Threaded connections, such as bolts and nuts, are used in a variety of applications, from plastic toys to large bridges. One of the similarities in connecting them is that these connections need to be kept together when we want them to stay together, but separated when needed. For example, the valve caps on the engine must remain in place while driving, but we also need them to be removable so that we can repair engine parts.

This article will explore different ways to ensure that these types of connections stay together when we need them. The term "thread lock" is generally used to ensure that the thread remains in place without loosening, and will be used frequently in this article.

There are many types of threaded connections, but there are mainly the following:

-Bolts and nuts-This is probably the most common and lowest cost threaded connection type. It is used in applications ranging from cars to cellular tower antennas. The following figure shows examples of bolts, nuts and flat washers.

-Screws and threaded holes-with this type of connection, we install the screw into a hole that has been drilled and tapped into the housing or mechanical part. The most common difference between a screw and a bolt is that the screw usually enters a threaded hole, while the bolt passes through 2 unthreaded components and is fastened with a nut. The following figure illustrates the screws fixed in the threaded aluminum extrusion.

We can see three cap screws, all of which are used for tapping aluminum parts. These can be used in place of bolts to achieve accuracy, ease of assembly and reduction of component clearance-possibly a combination of the factors we have already mentioned.

-Threaded parts-These are the most common shafts or housings (usually cylindrical), they are all screwed together. Flashlight bulbs and battery tubes are a good example. The figure below illustrates the concept of two cylindrical housings threaded together.

The last idea I want to talk about is the concept of torque and preload. The threaded parts are screwed together to generate some form of axial pretension. The torque is measured in units, such as in-lbs or Nm, but it is always just the force that "twists" the components together. For threaded parts, this torque collectively produces an axial force called preload. The following figure shows the pictures before and after tightening the bolts. The red lock washer is in an expanded state, but when the components are twisted together, the lock washer is compressed and behaves as a spring.

In some cases, the friction generated by this pre-tightening force is sufficient to hold the fasteners together. But in other cases, we need to use thread locks. Some of these conditions are high vibration, expected stress relaxation (plastic creep) and cyclic stress.

Thread locking can be divided into the following categories, which use mechanical objects to prevent loosening or adhesives to prevent loosening:

Mechanical hardware-These are the physical components used to prevent the loosening of threaded fasteners. Some common items used are:

-Lock washers-There are many styles of lock washers, but they are all placed under the head of the bolt or screw to prevent it from loosening and maintain the pre-tightening force. The figure below is a split lock washer.

This type of washer acts as a spring when tightening the bolt. Even if the screw is slightly loosened, the spring action will maintain a certain pre-tightening force. However, these components may be less effective under high vibration conditions. Another type of lock washer is Nord-lock, which relies on mechanical bevels and teeth to prevent loosening, as shown below:

The teeth bite into the bolt or screw head, and the material under the washer, and the direction of the bevel makes them "climb" in the direction of loosening the fastener. In other words, to loosen the bolt, you must increase the preload while overcoming the slope. These are effective but relatively expensive, and are usually used for larger hardware. There are many other lock washer styles, so I encourage you to explore other options.

-Nylon insert lock (nylock) nuts-They have round nylon inserts that lock the threads in place by deforming on the mating threads. They may not maintain the peak preload under all vibration conditions, but they usually maintain the nut even if it loses some preload. The image of the nylon insert nut is shown below.

These are usually used in high vibration environments for non-safety critical applications, and you still want to prevent the nut from loosening and falling off. For example, this type of fastener is used for my suspended mountain bike frame as shown below.

In this application, if the preload is lost, there is no direct safety hazard, because the screw still supports the load in the double shear. However, if the screws fall out, there may be safety issues, so we need to make sure that the bolts stay connected.

-Safety lines-the use of safety lines is very common in aerospace, defense and racing. It works by passing a wire through a hole in a screw, bolt or nut, and then anchoring it to another object. Security threads are usually applied between a series of fasteners to hold them together. The following figure illustrates this type of application in detail.

