Common faults and causes of the braking system in wheel loaders

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This post was last edited by dongsaili on 2014-1-13 at 15:14. The main problems with the braking system of wheel loaders are reduced braking performance, uneven braking efficiency among the wheels, and in severe cases, complete loss of braking function. According to Engineer Chen from the construction machinery recruitment team, the specific faults include: failed or ineffective braking, mechanical deviation when braking, braking drag, or abnormal noises from the brakes. 1. Poor or failed braking: Poor or failed braking refers to a situation where, when braking is applied under normal operating conditions, the braking torque is insufficient; as a result, the vehicle slows down slowly and cannot stop immediately. The degree of deceleration is low, the braking distance is long, and the braking effect is very poor. Even when the brake pedal is pressed all the way down once or several times in a row, the braking system still fails suddenly. The main reasons for poor performance or failure of pneumatic braking are as follows: (1) The air compressor does not function properly, resulting in low pressure or no air in the air reservoir. The cause of this fault may be a loose or broken belt on the air compressor, a leak in the air compressor’s exhaust valve, a soft or broken spring in the exhaust valve, or a leak in the piston or piston rings. (2) The pressure value of the pressure control valve is not adjusted properly, resulting in insufficient compressed air supply and reduced braking force. (3) A ruptured pipeline or loose connection results in insufficient working air pressure, thereby reducing the braking torque. (4) The brake valve diaphragm or the brake chamber diaphragm is damaged. (5) The free travel of the brake pedal is too large. (6) The gap between the friction pad and the brake drum is too large, or the friction pad is contaminated with oil. Start the engine and check the air pressure gauge on the dashboard in the cab. If the gauge reads “0”, press the brake pedal; if there is a hissing sound when releasing it, it indicates that the pressure gauge is faulty and needs to be replaced. If there is no air release sound, check the air compressor belt and the air pipe from the air compressor to the air tank. After the above checks, the condition is good. If the gauge reading is very low, the fault lies with the air compressor; it is necessary to check the condition of the exhaust valve or the interior of the cylinder and carry out repairs as needed. If the pressure reading indicated by the barometer is within the standard range, press the pedal to check for any air leaks from the brake valve to each wheel. If there is no air leakage, check the free travel of the pedal and adjust the gap between the brake shoe friction pad and the brake drum. The main reasons for poor performance or failure of hydraulic braking are as follows: (1) Excessive free travel of the pedal; (2) Insufficient brake fluid in the pump, or blocked compensation holes, as well as aging and deformation of the pump’s seals and gaskets, or these components being damaged. (3) Severe wear of the brake master cylinder piston and cylinder body leads to oil leakage; the return valve does not seal properly, and the spring of the outlet valve is broken. (4) The rubber seals of the brake caliper have aged and swelled; the piston is stuck, and severe wear between the caliper piston and the cylinder body causes oil leakage. (5) The brake shoes are severely worn, the brake clearance is too large, or the clearance setting is incorrect. (6) The brake drum is deformed, grooved, or worn thin; the surface of the brake shoes is oily and hardened due to ablation. The rivet is exposed. (7) Air enters the hydraulic system, or the temperature of the braking system is too high, causing the brake fluid in the pipes to vaporize and creating an air blockage. (8) Dented pipes, leaking joints, and aged, cracked, or clogged brake hoses. When hydraulic braking fails or performs poorly, it is necessary to check whether the gap between the friction pads and the drum of each wheel is too small or reversed (the gap on the drive side of the brake pad is smaller than that on the other side). If the gap is normal, then the condition of the drum wall and the surface of the friction pads needs to be checked. If everything is normal, check whether the brake shoe springs are too stiff, whether the rubber seals of the master cylinder or slave cylinders are swollen, and whether the fit between the cylinder walls and the pistons is loose. If everything is normal, then it is necessary to further check whether the brake hoses are blocked due to aging or similar issues. 2. Mechanical drift during braking: During normal service braking, when the steering wheel remains stationary, any mechanical tendency for the vehicle to drift to one side or slide sideways is referred to as braking drift. The deviation of a wheel loader when braking is caused by factors such as uneven braking torque between the left and right wheels. The main reasons for this are as follows: (1) The braking times of the brakes on the left and right sides of the coaxial system are not consistent. This is usually caused by uneven braking clearances or different contact areas between the brakes on the two sides. During braking, the friction pads on one side come into contact with the brake drum first, while on the other side, due to the larger clearance, the friction pads make contact with the brake drum later, resulting in asynchronous braking on the left and right sides. After this fault occurs, the brake clearances of the left and right wheels can be readjusted according to the specifications. (2) The braking torques of the brakes on both sides of the