How to fix the thread from twisting. Ways of fixing threaded connections
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 randomly unwind during the 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. Use of washers. 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 assumes protection against arbitrary unwinding. A soft metal washer is put on the thread, and then the nut is tightened, squeezing it out.
2. Washer and Grover. A more reliable way, practiced as well as 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 grover. After that, tighten the nut. Grover here serves as a kind of spring, creating tension that prevents unwinding.
3. Special lock nut. 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 a thread locker. The composition is applied to the place where the nut will be, after which it is put on the thread. This is a fairly effective way to prevent arbitrary unscrewing, but its effectiveness is affected by temperature changes, high humidity and exposure to active substances. Therefore, its scope is limited.
5. Application 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 cotter pin. This is the most reliable method, able to withstand almost any external impact and strong vibration. It is used in critical areas. The nut is fully tightened. Then, using a drill and a thin drill, a hole is made through it, along with the bolt. A hairpin is inserted into the resulting hole, the antennae of which are unbent, preventing falling out. Such a nut can only be torn off by a very strong rotational force in the direction of the thread.
Of course, we won't open up America by saying that a significant advantage of bolted connections over other types, such as welded and riveted connections, for example, is their ability to be dismantled.
However, this property is not only an advantage. It can lead to problems, such as accidental self-weakening during operation. Such accidental self-unscrewing, which in the literature is called vibration attenuation, is an important phenomenon. But the most annoying 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 the bolts, and he simply must take such reasons into account when developing reliable connections.
The information that we provide below just talks about the loosening of bolts and threaded fasteners from vibration. And these are the key facts for designers according to the theory of loosening threaded fasteners, as well as about self-unscrewing prevention methods.
Numerous specialty fasteners are described in many engineering publications that are 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 are the basic information about reasons for self-unscrewing threaded fasteners and methods to prevent this phenomenon.
On the causes of self-loosening of bolts, nuts and other fasteners
Of course Vibration is the main cause of bolt loosening.
However, a much more common cause of weakening is side shift of a nut or bolt head relative to the joint, which results in relative motion in the thread.
In the absence of such a phenomenon, the bolts do not loosen even if the connection is subjected to very strong vibration. In the process of detailed study, one can also determine clamping force required for bolts to prevent slipping in the joint.
Often the result of self-loosening of a bolt is fatigue failure, which reduces the clamping force acting on the joint. As a result, slip occurs in the joint, which leads to a bending load on the bolt and subsequently to the failure of the bolt from fatigue.
Tightened bolts (or nuts) rotate freely because there is relative movement between the external and internal threads. This movement neutralizes friction clamp and generates a tightening torque that is proportional to the thread pitch and preload.
Exist three common reasons occurrence of relative movement in the thread:
- Part bending, which leads to the appearance of forces on the friction surface. When slip occurs, the head and threads slip, resulting in loosening.
- Differential thermal effects arising from a temperature difference or a difference in the materials of the parts being tightened.
- Attached connection effort, which can lead to displacement of the joint surfaces, which will cause the bolt to loosen.
The fight against self-unscrewing
In the 60s. in Germany it has been studied that an alternating force applied perpendicularly prevents self-unscrewing.
The study of this issue led to the creation test facilities, which provided extensive information on the tightening action of self-tightening fasteners.
Settings such as Junkers machines(Junkers machines) (you can watch a video about these installations - see the bottom of the article) named after the inventor in the literature, have been used for the past 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 fixatives used by most large companies.
For example, ordinary spring washer is no longer used because it has been shown to actually promote weakening rather than prevent it.
There are many thread lockers. Despite the work of the American National Standards Subcommittee B18:20 on latching fasteners, there are three main categories.
Such as the:
- free rotation category
- friction clutch category
- category of chemical fixation.
To free rotation categories include simple bolts with a circular row of teeth under the head with a shoulder. The teeth are of an oblique type, which allows the bolt to rotate in the clamping direction, but lock into the bearing surface when rotated in the unscrewing direction. Whizlock belongs to this category.
friction clutch category can be divided into two subcategories: metallic and non-metallic. Friction clutch metal fasteners usually have twisted threads that provide 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 include binders 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 may be pre-applied to threads.
In order to determine what is most suitable for the application in each particular case, it is necessary to carefully study the scope and conditions of the future use of the fastener.
