I. Introduction 
The bridge crane is analyzed from 7 aspects that can cause mechanical failure, such as wire rope, drum and wire rope pressure plate, hook, reducer gear, brake, wheel and rail and safety accessories, and proposes measures to prevent mechanical failure of the crane. and recommendations. 
While overhead cranes bring high efficiency, convenience and speed in the production process of enterprises, accidents frequently occur due to unsafe factors of machinery, causing economic losses to the country and the people, and causing pain to the parties and their families. The main reason for the accident is the mechanical failure and misoperation of the machine itself. Studying mechanical failures, analyzing the causes, and formulating preventive measures are the main measures to reduce mechanical accidents of overhead cranes. This requires special equipment managers to pay attention to the hidden dangers of mechanical failures of cranes while standardizing the operation of operators, and formulate detailed plans according to crane conditions. Feasible precautions to ensure the safe operation of the crane. Through production practice, the mechanical failures and preventive measures of overhead cranes during operation are analyzed as follows. 

2. Steel wire rope
1 Failure analysis
During the operation of the steel wire rope, the stress of each steel wire rope is very complicated. Due to the different positions of the steel wires in the rope, some are in the outer layer and some are in the inner layer. Even if it is subjected to the simplest tensile force, the force distribution between each wire rope is different. In addition, the wire rope will generate bending stress when it goes around the spool, the pulley, and the squeezing force between the wire and the wire, so the force is accurately calculated. It is more difficult, and the static calculation method is generally used. 
The maximum static tension in the wire rope should meet the requirements of the following formula:
Pmax≤Pd/n
In the formula: Pmax——the maximum static stress that the wire rope can bear during operation;
Pd——breaking stress of wire rope;
n is the safety factor. 
Pmax=(Q+q)/(aη)
In the formula: Q——the rated lifting capacity of the crane;
q——the weight of the hook group;
a——the total number of rope branches carried by the pulley block;
η——the total efficiency of the pulley block. 
The calculation formula of the maximum allowable working tension of the wire rope is:
P=Pd/n
In the formula: P——Rated maximum static stress during wire rope operation
P≥Pmax is safe. It can be seen from this that the main reason for the wire rope breaking is overloading, and it is also related to the number of times of threading and winding on the pulley and drum. It is easy to be damaged and broken; secondly, the breaking of the wire rope is related to the bypass pulley, the diameter of the drum, the working environment, the type of work, and the maintenance. 
2 Preventive measures
2.1 The lifting capacity of the crane should not exceed the rated lifting capacity during operation. 
2.2 The wire rope of the crane should be selected according to the type of work and the environment. 
2.3 The wire rope should be lubricated regularly (determine the lubricating cycle according to the working environment). 
2.4 Do not subject the wire rope to sudden impact force during the operation of the crane. 
2.5 The wire rope in the environment of high temperature and corrosive medium must have isolation device. 

3. Reel and wire rope pressure plate
The drum is an important force-bearing component of the crane. During the use process, there will be thinning of the drum wall, holes and fractures. The reason for these failures is that the drum and the wire rope are in contact with each other and rubbing against each other. When the drum is thinned to a certain extent, it will break because it cannot bear the pressure exerted by the wire rope. In order to prevent the occurrence of mechanical accidents such as the reel, according to the national standard, the reel wall should be replaced in time when the wear reaches 20% of the original or when cracks appear. At the same time, pay attention to the sanitation of the operating environment and the lubrication of the drum and wire rope. 

4. Hook
The hook is the most commonly used fetching device for overhead cranes. It bears the entire load of the hoisting and transportation. In the process of use, once the hook is damaged and broken, it is easy to cause major accidents. The reason for the damage and fracture of the hook is that the hook is cracked, deformed, damaged and fractured due to friction and overloading. In order to prevent the hook from malfunctioning, it is necessary to strictly prohibit overloading during use. During the inspection process, pay attention to the opening of the hook and the wear of the dangerous section. At the same time, the hook should be annealed regularly. If the crack is found, it should be scrapped according to GB10051-88, and the hook should not be welded and repaired. The inspection of the hook by the special equipment manager shall determine whether the hook can be used according to the requirements of GB10051-88. 

Five, reducer gear
1 Failure analysis
The reducer is an important transmission component of the bridge crane. It transmits torque through gear meshing, and adjusts the high-speed operation of the motor to the required speed. During the transmission of torque, the gears will have broken teeth, pitting on the tooth surface, and glue on the tooth surface. Mechanical faults such as wear and tooth surface wear, etc., the reasons for the failure of the gear are as follows:
a. Fatigue fracture caused by short-term overload or impact load and repeated bending;
b. The tooth surface is not smooth, there are raised points to cause stress concentration, or the lubricant is not clean;
c. Lubrication failure due to excessive temperature;
d. Wear due to the entry of hard particles into the friction surfaces. 
2 Preventive measures
a. The crane cannot be loaded and used, and the starting and braking should be slow and stable. It is forbidden to suddenly reverse the car under non-specific circumstances;
b. It is necessary to replace the lubricant in time, clean the shell, and choose the appropriate type of lubricant;
c. Always check whether the lubricating oil is clean; if it is found that the lubricating oil is not clean, it should be replaced in time. 

