As a tool commonly used in fixtures, bolts are widely used, but long-term use will also bring many problems, such as loose connection, insufficient clamping force, and rusted bolts. During the processing and production of parts, the quality and efficiency of processing will be affected due to the loose connection of bolts. So how to prevent the bolt from loosening?
There are three commonly used anti-loosening methods: frictional anti-loosening, mechanical anti-loosening and permanent anti-loosening.
Anti-loose purpose: to work more effectively for a long time; to improve the reliability of related workpieces.
1. Double nut
The principle of anti-loosening nuts on the top: when double nuts are locked, two friction surfaces are generated. The first friction surface is between the nut and the fastened part, and the second friction surface is between the nut and the nut. During installation, the pretightening force of the first friction force surface is 80% of the second friction force surface. When impact and vibration loads are applied, the friction force on the first friction force surface will decrease and disappear, but at the same time, the first nut will be compressed to further increase the friction force on the second friction force surface. To loosen the nut, the first friction force and the second friction force must be overcome, because the second friction force will increase while the first friction force decreases. In this way, the anti-loosening effect will be better.
Down's thread anti-loosening principle: Down's threaded fasteners also use double nuts to prevent loosening, but the rotation directions of the two nuts are opposite. When impact and vibration loads are applied, the friction force of the first friction force surface will decrease and disappear.
The first nut (right-handed in the figure) will have a tendency to loosen, that is, the nut will turn to the left. However, the rotation direction of the second nut (left-handed in the figure) is opposite to that of the first nut, so the loosening force of the first nut is directly converted into the tightening force of the second nut. In this way, the nut will never loosen.
2. 30° wedge thread anti-loosening technology
There is a 30° wedge-shaped slope at the bottom of the 30° wedge-shaped female thread. When the bolts and nuts are tightened together, the cusps of the bolts are tightly pressed against the wedge-shaped slope of the female thread, resulting in a great locking. force.
Due to the change in the angle of the tooth shape, the normal force applied to the contact between the threads is at an angle of 60° to the bolt axis instead of the 30° angle like ordinary threads. Obviously, the normal pressure of the 30° wedge thread is far greater than the fastening pressure, so the resulting anti-loosening friction must be greatly increased.
Schematic diagram of Spirax thread structure
From the figure below, it can be seen that the force represented by the two arrows is Pɑ, the normal pressure of the traditional 60° angle thread is P=1.15Pɑ; and the 30° wedge thread has a 30° wedge-shaped slope at the bottom of the tooth. The angle and size of the normal pressure are changed, and the normal pressure P=2Pɑ.
In this way, the ratio of the normal pressure between the 30° wedge thread and the traditional 60° thread is ≈12:7, and the anti-loosening friction force increases accordingly. The wedge-shaped surface of the 30° wedge thread can also eliminate the problems of uneven stress on ordinary threads, tripping and seizure.
3. Self-locking nut
Self-locking nuts generally lock themselves by friction. The 30° wedge thread anti-loosening we mentioned above should belong to the category of self-locking nuts.
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It is divided into: high-strength self-locking nuts for road construction machinery, mining machinery, vibration machinery and equipment, nylon self-locking nuts for aerospace, aviation, tanks, mining machinery, etc., for working pressure not greater than 2atm working medium Gasoline, kerosene, water or air use the swimming self-locking nut on the product with a temperature of -50~100°C, and the spring clip self-locking nut.
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Insert a wedge between the nut and the screw to prevent loosening
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Japan Hadlock eccentric self-locking nut
4. Thread locking glue
Thread locking adhesive is an adhesive composed of (meth)acrylate, initiator, co-accelerator, stabilizer (inhibitor), dye and filler in a certain proportion.
For through-hole conditions: pass the bolt through the screw hole, apply thread locker to the thread of the mating part, assemble the nut and tighten to the specified torque.
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For working conditions where the depth of the screw hole is greater than the length of the bolt: apply locking glue to the thread of the bolt, assemble and tighten to the specified torque.
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For blind hole working conditions: drop the locking glue to the bottom of the blind hole, then apply the locking glue to the thread of the bolt, assemble and tighten to the specified torque; if the opening of the blind hole is downward, just apply the locking glue It can be applied to the thread of the bolt, and no glue is required in the blind hole.
