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As a professional manufacturer of hydraulic wrenches and bolt tensioners, TorcStark has done a lot of research and practice on bolt pre-tightening and anti-loosening, and has accumulated a lot of experience. Whether a bolt is tightened or not sometimes affects the safety of the entire project, so the prevention of bolt loosening is a very important thing. Now, TorcStark will introduce 12 ways to prevent loosening of bolts.

Common methods of bolt anti-loosening include: friction locking, mechanical locking and permanent locking.

The friction locking includes spring washers, self-locking nuts, and double nuts. The disadvantage of friction anti-loosening is that it is easy to loosen under impact force. It is necessary to mark the anti-loosening with a marker, which is easy to check. The relevant parts cannot be missing.

Mechanical locking includes cotter pin, slotted nut, stop washer, and wire in series. The mechanical anti-loosening method is more reliable, and the mechanical anti-loosening method should be used for important connections. The disadvantage is that the focus of the anti-loosening inspection is that the relevant parts cannot be missing or damaged.

Permanent locking includes spot welding, riveting, punching, and gluing. The disadvantage is that most of the threaded fasteners are broken during disassembly. Cannot be reused.

Now TorcStark will give you a detailed introduction to each bolt anti-loosening method.

Double Nut

The principle of the anti-loosening nut on the top: when the double nut is locked, two frictional surfaces are generated. The first frictional surface is between the nut and the fastener, and the second frictional surface is between the nut and the nut. During installation, the preload force of the first friction surface is 80% of that of the second friction surface. When shock and vibration loads act, the frictional force of the first frictional force surface will decrease and disappear, but at the same time, the first nut will be compressed, resulting in a further increase of the frictional force of the second frictional force surface. The loosening of the nut must overcome the first frictional force and the second frictional force, since the first frictional force decreases while the second frictional force increases. In this way, the anti-loosening effect will be better.

Down’s thread anti-loosening principle: Down’s thread fasteners also use double nuts to prevent loosening. However, the two nuts rotate in opposite directions. When shock and vibration loads act, the friction force on the first friction surface will decrease and disappear. The first nut will have a tendency to loosen, that is, the nut will rotate to the left. However, the rotation direction of the second nut 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. This produces a very good bolt anti-loosening effect.

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 bolt and nut are tightened against each other, the cusp of the bolt is 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 profile, the normal force applied to the thread-to-thread contact is at a 60° angle to the bolt axis, rather than the 30° angle like with ordinary threads. Obviously, the normal pressure of the 30° wedge thread is much larger than the tightening pressure, so the anti-loosening friction force generated is bound to be greatly increased.

It can be seen from the figure below that the forces represented by the two arrows are Pɑ, the normal pressure of the traditional 60° angle thread is P=1.15Pɑ; while the 30° wedge thread has a 30° wedge-shaped slope at the bottom of the tooth, The angle and size of the normal pressure have 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-shaped thread can also eliminate the problems of uneven force on ordinary threads, tripping and seizing.

Self-locking nuts

Self-locking nuts are generally self-locking by friction. The 30° wedge thread anti-loosening mentioned above should belong to the category of self-locking nuts. 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 For gasoline, kerosene, water or air, the use temperature is -50~100 ℃ on the swimming self-locking nut, and the spring clip self-locking nut.


Threadlocker is an adhesive composed of (meth)acrylate, initiator, co-accelerator, stabilizer (polymerization inhibitor), dye and filler in a certain proportion.

For through-hole conditions: Pass the bolt through the threaded hole, apply threadlocker to the threads of the mating portion, assemble the nut and tighten to the specified torque.

For the case 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.

For the blind hole condition: drip the locking glue to the bottom of the blind hole, then apply the locking glue to the thread of the bolt, assemble and tighten it to the specified torque; if the blind hole opens downward, just apply the locking glue It can be applied on the thread of the bolt, no glue is required in the blind hole.

For the working condition of stud bolts: drip the locking glue into the screw holes, then apply the locking glue on the bolts, assemble and tighten the studs to the specified torque; after assembling other parts, apply the locking glue to the studs To engage with the nut, assemble the nut and tighten it to the specified torque; if the blind hole opens downward, there is no need to drip glue in the hole.

For pre-assembled threaded fasteners (such as adjustable screws): After assembling and tightening to the specified torque, drop the locking glue into the thread engagement to allow the glue to penetrate itself.

Wedge Lock Lock Double Stack Washers

The radial serrations on the outer surface of the wedge lock washer engage with the workpiece surface it contacts. When the anti-loosening system is subjected to dynamic loads, displacement can only occur on the inner surface of the washer.

The extendable distance through the thickness of the wedge lock washer is greater than the longitudinal displacement that the bolt can produce along the thread.

Different from other existing locking methods, wedge locking uses clamping force instead of friction to tighten the bolt.

The 120-year-old HEICO-LOCK wedge locking system products include wedge lock washers, RING-LOCK wedge washers and wedge lock nuts, mainly made of carbon steel coated Dacromet and 316 Stainless steel, stainless steel materials such as 254SMO, C276 and 718 are also widely used.

Cotter pin, slotted nut

After the nut is tightened, insert the cotter pin into the nut slot and the bolt tail hole, and pull the end of the cotter pin open to prevent the relative rotation of the nut and the bolt.

Tandem steel wire anti-loosening

The anti-loosening of the series steel wire is to insert the steel wire into the hole of the head of the bolt, and connect the bolts in series to play a role of mutual restraint. This relaxation method is very reliable, but it is more troublesome to disassemble. This anti-loosening method is often used in aircraft and rockets. Single-strand wires are generally used in small screw groups that are closely spaced or in hard-to-reach places.

Stop gasket

After the nut is tightened, bend the single-ear or double-ear stop washer to the side of the nut and the connector respectively, and then the nut can be locked. If the two bolts need to be double-locked, a double-lock washer can be used to brake the two nuts against each other.

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 maintains a frictional force, generating a resistance torque and preventing the nut from loosening. At the same time, the sharp corners at 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.

Hot melt fastening technology

Hot-melt fastening technology does not require pre-drilling holes, and can be directly tapped under the closed profile to achieve connection, which is widely used in the automotive industry. This hot-melt fastening technology is a cold forming process in which the high-speed rotation of the motor is transmitted through the central tightening shaft of the equipment to the plastic deformation caused by frictional heat generation of the sheets to be connected, and then self-tapping and screwing.

The process steps and process of hot-melt fastening connection include six stages: rotation (heating) → penetration → through hole → tapping → screwing → tightening.

Discolored 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 a induction disc, and the tighter you tighten it, the darker the color.

When the strength reaches 90%, it changes from yellow to green, and after reaching 100%, it is black.


High-strength bolted 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 generates 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.


The above are TorcStark’s 12 classic design methods for bolt prevention. The above methods cover almost all methods of preventing loosening of bolts: from small bolts to high-strength bolts, from bolts on automobiles to aircraft and ships. If it were you, which method would you use to prevent bolt loosening first? Or if you have other ways to prevent bolts from loosening, leave your comments and let us communicate together.


TorcStark® is a professional manufacturer of hydraulic equipment. Mainly produce hydraulic torque wrenches, hydraulic bolt tensioners, hydraulic torque /tensioner pumps and other products.

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