12 kinds of bolt loosening methods, how many do you know?

Time：2024-07-03 15:45:56 / Popularity： / Source：

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 processing and production of parts, quality and efficiency of processing will be affected due to loose connection of bolts. So how to prevent 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 reliability of related workpieces.

01 Double nut

Principle of anti-loosening nuts on the top: when double nuts are locked, two friction surfaces are generated. The first friction surface is between nut and fastened part, second friction surface is between nut and nut. During installation, pretightening force of first friction force surface is 80% of second friction force surface. When impact and vibration loads are applied, friction force on the first friction force surface will decrease and disappear, but at the same time, first nut will be compressed to further increase friction force on second friction force surface. To loosen nut, the first friction force and second friction force must be overcome, because second friction force will increase while first friction force decreases. In this way, anti-loosening effect will be better.

Down's thread anti-loosening principle: Down's threaded fasteners also use double nuts to prevent loosening, but rotation directions of two nuts are opposite. When shock and vibration loads are applied, friction force of the first friction force surface will decrease and disappear.

First nut (right-handed in figure) will have a tendency to loosen, that is, nut will turn to the left. However, rotation direction of second nut (left-handed in figure) is opposite to that of the first nut, so loosening force of the first nut is directly converted into tightening force of second nut. In this way, nut will never loosen.

02 30° wedge thread anti-loosening technology

There is a 30° wedge-shaped slope at the bottom of 30° wedge-shaped female thread. When bolt and nut are tightened together, cusp of bolt is tightly pressed against wedge-shaped slope of female thread, thereby generating a large locking force.

Due to change in angle of tooth shape, normal force applied to contact between threads is at an angle of 60° to bolt axis instead of 30° angle like ordinary threads. Obviously, normal pressure of 30° wedge thread is far greater than fastening pressure, so resulting anti-loosening friction must be greatly increased.

Schematic diagram of Spirax thread structure
From figure below, it can be seen that force represented by two arrows is Pɑ, normal pressure of traditional 60° angle thread is P=1.15Pɑ; and 30° wedge thread has a 30° wedge-shaped slope at the bottom of tooth. Angle and size of normal pressure are changed, and normal pressure P=2Pɑ.

In this way, ratio of normal pressure between 30° wedge thread and traditional 60° thread is ≈12:7, and anti-loosening friction force increases accordingly. Wedge-shaped surface of 30° wedge thread can also eliminate problems of uneven stress on ordinary threads, tripping and seizure.

03 Self-locking nut

Self-locking nuts are generally self-locking by friction, and 30° wedge thread anti-loosening mentioned above should belong to 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., used for swimming self-locking nuts and spring clip self-locking nuts for products with working pressure not greater than 2atm, working medium of gasoline, kerosene, water or air, and operating temperature of -50~100℃.

Insert a wedge between nut and screw to prevent loosening

Japan Hadlock eccentric self-locking nut

04 Threadlocker

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 bolt through screw hole, apply thread locker to thread of mating part, assemble nut and tighten to specified torque.

For working conditions where depth of screw hole is greater than length of bolt: apply locking glue to thread of bolt, assemble and tighten to specified torque.

For blind hole working conditions: drop locking glue to bottom of blind hole, then apply locking glue to thread of bolt, assemble and tighten to specified torque; if opening of blind hole is downward, just apply locking glue on thread of bolt, and there is no need to apply glue in blind hole.

For stud bolt working condition: locking glue should be dripped into screw hole, then apply locking glue on bolt, assemble stud and tighten it to specified torque; after assembling other parts, apply locking glue to meshing part of stud and nut, assemble nut and tighten it to specified torque; if opening of blind hole is downward, no glue is needed in hole.

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

05 Wedge lock double stack washer

Radial serrations on the outer surface of wedge lock washers engage surface of workpiece they contact. When anti-loosening system encounters a dynamic load, displacement can only occur on inner surface of washer.
Expandable distance along thickness direction of wedge lock washer is greater than longitudinal displacement that can be produced by bolt along thread.

Different from other existing anti-loosening methods, wedge-type anti-loosening uses clamping force instead of frictional force to tighten bolts.
120-year-old HEICO-LOCK wedge anti-loosening system products include wedge lock washers, RING-LOCK wedge washers and wedge lock nuts. 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.

06 Split pin, slotted nut

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

Installation form of cotter pin is as follows:

Slotted nuts are used in conjunction with threaded rod eye bolts and cotter pins to prevent relative rotation of bolt and nut.

07 Series steel wire anti-loosening

Series steel wire anti-loosening is to pass steel wire into hole of bolt head, connect bolts in series, and play role of mutual containment. This loosening method is very reliable, but it is troublesome to disassemble.

This method of anti-loosening is often used in aircraft and rockets.

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.

08 Stop washer

After nut is tightened, single-ear or double-ear stop washer is respectively bent and tightly attached to the side of nut and connecting piece, so that 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.

09 Spring washer

Anti-loosening principle of spring washer is that after spring washer is flattened, spring washer will generate a continuous elastic force, so that threaded connection pair between nut and bolt will continue to maintain a frictional force, generate a resistance torque, and prevent nut from loosening.

At the same time, sharp corners of opening of spring washer are respectively embedded in the surface of bolt and connected part, thereby preventing bolt from rotating relative to connected part.

10 Hot melt fastening technology

Hot-melt fastening technology does not require pre-drilling, and can be directly tapped to achieve connection under closed profile, which is widely used in automotive industry.

This hot-melt fastening technology is a cold forming process that conducts high-speed rotation of motor through tightening shaft in the center of equipment to plastic deformation of plates to be connected due to friction and heat, then self-tapping and screwing.

Hot melt fastening process steps and process include six stages: rotation (heating) → penetration → through hole → tapping → threading → fastening.

11 Color changing bolt

Smart color-changing bolts, to be precise, this is an induction bolt called Smartbolt. Bolt head of induction bolt has an induction plate. The tighter you tighten it, the darker its color will be.

When 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 nut and connected parts, This pressure creates a frictional torque that stops nut from turning, so nut does not loosen.

Of course, after mastering anti-loosening design of bolts, tightening method and torque distribution during tightening should also be considered when manually clamping bolts.

Gud Mould Industry Co., Ltd