Created on:2021-08-25 13:43

Fault diagnosis and maintenance of forklift hydraulic_ Four faults of forklift

Fault diagnosis and maintenance of forklift hydraulic_ Four faults of forklift

(1) After the forklift lifts the goods, the fork automatically slides slowly

According to the working principle of the hydraulic system, the reasons for the slow decline of goods are as follows: first, the valve body and valve rod of the lifting slide valve are worn, and the matching clearance becomes larger. The pressure oil under the action of gravity flows back to the oil tank through the clearance of the slide valve, resulting in insufficient oil pressure in the lower chamber of the lifting cylinder and slow lowering of the fork; Second, the sealing of the piston of the lifting cylinder becomes poor, and the high-pressure oil leaks into the upper cavity of the lifting cylinder and is connected with the low-pressure oil, causing the fork to slide slowly.

The method to diagnose the cause of the fault is: remove the end connecting the oil return pipe on the upper part of the lifting cylinder with the oil tank, operate the lifting valve rod to lift the goods, and observe whether there is oil flowing out of the oil return pipe during the lifting process. If there is oil flowing out, it indicates that the piston sealing ring is poorly sealed; If no oil flows out, the fault is caused by the lifting directional valve.

Troubleshooting: disassemble the lifting cylinder and check whether the sealing ring is excessively worn, aged, scratched, lip turned up, etc. if so, replace it; If the cylinder wall or piston rod surface is slightly scratched, pitted, pitted or rusted, it can be gently ground manually with fine sandpaper or oilstone and lubricating oil; If the damage is serious, replace it with a new one. The repair of lifting directional valve shall depend on the wear of valve body and valve rod. If the mating surface is seriously worn, it shall be replaced with a new one; If possible, chrome plating, fine grinding, reaming or polishing can be carried out on the valve stem or valve hole to make the matching clearance reach the specified value.

(2) When the gantry and goods are in the backward tilt position, the gantry will automatically tilt forward slowly

The causes of this fault are similar to the former: one possible reason is that the valve body and valve stem of the inclined slide valve are worn, and the matching clearance is out of tolerance, resulting in slight pressure relief of the system; Another possible cause is the poor performance of the piston sealing ring of the tilt cylinder.

Common methods to judge the fault location: remove the oil pipe connecting the rod free cavity of the tilt cylinder and drain the hydraulic oil in the pipe; Operate the handle to keep the gantry and goods tilted back, and carefully observe whether there is oil flowing out of the removed pipe orifice; If there is a small amount of oil flowing out of the pipe orifice, it indicates that the piston sealing ring of the inclined cylinder is poorly sealed, and the pressure oil continuously flows from the rod cavity into the rod cavity; If there is no oil flowing out, the pipe orifice can be blocked by hand. If it is felt that there is internal suction (the piston and piston rod move forward under the push of the gravity of the goods, and there will be negative pressure in the rod free cavity of the tilt cylinder), it indicates that the fault is caused by the tilt slide valve.

Troubleshooting: the same as the previous method, if the piston sealing ring of tilt cylinder leaks, replace it; The tilt directional valve shall be replaced or repaired according to the wear.

(3) When pushing the lifting slide valve, the goods and goods fall too slowly

It can be seen from the working principle of the hydraulic system that the falling of the goods and the fork frame is to make the oil under the piston flow out through the shut-off valve at the bottom of the lifting cylinder under the action of its self weight, and then flow back to the oil tank through the reversing valve after the speed is controlled by the speed limiting valve. The lowering speed of fork and goods is required to be 300 ~ 600mm / s. The spring pressure of speed limiting valve is determined according to the design and generally does not need to be adjusted; The falling speed of the fork is too slow, which is mostly caused by the jamming of the slide valve of the shut-off valve. Therefore, the fault can be eliminated after cleaning and repairing the shut-off valve.

It should be noted that for simplicity, some repairmen usually remove the shut-off valve to reduce the flow resistance of oil. In fact, this is a wrong practice.

The function of the shut-off valve is to prevent the goods from falling rapidly if the high-pressure rubber hose suddenly breaks after the goods are lifted to a certain height. Figure m shows that when the oil from the lifting cylinder passes through the cut-off spool valve, the oil hole around the spool valve causes a pressure difference between the two chambers. When the pressure difference is less than the spring force, the spool valve does not act; If the high-pressure rubber hose breaks unexpectedly, a large pressure difference will be formed, which will make the slide valve move and block the oil hole around it. Only a small amount of oil is allowed to flow through the small hole at the end of the slide valve to make the fork drop slowly.

(4) Weak or unable to lift

There are many reasons for the inability of forklift to lift. In addition to the factors of insufficient engine power, leakage of hydraulic system pipeline and deterioration of hydraulic oil, there may be three reasons for the failure: insufficient pressure of safety valve in multi-way directional valve group, failure of gear pump or high temperature of hydraulic oil.

The insufficient pressure of the safety valve in the multi-way directional valve group is mostly caused by the weakening of the spring force in the safety valve, which reduces the system pressure and causes the pressure oil to flow back to the oil tank directly through the safety valve. The treatment method is to adjust the spring pressure or replace the safety valve; When used for a long time, the clearance between the inner wall of the pump body and the tooth top circle becomes larger, the gear end face is worn, the pump shell is deformed, etc., which will reduce the efficiency of the gear pump, and the pump should be replaced; If air enters the system, a large number of bubbles will be generated when the hydraulic oil moves rapidly, raising the temperature of the hydraulic oil. The solution is to add hydraulic oil and tighten the loose pipeline joints of the hydraulic system.

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