For bolt failure, designers should consider these factors during the design process. The first type of damage is because the diameter of the fastening bolt selected by the designer in the design is too small, and the number of bolts is too small, so that the strength of the bolt cannot meet the pressure generated during the explosion test, and the vulnerable part of the bolt is broken. The second type of damage is due to the material deviation of the cast aluminum casing selected by the designer during the design. The number of bolts is too small, and the strength of the thread on the aluminum casing cannot meet the pressure generated during the explosion test. The part of the thread that is most stressed is broken. At the same time, because the torque of the bolt cannot be clearly tightened in the design, the assembly personnel excessively tighten the bolt to overflow the bolt, which is also a cause of bolt damage. This damage to the thread often occurs during assembly. It is also because of the ambiguity of the tightening torque that the inspection unit cannot control the machining gap of the flat portion of the sample produced by the manufacturing unit with the planar flameproof joint.

In addition, the anti-looseness of the fastener is also a problem in the bolt design process. In China, the anti-looseness of fasteners usually adopts anti-loose measures such as spring washers, but there is no regulation and related calculation for the force distance of tightening bolts. It is impossible to unify the tightening of fasteners with multiple strong distances. In the sample, the screw on the aluminum casing was broken and the overflow buckle was caused by the above reasons. In Germany, fasteners do not require special anti-loose measures and are usually guaranteed with the specified torque. This will not only make the design of the fastener more standardized, but also reduce the unnecessary damage caused by human factors in the assembly. The tensile stress and shear stress of steel are greater than that of cast aluminum, so the strength of the thread on the aluminum casing should be considered in the design of the bolt. Then, we should determine the tightening torque of the bolt according to the specific force of the bolt. Next, let's talk about the anti-looseness of bolts and the accuracy of bolts and screw holes. Bolt Strength Design Stress Analysis of Bolts Currently, the flameproof enclosures are bolted to have two types of flameproof surfaces: flat and dead. For the planar structure, the bolt not only acts as a fixing but also ensures a plane clearance. For the joint type explosion-proof joint surface used for the electrical equipment of Class IIA and IIB, when the flameproof surface does not consider the plane portion, it is the force applied by the electrical equipment to apply the axial load F when performing the explosion test. The force of the bolt is increased from F0 to F, and the bolt is extended by the axial load increment F-F0, and the increased deformation amount is 1. The cover is affected by the bolt due to the axial intercept increment F-F0. The force is reduced from F0 to F0 (residual preload), and the deformation is also reduced accordingly. The reduction is 2. The force on the thread on the bolt is not uniform. When tightening with a nut with ten turns, usually About 1/3 of the force is concentrated on the first thread of the nut, and the thread after the eighth ring is almost unstressed. When the shell is fastened with bolts, the opposite is true. The first thread of the bolt is the most stressed, about 1/3 of the load. This is because the pitch error and the tooth shape error on the bolt work together. The first turn of the bolt is first engaged with the thread in the housing, while the force on the rear thread is reduced. Until almost no force. This corresponds to the failure of the housing thread that is seen during the usual test. In the test, the thread on the casing that is in mesh with the first and second coils of the bolt is often shear damaged. The accuracy of the bolt affects the accuracy of the bolt is affected by the accuracy of the single thread and the cumulative error of the thread, so the accuracy of the bolt should take into account the screw length and bolt accuracy. The tightening length of the fastening bolts used in the explosion-proof device is usually a medium-rotation length, and the accuracy of the bolt is generally 6 g. For the internal thread on the casing, the accuracy determines the connection accuracy of the fastener.

When checking the strength of the shell thread, we noticed that the stress on the bolt is not uniform. When the bolt engages the housing with 10 buckles, the last buckle of the screw hole is the most stressed, which is determined by the cumulative error of the thread. When we design the thread strength, we can also calculate the 10 buckles that can be engaged with the bolt. When the precision of the screw hole is not enough, the number of the screw holes on the bolt and the housing will be insufficient, and the last buckle of the screw hole will be stressed. Will increase accordingly, will affect the strength of the thread, even the bolt pre-tightening torque has been great, the upper cover and the housing are still not tightly connected. The accuracy of the screw holes is very important. Considering that the matching properties required for the fastening bolt and the screw hole should not change during use, the screwing length of the bolt should be medium or long, so the depth of the screw hole on the housing should be not less than 10 buckles. The accuracy should be no less than 5H.