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For the stud bolt working condition: the locking glue should be dripped into the screw hole, and then apply the locking glue on the bolt, assemble the stud and tighten it to the specified torque; after assembling other parts, apply the locking glue to the stud Assemble the nut and tighten it to the specified torque at the meshing part with the nut; if the opening of the blind hole is downward, no glue is needed in the hole.
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For pre-assembled threaded fasteners (such as adjustable screws): After assembling and tightening to the specified torque, drop the locking glue into the threaded joints and let the glue penetrate by itself.
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5. Wedge-type locking and anti-loosening double-stacked washers
The radial serrations on the outer surface of wedge lock washers engage the surface of the workpiece they contact. When the anti-loosening system encounters a dynamic load, displacement can only occur on the inner surface of the washer.
The expandable distance along the thickness direction of the wedge lock washer is greater than the longitudinal displacement that can be produced by the bolt along the thread.
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Different from other existing anti-loosening methods, wedge-type anti-loosening tightens bolts by clamping force rather than frictional force.
Video material, Wi-Fi is recommended to watch
The 120-year-old HEICO-LOCK wedge anti-loosening system products include wedge lock washers, RING-LOCK wedge washers and wedge lock nuts. The main materials are carbon steel coated with Dacromet and 316 Stainless steel, stainless steel materials such as 254SMO, C276 and 718 are also widely used.
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6. Split pin, slotted nut
After the nut is tightened, insert the cotter pin into the nut groove and the hole at the end of the bolt, and open the tail of the cotter pin to prevent the relative rotation of the nut and the bolt.
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The installation form of cotter pin is as follows:
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Slotted nuts are used in conjunction with threaded rod eye bolts and cotter pins to prevent relative rotation of the bolt and nut.
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7. Series steel wire anti-loosening
The series steel wire anti-loosening is to pass the steel wire into the hole of the bolt head, connect the bolts in series, and play the role of mutual containment. This loosening method is very reliable, but it is troublesome to disassemble.
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This method of anti-loosening is often used in aircraft and rockets.
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Single-strand steel wire is generally used in small screw groups with close spacing or places that are not easy to reach, as shown in the figure.
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8. Stop washer
After the nut is tightened, the single-ear or double-ear stop washer is respectively bent and tightly attached to the side of the nut and the connecting piece, so that the nut can be locked. If the two bolts need to be double locked, a double brake washer can be used to make the two nuts brake each other.
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9. Spring washer
The anti-loosening principle of the spring washer is that after the spring washer is flattened, the spring washer will generate a continuous elastic force, so that the threaded connection pair between the nut and the bolt will continue to maintain a frictional force, generate a resistance torque, and prevent the nut from loosening.
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At the same time, the sharp corners of the opening of the spring washer are respectively embedded in the surface of the bolt and the connected part, thereby preventing the bolt from rotating relative to the connected part.
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10. Hot melt fastening technology
Hot-melt fastening technology does not require pre-drilling, and can be directly tapped to achieve connection under the closed profile, which is widely used in the automotive industry.
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This hot-melt fastening technology is a cold forming process that conducts the high-speed rotation of the motor through the tightening shaft in the center of the equipment to the plastic deformation of the plates to be connected due to friction and heat, and then self-tapping and screwing.
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The hot melt fastening process steps and process include six stages: rotation (heating) → penetration → through hole → tapping → threading → fastening.
Video material, Wi-Fi is recommended to watch
11. Color-changing bolts
Smart color-changing bolts, to be precise, this is a kind of induction bolt called Smartbolt. The bolt head of the induction bolt has an induction plate, and the tighter you tighten it, the darker its color will be.
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When the intensity reaches 90%, it turns from yellow to green, and when it reaches 100%, it becomes black.
12. Preload
High-strength bolt connections generally do not require additional anti-loosening measures, because high-strength bolts generally require a relatively large pre-tightening force, such a large pre-tightening force creates a strong pressure between the nut and the connected parts, This pressure creates a frictional torque that stops the nut from turning, so the nut does not loosen.