6. Brake
1 Failure analysis
The brake is an important safety component of the bridge crane. It has the functions of preventing the falling of the suspended object and realizing the parking. Only the intact brake can guarantee the accuracy of the crane operation and safe production. During the crane operation, the brake will have insufficient braking force, The brake suddenly fails, the temperature of the brake wheel is too high, the brake pad smokes, and the brake arm cannot be opened. The reasons for these mechanical failures are analyzed as follows:
a. Excessive wear of the brake band or brake wheel; partial detachment of the brake band; loose adjustment of the main spring; grease between the brake band and the brake wheel; stuck or worn outside the living hinge Excessive parts; the lock nut is loose and the whole tie rod is loose; the impeller of the hydraulic push rod brake releaser is inflexible to rotate;
b. Serious brake pads or large pieces fall off, or the long-stroke electromagnet is stuck, the main spring fails, or the main components of the brake are damaged;
c. The gap between the brake and the gasket is adjusted too large or too small;
d. There is a stuck place in the hinge or the braking torque is adjusted too large, or the oil cylinder of the hydraulic push rod brake releaser lacks oil and contains air, or the grease used for the hydraulic push rod release does not meet the requirements, or the brake pads are not in line with the Dirt between the wheels. 
2 Preventive measures
Regularly check and maintain the brakes. The brakes of the hoisting mechanism must be once every shift, and the brakes of the operating mechanism must be once a day. The main inspections are as follows:
a. Check whether the hinges are stuck and worn, and whether there is any looseness in each fastening;
b. Whether the action of each moving part is normal;
c. Whether the hydraulic system is normal;
d. Whether the wear between the brake wheel and the brake band is normal and clean. 
According to the inspection situation, determine whether the brake is normal, and resolutely prevent the operation with illness. At the same time, the brake should be regularly lubricated and maintained. In order to ensure the safe operation of the crane, the brake must be adjusted frequently to ensure the working requirements of the corresponding mechanism. 

7. Wheels and Tracks
During the operation of the crane, the common faults of the wheel and the track are the gnawing of the wheel and the unequal height and slipping of the trolley. There are many reasons for gnawing, and the forms of gnawing are diverse. The light gnawing will affect the life of the crane, and the heavy gnawing will cause serious casualties. Therefore, the special equipment management personnel should pay enough attention to the gnawing. The main reasons for the road gnawing are the non-uniform friction caused by the installation error, the excessive wear of the parts in the transmission system of the cart, and the excessive key connection gap, which causes the brake to be out of synchronization. Therefore, the special equipment department of each unit must find a qualified unit for installation and maintenance when installing and maintaining cranes, so as to ensure the safety and operating life of the equipment; at the same time, special equipment managers should strengthen the usual inspection and management to avoid cranes gnawing If there is a mechanical failure of the road, in the inspection process, it is necessary to carefully and carefully find out the cause of the road gnawing, and take corresponding measures. The unequal height of the wheels of the trolley is an extremely unsafe factor in the operation of the crane. The unequal height of the trolley may cause a wheel to hang in the air or the wheel pressure is too small during the operation of the trolley, which may cause the vibration of the trolley body. The factors that cause the unequal height of the wheels of the trolley are caused by various reasons, but the main reason is that the installation error does not meet the requirements and the weight of the trolley itself is not uniform. High problem solved. 
During the operation of the crane, due to the unclean track, excessive starting, uneven track of the trolley, elliptical wheels, and unequal wheel pressure between the driving wheels, the trolley has a slip ring phenomenon, which requires the special equipment management personnel to check the process. We must be careful and careful, and find problems to be solved in time to avoid the phenomenon of the car slipping. 

Eight, safety accessories
The safety accessories of bridge cranes are completely protective devices set up from the perspective of protecting equipment and operators. The accessories are in good condition to ensure the safe operation of the overhead crane. 
The mechanical failure of bridge cranes is relatively complicated. To prevent mechanical failures, it is necessary to strengthen equipment management. At the same time, it is necessary to submit an annual review application to the local special equipment management and supervision department on time. Through the inspection and guidance of the special equipment supervision department, the unsafe factors of the equipment are eliminated. In the bud, to ensure the safety of equipment and personnel.