Large agricultural machinery, such as high-powered tractors and combine harvesters, use hydraulic steering systems. The fully hydraulic steering gear is an important component of the hydraulic steering system. It is manufactured with precision and has a complex structure. If maintenance personnel do not understand its working principle and operating requirements, drive recklessly, and blindly disassemble and replace parts, it often leads to frequent locomotive failures, decreased quality of use, increased maintenance costs, and extended maintenance time, seriously affecting the economic benefits of agricultural machinery owners. Therefore, it is required that we understand the structural principle of the fully hydraulic steering gear and pay extra attention to its use and maintenance. Only in this way can we reduce faults during operation and extend the service life of the steering gear.

1. Structural principle of Full Hydraulic Steering Systems

A steering gear is composed of a set of gear mechanisms that complete rotational motion to linear motion (or approximately linear motion), and is also a reduction transmission device in the steering system. It is an important component in the steering system. The operation of a fully hydraulic steering gear is fully hydraulic, which means there is no mechanical connection between the steering column and the steering wheel, and there is a hydraulic pipe or hose connection between the steering gear and the steering cylinder. When the steering wheel is turned, the steering gear delivers a relative amount of oil according to the proportion of the steering wheel rotation. This oil directly flows to the corresponding side of the control cylinder, while the oil on the other side returns to the fuel tank. Using the BZZI-80 steering gear as an example, explain the structural principle of a fully hydraulic steering gear. This steering gear is placed under the driver's cab and connected to the steering column of the steering wheel assembly.

(1) The structure of BZZI-80 type fully hydraulic steering gear is mainly composed of a follower rotary valve and a pair of cycloidal gear meshing pairs, which is a cycloidal rotary valve type fully hydraulic steering gear. The four oil ports of the steering gear are respectively connected to the single channel stable diversion valve, the left and right chamber oil ports of the steering hydraulic cylinder, and the return oil port of the hydraulic tank.

(2) The working principle of BZZI-80 fully hydraulic steering gear is that the valve core and valve sleeve rotate with the rotary valve in the valve body, playing a role in controlling the direction of oil flow. The rotor and stator form a cycloidal gear meshing pair, which acts as a metering hydraulic motor during power steering to ensure that the oil flow into the steering hydraulic cylinder is proportional to the steering wheel angle. After the engine is turned off, it can also function as an oil pump during manual steering, and the linkage shaft plays a role in transmitting torque.

In the middle position (i.e. when the steering wheel is not rotating), the oil from the one-way stable diversion valve flows through the inner cavity of the valve core and through another path to the one-way stable diversion valve. Along the way, it is turned back into oil through a three-way valve (flowing in the direction of low pressure). When turning power, the oil from the hydraulic pump enters the cycloidal gear meshing pair through the follower valve, pushing the rotor to rotate with the steering wheel and injecting a certain amount of oil pressure into the left or right chamber of the hydraulic cylinder, pushing the guide wheel to achieve power steering. The oil from the other chamber of the hydraulic cylinder flows back to the hydraulic tank.

After the engine is turned off, the steering wheel is operated with great force. Through the valve core pin, the rotor is linked to drive the oil pressure from one chamber of the steering hydraulic cylinder into the other chamber, pushing the guide wheel to achieve manual steering. The volume difference between the two chambers of the hydraulic cylinder can be supplied by the oil tank through the return port.

2. Precautions for disassembly and installation of Full Hydraulic Steering Systems

Several issues to be noted when installing a fully hydraulic steering gear:

(1) Do not disassemble the fully hydraulic steering gear at will, otherwise it may cause damage to the mating surface; When disassembly is necessary, be sure to pay attention to cleanliness and avoid scratching the mating surface.

(2) When disassembling, first remove the steel ball of the manual steering check valve. Then remove the valve sleeve and valve core. To prevent the steel ball from falling between the oil inlet hole and the valve ring groove, which may damage the surface of the valve sleeve. When assembling, follow the reverse order.

(3) All interior components must not be damaged or have residual oil stains and paint, and must be cleaned thoroughly. During assembly, do not allow dirt to enter the valve and apply clean new engine oil to the surface of the parts. When assembling the valve sleeve and valve core, they should be aligned smoothly and not collide with each other to avoid damaging the parts.

(4) Rubber seals must meet quality requirements and must not have burrs, burrs, or compression defects. It is strictly prohibited to use gasoline and kerosene during cleaning. It can be cleaned with warm soapy water with low alkalinity.

