Of course, we will not open America by saying that a significant advantage of bolted connections over other types, such as welded and riveted connections, is their ability to be dismantled.
However, this property is not only an advantage. This can lead to problems such as random self-loosening during operation. This random self-unscrewing, which in the literature is called vibration attenuation, is an important phenomenon. But the most unpleasant thing is that it is often underestimated by engineers.
Meanwhile, it is extremely important for the designer to be aware of the reasons for the possible loosening of bolts, and he simply must take such reasons into account when developing reliable connections.
The information we provide below just talks about loosening bolts and threaded fasteners from vibration. And these are key facts for designers according to the theory of loosening threaded fasteners, and also about methods to prevent self-unscrewing.
Many design publications describe numerous special fasteners suitable for threaded fasteners. However, such information about the self-loosening of threaded fasteners will confuse a designer who does not have theoretical knowledge.
Below is the basic information about reasons for self-unscrewing threaded fasteners and methods to prevent this phenomenon.
About the reasons for self-unscrewing of bolts, nuts and other fasteners
Of course The main reason for bolt loosening is vibration.
However, a much more common cause of weakening is lateral movement of a nut or bolt head relative to the connection, which leads to relative movement in the thread.
In the absence of such a phenomenon, the bolts will not loosen even if the connection is subjected to very strong vibration. In the process of detailed study, you can also determine clamping force necessary for bolts to prevent slipping in the connection.
Often the result of a self-unscrewing bolt is fatigue failure, which reduces the clamping force acting on the connection. The result is slippage in the joint, which results in a bending load on the bolt and subsequent fatigue failure of the bolt.
Tightened bolts (or nuts) rotate freely because there is relative movement between the male and female threads. This movement neutralizes friction clamp and creates a tightening torque that is proportional to the thread pitch and preload.
Exists three well-known reasons occurrence of relative movement in the thread:
- Bending parts, which leads to the emergence of forces on the friction surface. When slipping occurs, the head and threads slip, causing loosening.
- Differential thermal effects, arising from temperature differences or differences in the materials of the parts being tightened.
- Attached connection effort which can cause the joint surfaces to move, causing the bolt to loosen.
Fighting self-unscrewing
In the 60s In Germany, it was studied that alternating force applied perpendicularly prevents self-unscrewing.
The study of this issue led to the creation testing facilities, which provided extensive information on the tightening action of self-tightening fasteners.
Installations such as Junkers cars(Junkers machines) (you can watch a video about these installations - see the bottom of the article), named in the literature after the inventor, have been used for the last twenty years by most aerospace and automotive manufacturers to evaluate the performance of special self-locking fasteners.
As a result of long-term testing and careful study, scientists have improved a variety of fasteners used by most large companies.
For example, regular the spring washer is no longer used, as it has been shown to actually promote weakening rather than prevent it.
There are many thread lockers available. Despite the work of the American National Standards Subcommittee B18:20 on retaining fasteners, there are three main categories.
Such as:
- free spin category
- friction clutch category
- chemical fixation category.
TO free rotation categories include simple bolts with a circular row of teeth under a head with a shoulder. The teeth are of an inclined type, which allows the bolt to rotate in the clamping direction, but lock in the supporting surface when rotating in the unscrewing direction. Whizlock falls into this category.
Friction clutch category can be divided into two subcategories: metallic and non-metallic. Metal friction clutch fasteners typically have curved threads that produce torque; An example of this category is the Philidas nut. Non-metallic friction clutch fasteners have plastic inserts that act as thread clamps; An example is the Nyloc nut.
TO chemical clamp categories refer to adhesives that fill the space between the internal and external threads, thereby connecting them; an example is Loctite. Such binders are available in microencapsulated form and can be pre-applied to the thread.
In order to determine what is most suitable for each application, it is necessary to carefully study the scope and conditions of the future use of the fastener.
To summarize in a nutshell, the chemical clamp category, for example, provides the best protection against vibration loosening, thanks to its free-make-up clamp.
In general, in order to prevent fasteners from loosening, you must:
1. Make sure that on the intermediate surface of the connection sufficient clamping force to prevent relative movement between the head of a bolt or nut and the connection.
2. Check that the connection is designed to allow resistance to indentation and stress relaxation.
3. Make sure that only proven threadlockers are specified. This is especially true for thread sealants such as Loctite and flange fasteners such as Whizlock. Or torque-dominant fasteners, such as Nyloc.
Self-unscrewing fasteners is just one aspect of bolted joint design that every designer must keep in mind during the design process.
As you can see in the side photo, even if the threads are completely sealed with sealant, this will not eliminate the problem if there is not enough preload on the bolt to prevent the joint from moving. The photo shows an M12 bolt partially worn from shear.
Applying drawing analysis position to prevent vibration loosening of threaded fasteners is challenging.
Therefore, many serious companies, such as Bolt Science, have developed computer programs to help engineers overcome problems associated with the use of connections with threaded fasteners and bolts.
These programs are easy to use, and even an engineer with superficial knowledge in this field will be able to solve problems associated with the above task.
