CNC Turning Mechanical Machining Parts

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Block D, Dianlian Science Park, Matian St., Guangming District, Shenzhen 518106, China

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Analysis of Common Fastener Bolts and Nut Failures

Frame is the carrier of all parts of the automobile, the parts are directly or indirectly fastened to the frame, and the car as a sports equipment, by the load, road force, frame deformation, fasteners loose is a natural thing.

Fastener connection failure analysis first from the working machine analysis, assembly wrench tightening torque through friction into thread pre-tightening force, site quality control only check tightening torque. From this process, it can be analyzed that the factors affecting the failure of fasteners are tightening tools, tightening torque, thread structure, fastener quality, connecting parts, external forces, design, etc.

Tool value does not match the actual, the tool regularly effective inspection, compressed air pressure does not meet the requirements, check rectification, bolts, nuts online tightening, no special requirements of bolts, nuts by the wind wrench tightening can be, special requirements of the bolt, nut first use wind wrench screw Tightening, and then with a fixed-value wrench re-tightening, the reason for failure is simple, that is, the torque does not meet the technical requirements; Size difference, friction coefficient deviation, etc. , the connecting part factors are surface paint film quality, paint film thickness, gap between the joined parts, the elasticity of the joint, etc. , external force factors are mainly manifested in vibration, shock, force deformation, etc. , design factors are reflected in the fastener selection is not right, pre-tightening force is small or large, shear force, stretch length deformation can not meet the actual needs.

From the tightening process force analysis, tightening torque needs to overcome the tightening end friction moment, threaded sub-friction moment, the connecting part elastic deformation torque, etc. , before the remaining part is converted into clamping force. Therefore, under the same conditions of use, the key factors that determine tightening force should be the structure and essential quality of the thread itself, such as structure, material, heat treatment, surface treatment, quality control, etc.

It can be seen that the failure of fasteners is mainly manifested in loose, adhesive, deformed, unseeded, broken, resulting in the connection is not secure, disintegration, damage, and even caused traffic accidents, resulting in personal injury, property damage, such as the front of the transmission shaft fell off, the front end of the vehicle will be propped up to cause roll-over, steering key shaft bolts off caused steering failure. Fasteners in the automotive structure seems to be inspicuous small parts, fastener assembly is the main work of vehicle assembly, and many varieties, large quantities, is an important factor affecting the assembly of the whole vehicle, can not be underestimated.

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