Abstract: In wheeled mobile machines such as large trucks, forklifts, cranes, construction machinery, etc., in order to reduce the driver's steering wheel strength and improve the steering flexibility of the vehicle, hydraulic assisted power steering systems are often used. The hydraulic booster is installed on the steering gear to make the steering of the full hydraulic steering system flexible and light, stable in performance, low in failure rate and convenient in layout. However, there are often problems such as failure of steering, heavy steering, and deviation or drifting of the vehicle while driving. This paper mainly analyzes the common causes of faults in the hydraulic power steering system of forklifts, and proposes corresponding measures to provide reference for the maintenance and repair of engineering machinery such as forklifts. Key words: forklift; hydraulic power steering; fault analysis INTRODUCTION: Industrial trucks and forklifts play a very important role in the logistics system of enterprises. They are widely used in various sectors of the national economy such as stations, ports, airports, factories, warehouses, etc., which are mechanized loading, unloading, stacking and short-distance transportation. Efficient equipment. The limitations of the forklift's work site and its own working performance determine its ability to turn extremely frequently during operation. Therefore, its steering performance is closely related to the safe driving of the forklift, work efficiency and the labor intensity of the driver. The steering performance of a forklift depends to a large extent on the performance of the steering system. Research and improvement of the performance of the steering system are important for improving steering characteristics. The steering system of a forklift consists mainly of a steering gear and a steering gear. The steering gear is also divided into a spherical worm roller type, a worm crank pin type, a recirculating ball type and a worm worm wheel type. The steering transmission mechanism also has mechanical and hydraulic assisting functions. Hydraulic power steering system is a kind of steering system commonly used in forklifts [1], but since the hydraulic steering system has a higher failure rate than the mechanical steering system and the fault is difficult to judge, it is necessary to strengthen the knowledge of this aspect in order to properly use and maintain the hydraulic steering. System to reduce the incidence of failure [2]. Body: We know that the hydraulic power steering system adds a hydraulic booster to the mechanical steering system. The hydraulic booster is mainly composed of hydraulic pump, relief valve, distribution valve, steering gear, hydraulic steering cylinder, oil pipe, hydraulic oil tank, etc. (the structure is shown below). Figure 1 Schematic diagram of the structure of the hydraulic steering system It uses hydraulic oil as the power transmission medium, and the hydraulic pump generates power to push the mechanical steering gear to realize steering. Because it relies on hydraulic oil to transmit power, sealing technology plays a pivotal role and is the key to ensuring the performance of hydraulic systems [3]. In addition, the hydraulic pump works well, the amount of hydraulic oil and cleanliness, have a great impact on the hydraulic steering mechanism, so in the use and maintenance, these problems can not be ignored [4]. This paper only analyzes and judges the fault of the hydraulic power steering system. In fact, it is the leakage of the common hydraulic transmission part, the air or oil in the oil circuit, the poor working of the hydraulic pump [4], the steering gear and the cylinder failure. Heavy, deviation, no straight line, serpentine driving, steering wheel slip, steering inaccuracy and instability, steering wheel oscillating rebound or rotation, steering original damage, abnormal noise when turning, etc. are analyzed and judged [5]. Steering heavy (such as slow-moving steering, heavy steering or stuck when the forklift is driving at low speed) Symptom: Under normal conditions, the steering with a hydraulic booster is very light. If you feel heavy or suddenly turn to the direction during operation, it is abnormal. Causes of failure: 1. The hydraulic tank is short of oil or the oil filter is clogged. 2. There is air in the oil circuit. 3. The hydraulic pump is worn, the internal leakage is serious, and the suction port is blocked, which makes the hydraulic pump inefficient [6], or the drive belt is slippery. 4. The relief valve leaks, the spring is too soft or improperly adjusted. 5. The hydraulic steering cylinder or distribution valve rubber seal is damaged. 6. Each oil pipe joint leaks or the seal ring breaks and blocks the oil pipe. Diagnosis and elimination: 1. Check if the hydraulic pump drive belt is slipping or other geared transmissions are damaged. 2. Check if the oil level of the hydraulic tank is too low or too dirty. If the oil tank is short of oil, add enough water and the hydraulic oil should be replaced and clean the tank. 3. Check for leaks in steering gear, relief valve, hydraulic pump, hydraulic cylinder, and various oil pipe joints. If it leaks or is damaged, it should be tightly sealed or replaced. 4. If there is air in the oil circuit, lift or set up the steering bridge or remove the direct rod, start the forklift to run at low speed, repeatedly turn the steering wheel from one end to the other, so that the hydraulic cylinder is full. The stroke is reciprocated, and the air in the oil passage is gradually discharged. Finally add hydraulic oil to the specified height. 