How to fix a threaded connection and lock the nut so that it does not unscrew due to vibration? How to fix and lock a screw or bolt? Vibration resistance of threaded fastening. Vibration resistance (10+)
Fixing the threaded connection - Continued
Glue
Glue or a special fixing compound is applied to the internal and external threads in the place where they will be combined. After drying, the connection is locked. For such fixation, you can use glue that meets the following requirements: (1) It glues the material from which the bolt and nut are made (this can be steel, bronze, fluoroplastic, etc.) (2) It is quite elastic in the dried state and vibration resistant. (3) It is resistant to the environment in which it is intended to be used. For example, it is water resistant if the connection is in water. (4) It dries quickly. It should be remembered that under carving conditions, one-component adhesive rises at least four times slower than under standard conditions, so the instructions for drying time should be multiplied by four, or better yet by six for a margin. If the glue dries slowly, the work risks taking longer. For example, PVA in carvings takes a week to dry. Two-component adhesive (with hardener) cures for a nominal time.
I use the following glues and varnishes:
- Moment glue or rubber glue, if the connection will not be exposed to oils, solvents or their vapors, or sunlight.
- Any universal elastic glue "Liquid nails".
- Poxypol.
- Plumbing glue.
- Universal superglue.
- Primer GF-021 or Otex, if the connection will not be subjected to too much stress.
When choosing glue, consider the dimensions of the parts to be joined. For small parts you need liquid adhesives, such as superglue or Moment. For very quick fixation, you can use hot-melt adhesive, applying it with a hot-melt gun.
Flaws. This connection is quite difficult to disassemble. Sometimes the glue sets so much that it takes a lot of effort to unscrew it. But it is enough to provide a small initial offset. Then the process goes easily.
I like this method because it allows me to assemble threaded connections almost by hand, without tools. While the glue is not dry, it serves as an excellent lubricant and allows you to tighten without effort.
Teflon nut or insert
Inside the nut, a Teflon film is applied to the thread or part of it, or a winding is used - the external thread (bolt) is wrapped with a thin Teflon thread. Once tightened, Teflon creates increased friction and prevents vibration from loosening the connection. With the help of winding, you can fix almost any connection.
Flaws. The Teflon nut is disposable. After unscrewing, it must be replaced. The winding is also disposable. After disassembly, the remnants of the winding must be removed, and a new one must be wound before reassembly.
Common combinations
None of the options provide a 100% guarantee. Even the most reliable method - a cotter pin - can fail. The cotter pin may shear (break and fall out). Therefore, in particularly critical places, the described methods can be combined. You can put a groover between the nut and the locknut, glue a Teflon winding or nut, apply glue to the nut and locknut, etc.
For a threaded connection located in oily liquids (oil, gasoline, kerosene, diesel fuel...) and subject to vibration, only a cotter pin is suitable. All other options are unreliable. In industrial conditions, the reliability of other types of connections can still be ensured, but not in a homemade version.
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Let this be a purely technical note. Separately, I wrote about a funny story that happened due to - I recommend you read it. There is only technical information here, albeit a little. The knowledge gained from the note will be able to protect you from the inappropriate use of such a special liquid as a thread locker.
What types of clamps are there and on what principle do they work?
Thread lockers are quite dangerous liquids. Incorrect use of thread locker is guaranteed to lead to undesirable consequences.
Thread lockers come in red and blue (green). They differ in the principle of unscrewing:
- Red (one-piece) ones must be heated to a high temperature; without this, it will not be possible to unscrew the bolt with such a lock. Withstands higher temperatures. They are usually used on rotating and high-temperature components (for example, brakes or a bolt securing the main pulley to the crankshaft).
- The blue ones (detachable) are unscrewed with great force. Used on low-temperature units where the likelihood of unscrewing is not high. As a rule, they are protected from unscrewing when exposed to vibration (for example, attaching the bracket directly to the engine).
The classification does not always coincide with the color, so the main criterion is detachable/one-piece. Please also note that recently, along with detachability, clamps with varying degrees of fixation have also begun to be produced, while remaining detachable or non-detachable.
The operating principle is simple. A liquid is applied to the surface of the bolt, which, when dried, bonds the two metal surfaces. A primitive thread locker can be ordinary nail polish. But it should only be used where unscrewing is not critical. In critical assemblies, use only a specialized lock. Its cost is not high. Maximum 150 rubles per tube, which is enough for a large number of repairs.
How to apply
First, you need to be sure that the bolt in question simply requires the application of thread locker. To find out, you need to find the repair manual for your car and read it. Typically this information is written in the same section as the tightening torques.
If you are confident, then open the tube with the fixative and apply a strip of about 5 mm (usually one drop) to the thread, in the place of the bolt where the nut will be located. When you tighten the bolt, the fastener will distribute itself over the contacting surface.
How to unscrew
The blue lock should unscrew without problems. In extreme cases, you can slightly heat the part, but in practice this rarely happens.