(5) When the nut at the one-way valve (steel ball) is screwed into the valve body, it should be below the valve plane.

(6) The installation seat hole should be coaxial with the steering shaft, and there should be clearance during steering to prevent the valve core from being stuck. Generally, after installation, the steering wheel should be checked for flexible return.

(7) When installing the linkage, attention should be paid to the correct assembly relationship between the linkage and the rotor. Note that both the rotor and the linkage shaft end face are marked with "dots", and the two points should be opposite during assembly. If installed incorrectly, it may cause the steering wheel to rotate and cause accidents. When there is no mark, the center line of the pin groove at the upper end of the linkage should be aligned with the center line of the tooth groove of the rotor.

(8) The pipeline interfaces of the valve are marked with the words "left", "right", "in", and "back" respectively. Some valves are replaced with the words "A", "B", "P", and "0". When installing pipelines, the "left" and "right" interfaces should be connected to the "left" and "right" chamber pipelines leading to the steering hydraulic cylinder, respectively. The "inlet" oil port should be connected to the constant oil port pipeline from the single stable diversion valve output, and the "return" oil port should be connected to the return oil pipe leading to the hydraulic tank.

(9) When installing the valve, 50-100mL of hydraulic oil should be added to the oil port, and the valve core should be tested left and right. If there are no abnormalities, the machine can be installed.

3. Precautions for use

(1) During the steering process, when the steering feels heavy, a "hissing" sound can also be heard when the safety valve opens. The main reasons are insufficient oil supply or air mixed in the steering system oil circuit, or the safety valve spring of the one-way stable diversion valve is lower than the working pressure. At this time, do not continue to rotate to avoid damaging the shift pin or spring plate.

(2) If steering failure is found during use, the main cause is the breakage or deformation of the spring plate, shift pin, or linkage shaft, or the incorrect installation of the rotor and linkage shaft. At this time, the cause should be carefully investigated, and the steering wheel should not be forcefully turned, nor should it be easily disassembled to prevent damage to the steering gear. If the steering cannot return to the center position and the cylinder stroke is not proportional to the steering wheel, it is mainly due to the failure of the spring plate. After replacing the spring plate, the fault can be eliminated. If the steering cylinder does not operate, it is mainly due to leakage of the FK overload valve or internal leakage of the cylinder. Replacing the FK combination valve or troubleshooting whether the cylinder has internal leakage can solve the problem. The pressure oscillation increases and even cannot turn, mainly due to the bending or breakage of the shift pin, and the fracture of the linkage shaft pin groove. It is necessary to replace the shift pin or linkage shaft. During driving, there is a deviation and the cylinder response is slow when turning the steering wheel. This is mainly due to leakage inside the cylinder, the bidirectional overload valve in the valve block being blocked by dirt, and the overload valve spring in the valve block failing. The cylinder or overload valve needs to be replaced.

(3) The steering wheel cannot automatically return to the center position. When the pressure drop in the neutral position increases and the steering wheel stops rotating, the steering gear does not unload, the steering column is not concentric with the valve core, the steering column axially presses against the valve core, the steering column has too much steering resistance, and the spring plate breaks.

(4) The left and right steering feel is different. This is because the slide valve of the distribution valve deviates from the middle position, or although it is in the middle position, the gap size with the valve body shoulder is not consistent; There is dirt blocking inside the slide valve, causing different resistance when moving left and right; Improper adjustment of the distribution valve adjustment nut. This type of malfunction is usually caused by dirty oil and can be replaced with new oil. If it is confirmed that the oil quality is good, the adjustable distribution valve should be readjusted or the distribution valve should be disassembled to check whether the slide valve is centered and whether there are burrs at the shoulder. If necessary, corresponding adjustments should be made.

(5) After the engine is turned off or the oil pump is not working, do not use too much force to turn the steering wheel, and turn it slowly to avoid damaging the shift pin, spring plate, or linkage.

(6) Hydraulic oil has a significant impact on the performance and lifespan of a fully hydraulic steering gear, so it is essential to ensure the cleanliness of the oil. Regularly clean the hydraulic system, and filter the added hydraulic oil before purifying it. The fuel tank should be sealed, and the ventilation holes should be cleared and cleaned according to the prescribed time; Regularly fill the fuel tank with oil to prevent rusting in areas where there is no oil on the tank wall.

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