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Reasons Analysis And Prevention Measures Of Bolt
- Jan 18, 2017 -

Engineering machinery maintenance

Thread locking structure of threaded locking methods are commonly used: friction nut, mechanical looseness, damage screw locking. In General, the friction locking is simple and convenient, but less reliable. Important parts of the threaded connections, especially difficult check of excavator's internal thread connection, construction machinery repair check method should be used.

(1) the friction locking friction locking includes double nut and tighten, spring washers, self-locking nuts and other methods. Double nut and tighten locking method is simple in structure, suitable for smooth, slow and heavy, fixed connection. Working device pins can be used to repair construction machinery fixed, fixed, u-bolts. Because of the uneven pad elastic spring, bolt easily bent, curved bolt easily slip under impact and vibration. A type of excavator early M8, M10, M12 bolts, spring kapyong pad locking structure of the locking effect is poor, many bolt 500h not working. Later made more pads, good locking effect. Increase the gasket surface should be smooth and there are no harmful crack, scratch, Flash and bent, to avoid bending stress assembly bolts. Self locking nuts, locking more reliable, repeated after disassembly and Assembly, will not decrease anti-loosening performance. Loosing prevention methods that can be used to seat slide u-shaped handle, radio antennas, lighting and other smaller thread size.

(2) mechanical locking mechanics check for engine vibration damper bolt. The nuts welded in a bend of the bracket, through the bracket bending limit for locking ends.

(3) destroy thread locking thread before screwing, the fastening thread apply thread adhesive, belonging to destroy thread locking. Fastening thread apply thread after you glue and tighten the thread, thread adhesives harden, solidification, would prevent the loose thread. Threaded bolts of fastening is used in important parts, such as wheels, sprockets, engine bearings, engine shock absorbers, cab floor, cab shock absorber, counterweight, Rotary motor, travel motor, pivoting bolt.

4. preload

(1) should comply with the required accuracy of thread hole combination for 6H, its preload and fastening the fastening and are combined with the influence of friction coefficient of fasteners. Damage or corrosion of threaded fastening should tap or die before trimming (commonly known as "silk"), and screw, bolts with cleaning agents as well as paints and stains to clean residue combination.

(2) the preload should be appropriately increased preload can reduce the stress changes in amplitude. Preload but should not be too large, and must be controlled to within a specified range. This is caused because the preload is too general strength of bolt yield point is reached. When you use a torque wrench fastening bolt, bolt stress of (preload) generally yield strength of bolt 75%. Grade 10.9 bolts, tensile strength of 1000MPa, yield strength 900MPa preload is 675MPa. When you use a torque wrench when, theoretically, preload and tightening torque t f are as follows: t ≈ 0.2Fd: d for thread nominal diameter. In addition, the tightening torque is also associated with precision fastening tools. When using the socket wrench, when an ordinary wrench, tightening torque should be slightly smaller than when using a torque wrench.

(3) after the tightening after disposal of bolts fastened as required, in the bolt head (or nut) and the fastener surface color. After once the bolt is loose, to gradually tighten the fastening direction until the bolt head (or nut) coincides with the fastener surface of the tag again.

5. material

(1) when the fastener bolts screwed into the fastener, fastener materials, fastening bolt is screwed into the depth is different, according to fasteners for steel, cast iron, light alloys order bolts screwed into the depth increases gradually. Bolt is screwed into the depth of the steel should be slightly larger than the nominal diameter of the thread, but when the bolt diameter is less than 12mm, and admission to the battery terminals, unscrew hose clamps, clamps, radiator NET, bottom plate, it's screwed into the depth equal to nominal diameter. Cast iron foundry in matrix form after expansion of the graphite, can reduce the volume of contracted, reducing stress, passed to vibration can weaken, have very good resistance to vibration and shock absorption. The transition such as walking on the pilot valve, engine mounts, compressor bracket for cast iron production, its fastening bolt locking effect is good.

(2) screw threads material selection is particularly important. When the yield strength threaded too small, is likely to cause after the bolt is screwed into the deformation, causing early fatigue failure, bolts are easy to loose. Part of the yield strength of the material and thickness of the relevant, such as the Q235 steel, its thickness is thicker, smaller the yield strength, material should be aware. If the thread's tensile strength is too small, and when you use a torque wrench when tightening, can directly damage the thread. Therefore, the key position of threaded steel generally used Q345B, common thread steel is generally used for the location of Q235B.

(3) homemade increase pads increase the gasket is not used when tightening bolts standard parts, and have strict technical requirements, it can only be made. General 45th steel made increase the gasket material, the hardness is around HRC39, heat treatment temperature in the quenched and tempered. Homemade increase gasket not enough hardness after heat treatment, can cause deformation. When made to increase pad hardness is above HRC38, electro-galvanized, there will be 6h low temperature (200 degrees) dry to prevent hydrogen embrittlement, followed by slow cooling in furnace. Put these against loose measure used in aircraft design and Assembly processes, users, machine bolts loosened problems reduced, achieved good results and economic benefits.