The vacuum pressurized hydraulic braking system of forklift is mainly composed of brake pedal, brake master cylinder, vacuum booster, brake wheel cylinder, wheel brake and pipeline. As shown in the attached figure. Its working principle is as follows: the braking force acts on the brake master cylinder through the pedal and linkage mechanism to generate a fluid flow with a certain pressure at its outlet; After the fluid flow enters the vacuum supercharger, the pressure increases further and is finally transmitted to the brake wheel cylinder through the pipeline; The brake wheel cylinder generates thrust to open the brake shoe to both sides, and the shoe is close to the brake hub to generate braking force.
The forklift braking system often has four kinds of faults: braking failure, poor braking effect, braking deviation and braking drag. The diagnosis methods are described as follows.
1. Brake failure
It shows that the forklift cannot slow down or stop when braking. The diagnosis and troubleshooting methods are as follows:
If the brake pedal is pressed several times continuously, and the pedal does not rise and there is no resistance, first check whether the brake master cylinder lacks brake fluid. In case of lack, add brake fluid of the same model and remove the air from the pipeline; If there is no shortage, check whether each brake pipeline has leakage or damage, and repair or replace it as appropriate.
Press the brake pedal. If there is no movement, the connection between the pedal and the brake master cylinder may be disconnected. Check the connection mechanism and connect it properly.
If you step on the brake pedal, although you feel a certain resistance, the pedal position cannot be maintained, it sinks obviously, and the master cylinder leaks or sprays liquid, which is mostly due to the rupture of the seal of the brake master cylinder. Disassemble the brake master cylinder and replace the master cylinder seal.
If the above inspections are normal, the master cylinder seal may be out of position. At this time, disassemble the brake master cylinder and replace the master cylinder seal of the same model.
2. Poor braking effect
There are three phenomena of this fault: first, when the foot brake pedal is pressed several times, the forklift cannot slow down and stop; Second, the pedal height is normal when stepping, but I feel weak and can't stop immediately; Third, when stepping on the pedal, I feel that the pedal is high, hard or even rebound, but the braking effect is poor. The troubleshooting methods are as follows:
If the brake pedal is pressed several times continuously, the pedal will rise gradually, and the pedal will continue to step down without raising the foot after rising. You can feel the elasticity of the pedal. You can release the pedal and step it again for a while. If it is still the case, there is air in the brake system and it should be vented.
The air shall be discharged from the brake master cylinder to each brake slave cylinder. The air discharge sequence of each sub cylinder shall start from the sub cylinder nearest to the brake master cylinder and discharge one by one. Exhaust method: one person is responsible for stepping on the brake pedal and the other person is responsible for exhaust. First step the brake pedal to the bottom without loosening, and then loosen the exhaust screw. After the brake fluid mixed with air is ejected and the pressure drops, tighten the exhaust screw and loosen the brake pedal. After the brake pedal is fully returned, press the brake pedal to the bottom again and continue bleeding. Repeat this several times to completely exhaust the air.
If the brake fails to work when one foot of the brake pedal is pressed, the pedal position increases gradually after several steps of the brake pedal, and the braking effect is good, indicating that the free stroke of the pedal is too large or the clearance between the friction plate and the brake hub is too large. At this time, check and adjust the free stroke of the pedal to make it within the specified range, and then check and adjust the clearance between the friction plate and the brake hub.
If you continuously press the brake pedal, the pedal position can gradually rise. If you don't raise your foot and continue to press down, you don't feel the elasticity of the pedal, but you feel sinking, indicating that there is oil leakage in the brake system or the oil outlet valve of the brake master cylinder is not tightly closed. At this time, check whether there is liquid leakage at the pipe joint. If there is leakage, fasten, weld or replace it.
When stepping on the pedal, if the pedal position is very low, step on the pedal several times, and the position still cannot rise. Generally, the vent hole of the brake master cylinder or the brake fluid compensation hole is blocked, so it should be checked and dredged. If the pedal height meets the requirements, it is not weak and sinking when stepping, but the braking effect is poor, it is a wheel brake fault, and the wheel brake shall be disassembled for maintenance.
If the brake pedal is high, hard or bounces against the foot when using foot brake, the vacuum booster may be faulty. First check whether each vacuum pipe is intact and whether the connector is reliably connected, and then check the vacuum booster. During inspection, step on the brake pedal first, then start the engine and idle for a few seconds. If you feel that the brake pedal drops a little, it indicates that the vacuum booster is in good condition; If the brake pedal does not lower by itself, it indicates that the vacuum booster is not working well, and the vacuum booster should be replaced.
3. Brake deviation
The fault is that the wheels on both sides cannot act at the same time or the braking force is different, which makes the forklift deviate to one side when braking. The troubleshooting methods are as follows:
During the road test, if the forklift deviates to the right when braking, it indicates that the braking of the left wheel is slow or the braking force is insufficient; On the contrary, it means that the braking of the right wheel is slow or the braking force is insufficient.
If it is determined that the braking of a wheel is slow or the braking force is insufficient, check whether the brake pipeline of the wheel is flat or leaking. If so, find out the cause and eliminate it. If the wheel pipeline is intact, exhaust the wheel cylinder. If the brake deviation is eliminated after exhaust, it indicates that there is air resistance in the wheel cylinder or pipeline. If there is no air resistance, check and adjust the clearance between the wheel friction plate and the brake hub.
If there is no problem in the above inspection, it indicates that the fault is inside the wheel brake. Disassemble and inspect the wheel brake, check the wear of the brake pad and the operation of the brake wheel cylinder, and repair or replace it if necessary.
4. Brake drag
This fault shows that after lifting the brake pedal, all or individual wheel brakes cannot be released immediately and completely, so as to affect the restart or driving of the forklift. The troubleshooting methods are as follows:
After the forklift works for a period of time, touch the brake hub of each wheel with your hand. If all wheel brake hubs are heated, the fault occurs in the brake master cylinder; If individual wheels are hot, it indicates that the fault is in the wheel brake.
If it is judged that the fault location is the master cylinder, first check the free stroke of the pedal. If the free stroke does not meet the requirements, the free stroke of the pedal shall be adjusted according to the regulations; If the free stroke meets the requirements, open the oil storage chamber cover of the brake master cylinder and observe the oil return of the master cylinder. If the oil cannot be returned or the oil return is slow, disassemble the master cylinder for maintenance and replace it if necessary.
If the fault lies in the wheel brake, loosen the exhaust screw first. If the brake fluid ejects rapidly and the brake shoes return, the pipeline is blocked and the pipeline should be dredged; If the brake shoes cannot return, adjust the clearance between the brake pads and the brake hub.
If the above inspection and adjustment have no effect, remove the brake hub, check the condition of the cylinder piston seal, return spring and brake shoe pin, and repair and replace it if necessary.