It's often worth some effort with the red lock. The first problem is to recognize the presence of the red fixative. If you know that red thread locker is applied to any components that need to be repaired, and you take the car to a repair shop, warn the technicians so that they do not inadvertently break the head of a nut or bolt. Usually this is recognized by the fact of unscrewing. Heating the part with a gas burner helps a lot in dealing with the red retainer. The danger here is that there is rarely anything plastic near the heated part. To avoid melting anything, make a screen of asbestos, or at least tin, so that direct flame does not reach beyond the heated part. And make sure there is nothing plastic adjacent to the heated part. When the part has heated up, it is necessary to quickly unscrew the bolt, since when it cools down, the red lock may seize again.
Good luck with the renovation.
Threaded connections are quite reliable and efficient. They are considered one of the most common and cost-effective. However, when exposed to vibration, there is a possibility of weakening them. This can often be seen in the example of nuts that loosen randomly during operation of various devices. Therefore, methods have been developed that can reduce this effect or completely eliminate it.
Ways to avoid loosening the nut
1. Using a washer. This method is one of the most common. It is used in the manufacture of household appliances and some types of industrial equipment. It does not have a high degree of reliability and is protected from arbitrary unwinding. A soft metal washer is placed on the thread, and then the nut is tightened, squeezing it out.
2. Washer and Grover. A more reliable method practiced by the transport industry. Protects the threaded connection from arbitrary unwinding even in the presence of slight vibration. First, a regular washer is put on the thread, and then a groover. After this, tighten the nut. The Grover here serves as a kind of spring, creating tension that prevents unwinding.
3. Special nut with lock. The method using a special nut cannot be called the most effective. However, it takes place in world practice and is used in some types of production.
4. Using thread locker. The composition is applied to the place where the nut will be located, after which it is put on the thread. This is a fairly effective way to prevent random unscrewing, but its effectiveness is affected by changes in temperature, high humidity and exposure to active substances. Therefore, its scope is limited.
5. Use of two or more nuts. One of the most reliable methods. Two nuts are screwed onto the thread at once. After clamping the first, the second is tightened separately, holding the first in its original position and even trying to unscrew it a little.
6. Fixation with a cotter pin. This is the most reliable method, able to withstand almost any external influence and strong vibration. It is used in particularly critical areas. The nut is fully tightened. Then, using a drill and a thin drill bit, a hole is made that pierces it along with the bolt. A pin is inserted into the resulting hole, the antennae of which are unbent, preventing it from falling out. Such a nut can be torn off only with a very strong rotational force in the direction of the thread.
Threaded connections are certainly convenient, but have a serious drawback. Under external mechanical influence, the fastening may be weakened, up to complete separation.
To maintain reliability, a thread locker is required. There are many mechanical devices that keep the nut (screw) from unscrewing. Read about liquid fixatives below.
A cotter pin is a rod bent in half with an eye at the bend. It is installed in the bolt hole, and the nut has special grooves for fixing. After tightening the connection, the free ends of the cotter pin are separated - the fastening is fixed. 
Used on nodes that require high connection reliability. For example, a wheel nut on a car. A lock washer with fixation is placed under the nut and is secured against turning on the stationary part of the structure. The washer has a petal that bends at an angle of 90° and presses against one of the faces of the nut. 
Spring type lock washer. It is one coil of spring, which is placed under the nut when tightening the connection. 
The action is based on creating a thrust force. Excellent resistance to unwinding from vibration. Lock washer with teeth. On the outer diameter of the thin washer, notches are made, rotated at a certain angle. 
In addition to the spacer effect, the teeth increase resistance when unwinding the thread. Nut with an oval cross-section or a plastic sleeve. Compresses the thread, increasing the force when unscrewing the bolt. Resists vibration well, but may loosen the connection under rotational load. 
All these fasteners have a common drawback: they make changes to the design: replacement of standard fasteners is required, or the purchase of additional parts. How to fix a standard connection, without unnecessary components?
A simple solution - liquid threadlockers
Mechanical thread locker is not suitable for all types of connections. Quite often, when assembling a unit, it is not possible to install a third-party object into the structure.
In such cases, adhesive fasteners are used: the fixing compound itself is not able to hold the parts, but it is easy to prevent the nut (bolt) from unwinding.
Previously, varnishes and paints were used to fix threaded connections. When tightening, a drop of durable paint was simply applied to the threads, and the nut did not unscrew without applying additional force.
Video testing of threaded fasteners
However, this method was not reliable - after all, paint and varnish compositions are not intended for mechanical fastening. In addition, in the space between the threads, where air access is limited, the composition does not harden, or occurs too slowly.
Therefore, the practice of “dropping paint on the thread” has long been abandoned. Today, special adhesives or sealants are used: so-called anaerobic thread lockers.
Operating principle of anaerobic fixators
For reliable fastening, the adhesive composition must go from a liquid state of aggregation to a solid state: that is, it must polymerize. All sealants and adhesives work on this principle: while the composition is in a sealed container, it is liquid.
After application to the surface to be treated, the chemical components react with oxygen and hardening occurs. The problem with threadlockers is that the main working surface remains sealed (inside the threads).
There is no oxygen access, polymerization does not occur. What is needed for the normal completion of the reaction? This property is characteristic of anaerobic microbes that develop in conditions of limited access to oxygen.