Quality Control in The Tightening Of Bolts

Self-loosening or loosened fasteners are frequently described as one of the most common reasons for manufacturing or a problem with the construction or project you have created. According to a survey by the United States automobile service manager, it was disclosed that 23 percent of all service problems could be traced to a loose fastener. More importantly, around 12% of new cars were also found to have loose fasteners. This loosening of fasteners can be a big issue in all mechanical industries as it will frequently also mean that the functioning of the machine must stop.
If you want to avoid the loosening of bolts, the best thing to try is quality controlling the tightness of the bolts. There are many elements to this process and many different things to consider, but being able to quality-control the bolts can help you make sure that your projects will remain steady in the long run.

Is the fastener being used tight enough when the bolt stops tightening?

One of the most common misconceptions is assuming that just because you are unable to tighten a bolt any further, that bolt is properly tightened. In reality, though this could not be further from the truth. This is because it is not just about the amount of tightening but also the method of tightening. A single both that is inaccurately or incorrectly tightened could potentially lead to the entire fastener failing over time. High tightening torque specifically will mean that your fastener has an increased risk of a bolt shank or thread stripping failure. Low high tightening torque on the other hand could mean that the bolt tension does not mean the necessary functional requirements. As a result, the fastener is not going to be able to perform its function in the long run.

How to tighten a bolt correctly?

Tightening a bolt correctly will mean that you are using the correct torque, that is rotating force in order to tighten the bolt. To determine the correct torque, you will need to take into account the bolt diameter, bolt grade, and the nut factor, also referred to as the friction coefficient. These three components will help you better understand how much tension is needed for your fastener to be properly tightened.

What is the most common method for tightening threaded fasteners?

The most used controlled method of tightening threaded fasteners is through tightening while achieving a specified torque. Torque control, as this method is called has one problem which relates to the clamping force generated. In this method, it is important to remember that the applied torque widely depends on the fastener's design, and often times the correct torque might even be disclosed in the bolt instructions.
While torque control is not a perfect method it continues to be popular for those who want to tighten their bolts as efficiently as possible.

What are the methods for quality controlling the tightening of bolts?

There are three popular methods for quality control of tightened bolts. These vary in both their popularity and applicability and depending on the project you are working on you might find that one or the other might work best in terms of controlling how tight your bolts are. These three methods are:
1. Dynamic Torque Auditing
2. “Crack-On” and “Crack-Off” Methods of Torque Auditing
3. Marked Fastener Method of Torque Auditing

Q: What is Torque Auditing?

A: Torque auditing is a method that ensures that a torque-controlled tightened bolt complies with the specified torque set by the quality control in a company or production team. Usually, this technique is used by manufacturing companies who want to perform in-house audits to ensure that their fasteners and products are up to par. There are two approaches to this, the first is dynamic torque auditing which requires an in-line transducer to be attached to the tightening tool so that during the installation it can be ensured that the torque is measured correctly. There are also approaches that would require an operator or inspector to inspect the torque after the installation has been completed.

Q: What is Dynamic Torque Auditing?

A: The dynamic method usually requires powered torque tools that can be used along with data collection systems. This method will essentially allow you to automatically collect all the data during the tightening process which you can then process to determine if the bolts have been tightened properly. One of the big disadvantages of this method is the fact that it can be rather high in cost and requires a lot of specialized technical skills and expertise. It is also not a method that you can apply to check the quality of a fastener after assembly usually.

Q: What is Torque Auditing after Assembly?

A: Dynamic Torque Auditing which was covered apart is only an effective way of quality control if you are performing the check before assembly is complete. However, there are occasions in which you might find yourself needing to check the quality of a fastener that has already been assembled. As such, there are different methods that you can use for this process. These three include:
1. The “crack-on” method in which the socket is moved away from the tightened position in the tightening direction.
2. The “crack-off method” in which you move the socket in the untightening direction.
3. The “marked fastener method” requires you to retighten the socket to a mark position.

When using any of these three methods the most important thing is applying the proper torque. To successfully evaluate the tightening of your fastener you are going to need to apply the torque in a slow and deliberate way in order to ensure that the bolts are securely put in place.
Another important aspect of this process is that you should ensure the torque reading is checked as soon as possible after the tightening operation, and before any other process takes place. This will help ensure as little variation to the threads as possible making it more certain that you are evaluating the quality of your tightened bolts appropriately. It is also important to remember that torque values can vary up to 20% if they are left for two days.

What are the "Crack-On" and "Crack-Off" Methods of Torque Auditing?

The "crack-on" and "crack-off" methods are two variations of a similar method for controlling the quality of a tightened bolt. In both the case of the “crack-on” and the “crack-off” methods there are two ways in which the check can be performed.
The first of these is completely manual and it will require an operator with the knowledge to assess or “feel” the breakaway point when it occurs.
The second does not require an operator instead you will need to use electronics and a strain-gauged torque wrench that will help you automatically record the breakaway torque. This method can often yield better results as it removes the operator's performance as a variable and instead streamlines the process.

What is the Marked Fastener Method of Torque Auditing?

The Marked Fastener method is considered to be a lot more consistent and accurate than the other methods for quality controlling or checking the applied torques after the installation has been completed.
In order to effectively use this method, you will need to mark the position of both the bolt and the nut in relation to the joint. From there loosen the angle of the fastener by around 30 degrees. From there measure the torque required for the fastener to return to its previous marked position. This is the most reliable method because it works entirely with the previous torque.

What can affect the tightening of bolts?

Even when you have finished with quality controlling a fastener or bolt torque there are certain things you will need to keep in mind to ensure that your fastener will stay in place. The most important variables to remember when it comes to bolts losing their tightness include temperature changes, humidity, corrosion and its effect on torque value and friction, and vibrations. All of these elements could effectively lead to your tightened bolts losing some of their tightness even after they have been quality controlled.

Conclusion:

When it comes to quality controlling bolt tightness there are different options available. The method you will use will widely depend on whether you are checking the condition after installation or before. This can widely affect the type of data you can gather as well as change the methods you have at your disposal. What is important to remember is that even after you have finished checking the tightness of a bolt things can change as many different variables will often cause your fasteners to self-loosen. Thus, maintenance is also an important step to ensuring quality.

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