifting, hydraulic support of trailers, rapid automatic lifting and lowering of springboards, automatic locking devices, etc. are gradually adopted to form a high-quality, high-level heavy equipment transportation system with Chinese characteristics, for national defense modernization and national economy. Development contributes.

Air into the hydraulic system should pay attention to the following aspects: a. Adequate sealing measures for hydraulic components and piping joints The location of the suction and discharge ports of the system is reasonable and cannot exceed the liquid level c. Design the tank with the correct capacity so that the air entering the oil has enough time to precipitate d. An exhaust valve is installed at a suitable position in the system to discharge the air e mixed in the oil at any time. The oil tank is provided with an appropriate resistance to the oil return port of the defoaming network so as to prevent the oil return from being too rapid, so that a negative pressure is formed in the oil inlet chamber and air dissolved in the oil is precipitated.

3.3 Back pressure is set in the return line. As can be seen from equation (4), setting proper back pressure can effectively control piston creep. However, excessive back pressure also tends to increase power consumption. Therefore, the setting of back pressure should be determined according to the actual situation.

In addition, the wear of the hydraulic components or the contamination of the oil can cause the area of ​​the throttle opening of the system to change, causing pressure and flow to fluctuate, causing crawling failures. Therefore, hydraulic oil contamination should be reduced and wear elements repaired or replaced in time.

4 Conclusion In conclusion, there are many causes of hydraulic system crawling. The cause of the hydraulic system should be analyzed according to the actual conditions on the site, so that effective measures can be taken to control it in time to ensure the normal operation of the hydraulic system.