To summarize in a nutshell, for example, the category of chemical clamping provides the best protection against vibration weakening, thanks to the free make-up retainer.
In general terms, in order to prevent loosening of fasteners, it is necessary:
1. Make sure that on the intermediate surface of the connection sufficient clamping force to prevent relative movement between the head of the bolt or nut and the connection.
2. Check that the connection is designed to be resistance to indentation and stress relaxation.
3. Ensure that only approved screw locks are listed. This is especially true for thread sealants such as Loctite, flanged fasteners such as Whizlock. Or fasteners dominated by torque, such as Nyloc.
Self-loosening fasteners is just one of the aspects of bolted joint design that every designer must keep in mind during the design process.
As can be seen in the side photo, even if the threads are fully sealed with sealant, this will not fix the problem if there is not enough preload on the bolt to prevent the joint from shifting. The photo shows a partially worn M12 bolt from shear.
Applying the position of drawing analysis to prevent vibration loosening of threaded fasteners is a challenge.
Therefore, many serious companies, such as, for example, Bolt Science (Bolt Science) have developed computer programs to help engineers overcome the problems associated with using connections with threaded fasteners and bolts.
These programs are easy to use, and even an engineer with a smattering of knowledge in the field will be able to solve the problems associated with the above task.
Almost everyone knows what a bolt, nut and washer is. (Exceptions are Neanderthals or babies!)
Especially for Neanderthals, babies and just those who want to remember abstruse words.
Bolt - a threaded fastener in the form of a cylindrical rod with a head, part of which is threaded for screwing a nut
Conditional graphic representation of a bolt. The figure shows a bolt in version 1 according to GOST 7805-70 and GOST 7798-70:
Photo of a bolt with a nut in profile:
Nut - a type of fastener with a hole in which a thread is cut
Usually, nuts are made in a hexagonal shape for a wrench, but they can also be square, round with a notch, with ledges for fingers (“lambs”) or other shapes. The main purpose of nuts, together with a bolt, is to connect parts.
Washer (from German Scheibe) - a part placed under a nut or head of a bolt (screw) in order to create a larger bearing area, reduce damage to the surface of the part, and also prevent self-unscrewing of the fastener
Washers are: round, oblique, crown, spring (grover), locking, quick-release, sealing, end, spherical, etc.
Now, most of all, washers preventing self-unscrewing are of interest.
Spring washer (Grover (German Grower), Grover's washer) - a metal part of machines and mechanisms, in the form of a cut ring. One of the most common parts to prevent self-loosening of threaded connections. Inserted between the nut and the mounting surface.
Well, so we got acquainted with the “main characters”, or someone like me, for example, just remembered buzzwords
In principle, all of the above products should be enough to prevent self-unscrewing and securely fix the product. Many have enough, and many catastrophically not enough.
My uncle had a “motor on wheels”, and so bolts and nuts constantly fell off from him. (From the engine, not from the uncle)
And no matter how he tightened them, and what he just didn’t invent (with the exception of welding), nothing helped. The vibration did its dirty work, loosening even the most heavily tightened joints assembled with any kind of washer (grover, crown washer, notched washer, etc.).
Then I was not interested in why everything was happening like this, but now, in connection with my professional activities, it became interesting. And here's what I found out.
Threaded connection - detachable connection of machine parts using a screw or spiral surface (thread)
This compound is the most common due to its many virtues. In the simplest case, for connection, it is necessary to tighten two parts that have threads with matching parameters. To disconnect (plug) it is necessary to perform the steps in reverse order.
Almost any new threaded connection has a backlash. Try it yourself and you will understand everything. The nut does not sit tight on the bolt! Exceptions are nuts with a plastic ring, or some similar fixing methods.
So, it is because of this backlash that the bolt / nut is unscrewed. Growers and other similar washers may not always help. Yes, and storing a whole bunch of growers in a garage or warehouse is not always convenient. There can be a great variety of sizes alone, but there are no universal ones! More precisely, there is, but this material is from a different category.
Allow me to introduce:
Anaerobic fixative is a one-component material that cures at room temperature, provided there is no contact with oxygen.
The liquid curing component remains inactive as long as it is in contact with atmospheric oxygen.