coaxial shaft are different, resulting in varying gear shifts of the vehicle and thus causing braking deviation. This is caused by oil leakage from a brake caliper, severe oil contamination on the brake friction pads, differences in the coefficient of friction, or unequal tire pressures on the left and right sides. The friction pads can be cleaned with gasoline, the tire pressure at high speeds should be checked, leaks should be repaired, and these issues should be addressed separately. (3) When the brake pedal is not pressed, the walking machine automatically slides to one side. This is mostly caused by deformation of the front suspension on one side, deformation of the chassis floor of the front suspension, a significant decrease in the elasticity of the coil springs in the front suspension, as well as interference or misalignment between various components such as the frame when the vehicle is braked. In such cases, the cause is identified and then repaired. (4) The wheels turn automatically to one side and drift during braking. This is mainly caused by differences in the roughness of the working surfaces of the brake drums and friction pads on either side, or by blockages in the brake line connections on one side. The root causes should be identified separately and fixed. (5) Uneven tire pressure on the left and right sides causes deviation. The tire pressure on the left and right sides must be equal; otherwise, the actual rotational radius of the wheels on each side will differ, resulting in unequal distances traveled along a straight line and thus skidding. The tires must be inflated to the specified standards. In addition to the reasons mentioned above, inaccurate wheel alignment and uneven wear between the left and right tires can also cause deviation and skidding, as the difference in resistance exerted by the road surface on the left and right wheels plays a role in this. In such cases, once the cause is identified, the components should be adjusted or replaced in accordance with the relevant specifications. 3. Braking drag: Braking drag refers to the difficulty in starting the vehicle again after applying mechanical brakes, or the engine shutting down during startup; it may also require increasing the throttle to start the vehicle, and once it does start, the vehicle’s speed cannot increase easily and acceleration is difficult. The fundamental cause of braking drag is the incomplete disengagement of the brakes after they are released. The specific reasons for this include excessive brake clearance and too little free travel of the pedal; when the brake pedal is released, the braking force is not fully removed, causing the friction pairs to remain in a friction state for an extended period of time ； The return spring of the brake shoe deforms due to heat, loses sufficient elasticity or becomes ineffective, which prevents the brake friction pad assembly from returning to its original position in a timely manner, resulting in incomplete release of the brake ； Blockages in the exhaust port or filter of the brake valve, broken return springs of the valve spool, etc., can result in poor exhaust during braking ； The brake valve or quick-release valve is not functioning properly, resulting in slow exhaust or incomplete exhaust ； In the air-boosted oil braking system, the hydraulic system retains pressure due to blocked pipelines, and the automatic return mechanism for the brake caliper piston fails as a result of broken components or loose fitting of the shafts. Furthermore, deformation of the disc brake friction pads, or jamming of the spline teeth on the fixed or rotating disc, can also cause incomplete release of the brakes. 4. Abnormal noises from the brakes: A harsh noise is generated by the braking system during braking, mainly due to the out-of-round shape of the brake drums, an insufficient friction contact area, and excessive gaps in the middle section. The common reasons are as follows: (1) The brake drum is out of round, causing collisions to occur at the moment the brake pads make contact with the drum, which results in a sharp knocking sound. During maintenance, the brake drum is removed, bored in accordance with specified standards, and its balance must be checked, with the imbalance amount kept within 200 g/cm. (2) When the surface of the brake friction pad is too smooth, resulting in a low friction coefficient, and the braking pressure is high, friction noise is generated as the smooth surface slides against each other. Or, foreign objects inserted between the friction pairs compress the friction surfaces, which can also result in friction noise. During maintenance, remove the brake drum, clear out any foreign objects, and sand the friction pads with coarse sandpaper so that the contact area between them and the friction surfaces is over 70%. (3) The brake friction pads are severely worn, with grooves and irregular shapes on their surface; as a result, they cannot make full and effective contact with the brake drum during braking. Alternatively, the brake support plates may be deformed, disrupting the coaxial alignment between the drum and the pads, which leads to localized friction and collisions and thus noise. During maintenance, the friction pads should be replaced and the brake support plate adjusted. Furthermore, when the brake pedal is released, the braking force is not completely removed, which causes the friction pair to remain in a friction state for an extended period. Additionally, factors such as the return spring becoming deformed due to heat, its elasticity decreasing or disappearing, and the inability to ensure that the brake pad assembly returns to its original position in a timely manner can cause the brake drum to heat up. Once the brake drum heats up, it is necessary to carefully investigate the cause and address it.