5. Check that the hydraulic pump, relief valve (proportional valve), hydraulic steering cylinder are good, and repair or replace if damaged [5]. When the vehicle is driving straight, the steering wheel is floating (swinging) or the vehicle is deflected Fault phenomenon: When the vehicle is driving straight (as shown on the left, the left and right steering oil channels are not flowing, the steering cylinder is in the middle), the steering wheel will float, or the car will be difficult to maintain the forward direction, always going to one side. Figure 2 Schematic diagram of the operation of each part when the forklift is running straight The cause of the failure: 1. The positioning spring piece between the valve core and the valve sleeve of the steering valve in the steering gear is damaged or too soft, and it is difficult to overcome the resistance when the steering gear is reversely driven, so that the valve sleeve cannot automatically return to the neutral position. 2. The movement of the spool is blocked due to the contamination of the hydraulic oil. 3. The steering axle cylinder is not in the middle position because the forklift is not adjusted in the middle position, or the spool is not in the middle position due to the positional deviation of the spool and the valve body step. 4. The flow control valve is stuck so that the hydraulic pump flow is too large, the hydraulic pressure piping is unreasonable, or the two-way overload valve or the two-way charge valve has one side leakage, and the throttle loss of the hydraulic system is too large, so that the hydraulic cylinder left cavity or The right chamber pressure is too large [7]. 5. Other causes of this failure may be loose steering connection, air in the oil, improper positioning of the front wheels, differences between the tires [2] or uneven adjustment of the brakes of the two front wheels. Diagnosis and elimination: First check the hydraulic oil for dirt. If the new car or the overhauled vehicle does not carefully perform the oil change regulations for maintenance, the oil is often dirty. For vehicles that are used for a long time, it may be caused by the failure of the flow control valve or the relief valve reaction spring. The steering wheel can be turned without starting the engine, and the feeling of the hand can be used to judge whether the spool is open or not. If the slide valve is not open, it should be dismantled. If the spool of the steering cylinder is not in the middle position, you need to reset the center of the forklift to keep the forklift tires in front. If the connection is loose or the positioning of the tire and the pressure of the model are caused by the difference, the steering connection mechanism needs to be tightened, and each tire type should be inspected or the front and rear tires should be repositioned. If the deviation is caused by uneven braking, the brake should be corrected. Vehicle snake Symptom: When the steering wheel of the vehicle is hit or not moving, the vehicle moves forward along the serpentine shape (S shape). The cause of the failure: 1. There is air in the oil, the pump suction pipe and the joint leaks and cavitation occurs; 2. The oil suction pipe partially shrinks or bends to produce poor oil flow; 3. The fuel tank liquid level is low; 4. The cylinder pin The looseness of the child or the bending of the rod; 5. Leakage in the cylinder or leakage of the two-way charge valve of the two-way overload valve. Diagnosis and elimination: This phenomenon is mostly caused by leakage of the oil system into the air. Refer to the above symptoms for diagnosis. If the joints or hydraulic equipment leak, seal the remedy or replace the equipment. If the fuel tank is short of oil, it should be refueled to a reasonable height. If the tubing is partially contracted or bent, the tubing trend should be re-planned or replaced. If the hydraulic components are loose or damaged, they should be tightened or replaced to eliminate the malfunction and keep the vehicle straight. Left and right steering Fault phenomenon: The degree of resistance is different when the vehicle is turning left and right. Cause of failure: 1. The spool valve of the distribution valve deviates from the intermediate position, or although it is in the middle position but does not match the gap size of the valve body shoulder. 2. The internal friction of the steering bridge or the distribution valve slide valve makes the left and right movement resistance different. 3. The adjustment nut of the adjustable distribution valve is not properly adjusted. 4. The steering tubing leaks or the seal at the joint is damaged. 5. The large and hard contaminated particles enter the center of the stator and rotor or the serious damage of the steering gear caused by severe grinding, pin breakage and thermal shock. Diagnosis and elimination: This type of failure is caused by dirt and oil, and should be replaced with new oil [8]. If the oil is not dirty, re-adjust the adjustable dispensing valve or disassemble the dispensing valve to check that the spool position is centered. If the tubing leaks or the seal is damaged, it should be repaired or replaced. If the steering components are damaged, replace the damaged components. Similarly, when the vehicle is inaccurately turned, it can also be solved by referring to the above fault analysis. Only one side steering Fault phenomenon: Only one direction is easy to use when turning, and the other direction is not. The cause of the failure: only one direction of steering, indicating that the forklift's steering control valve is stuck when moving [6]. Analysis and diagnosis: For this fault, the hydraulic steering cylinder (integral cylinder and valve) should be removed for cleaning, so that the valve can be moved without jamming.
Fault Analysis of Hydraulic Power Steering System of Forklift
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