If the fixative is deprived of access to atmospheric oxygen, for example, when joining parts, rapid curing occurs - especially when in contact with metal at the same time.
This curing can be represented as follows: when the supply of atmospheric oxygen stops, free radicals are formed under the action of metal ions (Cu, Fe), these free radicals contribute to the start of the polymerization process:
Fixative polymerization by anaerobic reaction: with constant exposure to oxygen, the fixative remains in a liquid state (1)
When the fixative enters the gap, the supply of oxygen (2) stops, peroxides are converted into free radicals, reacting with metal ions.
Free radicals stimulate the formation of polymer chains (3)
The cured state (4) is a solid structure with crosslinked polymer chains.
What does it look like in real life and how does it work? I'll try to demonstrate and comment now
And here is our bolt and nut, in company with:
A cleaner is necessary for a stronger connection, as it removes grease and other contaminants without leaving any residue.
These cleaners leave behind a film that impairs the adhesion of the fixative.
While the degreased surfaces are drying, I will try to show what an anaerobic fixative looks like in real life.
For clarity, I took a universal, medium degree of fixation. Why did I choose Permabond A130? Yes, he was just the closest J.
But in fact, all fixators are divided into the following groups:
- Low Strength Retainer - . The connection assembled with this clamp can be disassembled with ordinary tools, without much effort.
- Medium Strength Fasteners - , Permabond A113 ; Permabond A 130. The connection, assembled using these clamps, can be disassembled with ordinary tools, with force
- High strength retainer - Permabond HM129, . The connection assembled with the help of these clamps can be disassembled by special. tool ("grinder" or gas welding machine J) or by heating the joint above +30 ° С
The process of polymerization (curing) of the thread lock, from 5 to 40 minutes. This time depends on the retainer itself, on the metal that the retainer will come into contact with, and on environmental conditions.
The fixing time, on different metals, is different. The most active metal is copper and its alloys (fixation time from 5 minutes), and the most inactive metal, stainless steel and galvanized coating (fixation time from 30 minutes). For inactive metals, I recommend using.
Also, when choosing a latch, you should be guided by other parameters:
- Fixative viscosity (thick or thinner)
- Connection operating conditions (working temperature, working environment, collapsible / non-collapsible connection)
- Desired hold time (requires/does not require connection adjustment)
- Surface cleanliness (degreased or oily surface)
But back to our fixer.
This is what Permabond A130 fixative looks like in liquid form, on a flat surface:
And so, Permabond A130 fixative in liquid form, on an inclined surface:
Remember, I wrote that the anaerobic fixative requires metal and the absence of air. And there is another way, Permabond A905 activator. It imitates the presence of a very active metal and therefore cures even in air:
And this is what the anaerobic fixative turns into after curing:
Reminds me of plastic.
While I'm here "la-la", the cleaner has evaporated and you can proceed to further work.
We take our bolt, take, for example, Permabond A130 fixative and apply it to one of the surfaces.
For blind holes, apply adhesive directly to the bottom of the hole, not to the fastener. If there is a gap, then apply glue to the internal threads of the hole instead:
I applied the fixative to the bolt, as it is more correct, and this is what happened:
Then, calmly tighten the nut:
And here's what happens in the end: 
See that little blue streak? That's right - this is an excess of fixative. More than once I heard indignation from mechanics, the fixative is bad, because it has not hardened.
Certainly! And how does it harden in air and minimal contact with metal?
There are 3 options for dealing with excess fixative:
- Apply fixative in the right amount
- After assembly, remove excess with rags
- Use an activator, and after curing, remove, for example, with a wire brush
We decided on the excess, and now let's get back to what happens between the bolt and the nut.
From a liquid state, Permabond A130 turns into a kind of plastic (we already saw this a little higher) and the initial transformation process took 20 minutes
There are several concepts associated with the polymerization process of anaerobic fixatives:
- Initial strength - the strength at which the product is fixed. It's usually a few minutes
- Working strength - the time after which a newly glued joint can be put into operation. By this time, the joint will have reached ~60% of its final strength, and therefore it can already be subjected to normal loads. Working strength is achieved from several minutes to several hours
- Full strength - strength at which 100% polymerization and the manifestation of all declared characteristics are achieved
And an hour later, when working strength appeared, I picked up a miracle tool and ...