We can see that due to the direction of the wires, one nut must be loosened and the other must be tightened. It is very important to remember this, because a safety wire installed in the wrong direction is almost completely useless.

-Locknut/Safety Clip/Retaining Pin-There are multiple iterations of the locknut, but they all operate in a similar way. Some kind of pin passes through a threaded shaft or bolt to prevent the nut from retracting. The image below illustrates a castle nut:

In this application, a hole is drilled in the bolt so that the split pin can pass through the slot in the cage nut. This prevents the nut from rotating in either direction to prevent loosening, but it still allows the pin to be removed and the nut to loosen if necessary. The following figure is an example of a lock nut used to hold the hub in place:

In many cases, adhesives are used to hold the threads in place. In many cases, they are even used in combination with the above-mentioned mechanical methods. In the oil and gas industry, where extreme vibrations occur in the drilling environment, lock washers and adhesives are often used. The adhesive can be broken down as follows:

-Threadlocking agents (Loctite, Vibratite)-this category can contain a variety of adhesives, but they are all used to bond the threads in place. Loctite is the most well-known brand in the United States, but there are also many options. The figure below shows an example of Loctite 242 (blue) applied to a bolt.

It should be noted that Loctite has a variety of grades, some of which are not removable. You must make sure to use the correct level for your application.

-Dry patch-literally means a dry patch pre-applied to the thread as a thread locker. In some cases, it is just a dry nylon, and in other cases, it is a reactive adhesive. The image below shows the dry patch of the threadlocker/sealant combination used to seal the pneumatic relief valve.

The dry patch can be used to eliminate the process variability associated with manual dispensing of threadlocker, or it can be used to improve the logistics of loose hardware that requires threadlocker. For example, I recently assembled some furniture, and all the hardware has dry spots. The manufacturer knows that this will increase the reliability of the component and eliminate the need for the end user to use liquid thread locker.

I have summarized these methods in a quick reference chart, which will help you start looking for the right thread locker.

As we have seen here, under normal operation, there are many ways to prevent the threads from loosening. However, it is important to choose the right method for the application. Although the diagram above will help you get started, you should do more research before you decide on the final solution. When choosing your method, browsing through these options with vendors and colleagues will help match your application to yours. If you test several different options to see which options provide you with the best combination of attributes, you can further optimize your design.

This article is provided by Fictiv, the most efficient parts manufacturing platform. Supported by a distributed network of rigorously vetted suppliers, an online interface makes it easy for customers to get real-time quotes, view manufacturing feedback, and manage orders-all through a single service.

I don't know if Spiralock counts as one of these technologies, but Spiralock should be more widely known. The Spiralock thread places a shoulder that is ground to a slightly different angle on the entire thread, so the entire bolt or screw is subjected to a compressive load along its bearing. When the screw is tightened, it is locked in place by friction while still allowing easy rotation until it is Tighten up. Finite element analysis shows that this eliminates the stress concentration along the topmost thread that occurs in conventional screws and bolt threads. This was invented more than ten years ago. I really think it deserves to be more widely known, because stress concentration is the source of material fatigue and failure, and because the stress concentration propagates cracks, the thread on the screw basically causes the screw to fail at the root of the thread.

To continue this idea, here is a video explaining the Spiralock thread.

I can add that the spring lock washers mentioned here and all other spring washers are no longer part of the DIN specification because the proven effectiveness is as low as zero. However, there are many other lock type washers (including nordlocks mentioned).

Correct, the spring washer is invalid, which has been proven since 1969. http://www.boltscience.com/pages/helicalspringwashers.htm

I don't care what tests are performed. Based on my past experience, using spring washers has never disappointed me. More than once. I have used them for more than 20 years. If I use bolts and nuts instead of bolts and nuts, it's completely different.

I also think that the tension bolt is a thread locking mechanism. 

You omit the locking spiral, the locking plate (insert a metal plate under the bolt head, and bend one corner up to the plane of the bolt head and down to the side of the flange to prevent the bolt from retracting), and fast and Dirty but very effective tack welding.  

These are also great locking mechanisms, thanks for sharing!

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