Strength Of Threaded Fasteners

When it comes to creating a strong enough joint that will hold the strength of the fastener is key. It is not common for a fastener to completely fail, but on the occasion that it does happen the effects could be disastrous not only on the project that was being constructed but also possibly in its surrounding area. This is precisely why threaded fasteners undergo a lot more extensive testing to ensure their strength and quality.

While the testing does help ensure the quality of the fasteners and therefore the joint, there are certain situations where testing does not necessarily take place. Usually, this is because the process of testing is both impractical and too expensive to be justified. This is why the quality of your material is often better ensured by choosing quality vendors like AYA FASTENERS to purchase your items for. This could help ease your mind without you having to worry more extensively about the type of testing you might need to take on before you can complete your project.

There are three different modes of threaded fastener failure that you might witness. The first one is when there is a failure through the shank or the threaded part of the fastener. The second on when there is thread stripping of the external thread and the third mode of failure is when there is thread stripping in the internally threaded member.

This type of failure is far more common than people expect. IT usually occurs when there is a fracture in the male thread which is usually caused by the static load being too much for the strength of the thread. In order for the stress area to be determined there needs to be a measurement of the mean of the minor and pitch thread diameters.
This is where an engineering handbook can come in handy as it will usually have within it the table of the stress areas for the different thread sizes that you might be dealing with.
An important thing to remember when it comes to this type of failure is that there are two different types of stresses that you will need to consider. As the bolt is tightened the shank sustain is going to cause direct stress, but there will also be torsional stress caused by the torque acting on the threads. This second type of stress is not usually added in the tables of bolt tightening torques even though it can be the reason for 20% or more of the stress felt by the thread.
There are also certain programs that can be used for the computation of stress. These require some more measurements but can also be more accurate as they provide an analysis of the torque tightening of threaded fasteners.

Thread stripping is a frequent problem with many different types of fasteners, especially those that require low-tensile material.

• In general stripping of all types, both internal and external threads should be avoided in order for a fastener to remain intact for as long as possible and for a reliable fastener to be achieved. It is also clear that in cases where the bolt breaks because of the intense tightening then it is necessary for that bolt to be switched before one can proceed.

• The danger however is that stripping it usually tends to be a far more gradual process. This is where the biggest danger lies as it is possible for stripping to occur after the mechanism has been put in place or has started being in action. The stripping will then cause it to break within an undisclosed period of time which as an effect could lead to a lot of disastrous consequences, especially if the design breaking down causes someone to be injured.

 

What might affect the stripping strength of a thread?

 

There are many different factors that one must account for in order to determine the thread stripping amount. The following five factors play a big role in determining the strength of stripping.

1. The variations in thread dimensions. This can have an effect on the internal and external thread stripping strength depending on whether the variations are major, pitch, or minor.
2. The tensile and shear strength variations in the materials used.
3. Radial displacement of the nut, also commonly referred to as nut dilation, and its effects on the thread's shear strength.
4. The bending of the threads.
5. Production variation in the threaded assembly and its effects on thread strength.
Something to also remember is that the strength of a nut and bolt should not be considered separately as the two elements are completely interdependent. Another thing to consider is the effects of thread bending, nut dilation, or bell-mouthing in the general stripping strength.

What is thread bending?

Normally during the tightening process, there is a certain degree of thread bending that is caused by the tensile force. This process can reduce the area available to both the internal and external threads.

What is Nut Dilation?

During the tightening process, the tensile force present can often lead to a wedging action which can lead to radial displacement or nut dilation. This is common both with threaded and conventional nuts. Much like with thread bending as a result of the dilation, the shear area of both the internal and external thread can be reduced.

What is bell-mouthing?

Bellmouthing occurs when a drilled hole is tapped. The taper on the hole, which is common in all cases of drilled holes to a degree, will usually only extend for around half the diameter of the start of the hole. This is usually caused by the instability of the drill at the time of the hole being drilled. However, it is possible to reduce this as with a closer fitting that is well-aligned it is possible to have a more accurate drill. With holes, the variations while drilling could lead to variations in the lengths of engagement and fine pitches.
Generally speaking, finer threads are expected to cause more bell-mouthing, while having a sharpened drill can be the key to reducing bell-mouthing.

 

What are the Strength Specifications of Metric Threaded Fasteners?

With threaded fasteners, there are specific strength specifications for the different classes of material. These specifications and classes will depend on whether you are dealing with steel bolts, screws, or studs. Normally the strength specifications will cover the following four areas.

• 1. Tensile strength

Tensile stress is the maximum amount of stress that the material can withstand before fracturing or breaking.

• 2. Proof of strength

Proof strength is determined by the maximum stress an object can withstand before it starts appearing deformed in order for the stress to be released.

• 3. Yield strength

Yield strength is the amount of tensile stress a product with 0.2% plastic or permanent deformation is able to withstand.

• 4. Property class

 

The property class is normally visible on a stamp at the head of the bolt or screw. The two figures on it should clearly indicate the tensile and yield strength.
With stainless steel threaded fasteners, the strength specifications are slightly different as there is no yield stress exhibited. Instead, a 0.2% proof test is used in its place.

 

 

What is the weakest point in a bolt?
Normally the weakest point in a bolt is going to be the thread root. This is the area where a large amount of stress could cause a lot of damage.

Is there a way of reducing the stress area?

The stress area is defined by the materials you are using. Generally speaking, the finer the threads, the higher the stress area, the coarser the stress the more reduced the stress area will be. It is always possible to calculate the tensile stress area with the measurements of your bolts and nuts.

Conclusion:

There are a lot of different things that need to be considered in the strength of threaded fasteners. The most important of these include the tension that each component can take not on its own, but rather when joined. This is because the nuts and bolts are completely interdependent in the joint. The material you are using in the two individual components could also have an effect on the strength which is why it is crucial to use the strength specification tables to determine which products can withstand the type of stress you will be putting them under. This will help reduce the chances of fasteners failing.

AYA FASTENERS is one of the most reputable leading fasteners manufacturers and suppliers in China, we focus on assembly and fastening materials, supplying global customers with high-quality screws, bolts, washers, and nuts products over 20 years.

AYA has been deeply involved in the Fastener industry with a single-minded and dedicated attitude, devoted itself to providing our customers with more industry-specific, professional, standardized, and precise Fastener solutions. If you want more information, please contact AYA FASTENERS for the details.