Opened the connection. With effort, but untwisted. The latch was of medium strength.
By the way, it is very important to take into account the diameter and the involved length of the fasteners, if they need to be dismantled in the future.
Doubling the diameter will increase the strength six times!
As you can see, the retainer filled all the voids between the threads. The same thing happened with the thread on the nut:
Having filled all the voids, sticking and gluing both surfaces, the Permabond A130 fixative made it possible to securely fix the threaded connection
Compared to washers, fixation occurs over the entire surface of the thread, and not just under the bolt head or nut. And believe me, the vibration, doing its dirty work, will not be able to unwind this connection. And in case of very active vibration, we have high-strength clamps!
Permabond A130 and Permabond A1046 can be replaced with fixative.
Thanks to a complex of unique properties, EFELE 133 simultaneously glues surfaces together, provides reliable thread fixation and 100% sealing of the joint.
EFELE 133 protects threaded connections from moisture, corrosion and the negative effects of petrochemicals, alkalis, gases and acids, corrosion, prevents spontaneous unscrewing
EFELE 133 Anaerobic Sealant provides an instant strong connection and can be used in almost any operating conditions.
Let this be a purely technical note. Separately, I wrote about a curious story that happened due to - I recommend to read. Here is only technical information, albeit a little. The knowledge gained from the note will be able to protect you from the misuse of such a special liquid as a thread locker.
What are fixators and how do they work?
Threadlockers are dangerous liquids. Improper use of thread lock is guaranteed to lead to undesirable consequences.
Thread locks are red and blue (green). They differ in the principle of unscrewing:
- Red (one-piece) must be heated to a high temperature, without this it will not be possible to unscrew the bolt with such a lock. Withstand higher temperatures. They are used, as a rule, on rotating and high-temperature components (for example, brakes or a bolt securing the main pulley to the crankshaft).
- Blue (detachable) unscrew with great effort. They are used on low-temperature units where the probability of unscrewing is not high. As a rule, they are protected from unscrewing when exposed to vibration (for example, fixing the bracket directly to the engine).
The classification does not always match the color, so the main criterion is detachable / one-piece. Also note that recently, along with detachability, clamps with varying degrees of fixation have also begun to be produced, while remaining detachable or one-piece.
The principle of operation is simple. A liquid is applied to the surface of the bolt, which, when dried, binds the two metal surfaces. A primitive thread lock can be ordinary nail polish. But it should be used only where unscrewing is not critical. In critical nodes, use only a specialized fixer. 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 this bolt simply requires the application of a thread lock. To find out, you need to find the repair manual for your car, and read it. Usually this information is written in the same section as the tightening torques.
If there is confidence, then open the tube with a lock, 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 latch itself will be distributed over the contact surface.
How to unscrew
The blue retainer should unscrew without problems. In extreme cases, you can slightly heat the part, but in practice it rarely comes to this.
With a red fixative, it is often worth the trouble. The first problem is to know about the presence of the red fixative. If you know that a red thread lock has been applied to any units to be repaired, and you are giving the car to, warn the craftsmen so that they do not inadvertently turn the head of the nut or bolt. Usually this is recognized already by the fact of unscrewing. It helps a lot to cope with the red fixative by heating the part with a gas burner. The danger here is that there is rarely anything plastic next to the heated part. To avoid melting anything, make a screen of asbestos, or at least tin, so that direct flame does not fall outside the heated part. And make sure that there is nothing plastic adjoining the heated part. When the part has warmed up, it is necessary to quickly unscrew the bolt, since when it cools down, the red retainer may seize again.
Happy repair.
Author: elremont from 26-06-2015* lock nuts / bolts *
Fixing the nut in some cases is very important. So I'll list a few fixing methods for you and show you exactly how they are used. This is not a complete guide, there may be more methods, but these are the majority of the methods that I am aware of.
*Lock-nut*
The basic way to secure a standard nut is to thread the nut onto the bolt and then tighten to the specified torque with a torque wrench, but for this demonstration we'll just hand tighten it and you could use a counter nut.
That is, the locknut is designed to fix the nut and stop it from moving. After you tighten the first nut, the second nut or locknut will be placed behind the first and then we will tighten it. This will stop the nut from loosening. In extreme cases, with high vibration, this type of nut can actually come loose. This happens quite often when using two regular nuts, although you can buy a specially designed locknut like this one, made specifically for this purpose.
* thread locking compound *
You can use thread locking compound. It is a highly fluid anaerobic liquid. Since it displaces the air around it, it is actually used as an adhesive. You simply smear it onto the thread where you want it to be. Then you tighten the nut... Tighten and then wait for it to dry, this will compress the nut and stop it from moving. Thread locking compound is one of the most common ways to secure a nut or bolt. This is a very efficient way. There are quite a few different brands of compounds, so you have to make sure it's the right one for you. Some of them are so strong that you may need an impact wrench to loosen them. Loctite is a very common brand. I'm sure most people have seen it. So this is a quick and easy way to fix a nut.
* Spring washers *
There are also spring washers, they are made of spring steel, and they are designed so that when the nut is tightened, a load constantly acts on it, which can prevent it from unscrewing. Usually a spring washer is used along with a regular washer. First put the usual, and then the spring. They don’t always put a regular washer when using a spring washer, it depends on the specific location. After that, we bait the nut, and tighten it to the specified torque. This is a very common thread locking method, but in extreme cases, high vibration can cause the connection to come loose. I saw this, the spring washer broke into several parts, which meant that the nut could now be unscrewed. But in most cases, such a washer presses on the nut, which makes it difficult to unscrew it. Therefore, once everything is tightened, it is difficult to unscrew it. And it will work, but it's not ideal for high vibration, vibration weakens this connection.
* Serrated washers *
Serrated washers can be used to prevent some nuts and bolts from coming loose, but they aren't perfect either. The problem is that if you tighten them too much, it can flatten the teeth and they end up just turning into regular washers. Once flat they are pretty much useless. But you can use them in certain circumstances, they are very popular in washing machines because they are very cheap. So you just put the washer on, screw on the nut, and turn it in to the specified torque with a torque wrench. It is important not to overtighten them so as not to flatten the washer, otherwise you will end up with something like this. As I said, they are very cheap, which is why they are used in many washing machines and the like. They can prevent nuts and bolts from loosening, but this is a local solution.
* Self-locking nut *
The self-locking nut has a nylon ring at the top that catches the bolt when you tighten it. That is, you can start turning fairly easily, just like a regular nut, but once you get to nylon, then it becomes difficult to turn, you will need a tool to screw the nut onto the bolt. This can take quite a bit of time because the wrench will need to be turned all the way, with a regular nut you can just turn it down quite easily by hand. So self-locking nuts are very useful, they are used in many places, but it takes a lot of time to turn them. Self-locking nuts are useful, you will find them in all machines, but they are not very suitable for critical machinery, as they can be loosened by vibration.
* Deformed knot *
Sometimes you can't see a nut with a deformed thread on the inside, it's actually only slightly deformed, so it's pretty easy to screw it on initially until you get to the deformed spot, and then it's very tight. You will then need a wrench or socket to be able to tighten the nut. Once you've tightened everything holds up very well, but in extreme conditions, again with high vibration, they can come loose.
* Nord washers *
They are unique in that they have cams on the face of the washers. They should be placed in pairs, like this. That is, you put two pieces together with the cams out and tighten the bolt on them. You can put a nut. Tighten to the specified torque. Once the nut is tightened, these two washers will grab each other and the joint will be very difficult to unscrew. They are very good at vibrating. It is highly unlikely that any of these washers, if properly seated, will come loose with vibration.
* Castellated nut *
They are often used in critical machinery such as aircraft. On board the aircraft, after tightening it, a piece of wire will be threaded through it, and this will prevent the nut from ever being unscrewed. So, to use one of them, you bait it on a bolt, tighten it to the specified torque, or as long as it drags. Now I will drill through the bolt with a small drill bit. But such a drill will work better for larger nuts and bolts than this one. It's stainless steel, so I'll work carefully and slowly. I'll put some more lube on the bit. Once you've done that, you can put the pin in and wrap around it and it will keep the nut from coming loose until you pull the pin out.
Hope this demo was helpful to you. I have outlined some of the methods you can use to stop nuts and bolts from loosening. There are also washers that I don't have, but it's just a washer and a tab that flexes and prevents the nut or bolt from coming loose.
_