A New Approach To The Tightness Checking Of Bolts
Feb 06, 2023
One of the biggest mysteries when it comes to bolt tightness is how it sometimes appears that the bolt you just tightened, is far less tight than what you originally remember tightening it to. While the first time this happens you might feel confused, in reality, it is a completely normal thing to happen. What many people are not aware of is that the torque of a newly tightened bolt is in fact different depending on whether you are trying to tighten or untighten it. Naturally, this will leave you with one question in mind.
Exactly how different is the torque when tightening and untightening a nut, bolt, or fastener? EMAIL: SALES@AYAFASTENERS.COM
In reality, the difference can be quite vast, with the torque being between 10 and 30 percent less than what it would have been if you were trying to tighten it further. Therefore, if you were simply trying to untighten the fastener to figure out whether the tightness had been enough originally, you will most likely find that this technique is not the way to go about it
Should I untighten the fasteners to check the tightness? EMAIL: SALES@AYAFASTENERS.COM
The short answer to this question is No. For the most part, untightening a tightened fastener is far easier than it would be to tighten the fastener further. Therefore, you will get a false view of how tight the fastener is depending on whether you are trying to untighten it or tighten it further. As a result, you most likely will need to use a different technique if you want to actually find out how tight a bolt is.
What causes this difference in the torque when tightening and untightening bolts? EMAIL: SALES@AYAFASTENERS.COM
If you are trying to understand better the reason why tightening and untightening torques are so different you will need to understand the role that friction plays in these processes. When trying to tighten a fastener, you are automatically trying to overcome the friction in the threads. Therefore, as you are tightening the bolt, by rotating the bolt head the process will require more and more friction with each rotation. Normally, the proportion of torque needed to overcome friction falls somewhere between 85 and 90 percent. When untightening the friction that needs to be overcome is lesser. This is also the reason why it is possible that your fasteners will come loose over time.
What is a clamping force, and why is it important? EMAIL: SALES@AYAFASTENERS.COM
When tightening torque there is a certain clamp force that is produced. In many cases, the clamping force can be the key way of determining the integrity of the structure and assembly you are connecting. This is why normally a lot of attention is put on making sure that the connections are made in the most controlled manner possible. Part of that control also includes predicting what the clamping force is going to be.
How can you control the tightening of a threaded fastener? EMAIL: SALES@AYAFASTENERS.COM
The most popular technique to control the tightening process is by using a specific tightening torque. As the relationship between the applied torque and clamp force is linear, whatever change you make to the torque will equally be reflected in the clamping force. For example, if you choose to double the torque then the clamping force will also be tackled. If on the other hand, you actually choose to halve the torque then you will find that the clamping force is also halved.
However, after the fastener has been tightened it is possible for the clamping force to decrease.
What causes the clamping force to decrease after tightening? EMAIL: SALES@AYAFASTENERS.COM
The clamping force decreases after you have finished tightening the fastener can be caused by a number of different things. In fact, the decrease in clamp force does not need to be related to a rotation of the thread, and it could instead be the result of stress relaxation, creep, self-loosening, or embedding.
Does the decrease in clamp force mean that further checks are necessary? EMAIL: SALES@AYAFASTENERS.COM
The decrease in clamp force can be a concern as it could lead to loosening bolts. Loosening bolts could cause a problem as the assembly is going to naturally not be as sturdy as it should have been. As such it is necessary for further tightness checks and quality control checks to take place in order to ensure that the construction or assembly is going to remain up to par. While oftentimes people will choose to measure tightness by looking at the measured torque value, in the majority of cases looking at the clamping force can actually be a better assessment factor. There is only one problem with that. Currently, there are no cost-effective methods of assessing the clamping force, as such most testing is usually completed through torque-based methods.
What are the torque-based methods predominantly used for checking tightness? EMAIL: SALES@AYAFASTENERS.COM
There are three main approaches to the process known as torque auditing or tightness checking. These are:
On-torque method. The On-torque method requires the rotation of the bolt or nut, by a 2 to 10-degree angle in the tightening direction.
Off-torque method. The off-torque method simply requires the untightening of the bolt or nut. Normally this is easier than trying to further tighten a bolt or nut.
Is there a better method for checking tightness? EMAIL: SALES@AYAFASTENERS.COM
In this article, we will suggest a method that will allow you to assess the clamping force instead of focusing on torque. This should be a better method as the clamping force does not change over time regardless of the conditions that the connection is being kept in. As such, it is far more likely to provide you with an actual depiction of the tightness levels.
This method requires you to go through a three-step process of tightening-untightening-retightening the bolt. This is also often referred to as the 'On-Off-On' method. One of the key reasons why this method works better is because it can provide information regarding the clamping force rather than the torque. The clamping force is a much better way of ensuring the structural integrity of your connection, which is why it is a much more fitting way of checking the tightness of a connection.
When auditing tightness through the ‘On-Off-On’ method you will find that there are two ways of making the process even more accurate.
The first of these is to use well-lubricated fasteners so that there is a lower friction value. The second would include conducting the auditing process within a short timeframe after the installation of the fastener.
Non-lubricated fasteners with high friction are less likely to allow you to get an accurate reading when checking the tightness of a connection. Similarly, the longer you wait to actually check the tightness following the fastener installation, the most likely you will be to get an inaccurate result. This is mainly because it is more likely that corrosion has occurred between the original installation and the time of testing.
The “On-Off-On” tightness auditing method’s biggest advantage is that it can provide you with information on the tightness of a connection without any significant change taking place in your work practices. It will also not cause any problems that could lead to the integrity of the joint being harmed.
Conclusion:
When it comes to tightness checking, on-torque techniques appear to be the most commonly used. However, these techniques do not always provide an accurate view of how tight a connection which in turn could cause more harm than good. As such, a different technique is often necessary if you want to ensure that your connection is strong. A clamp force measuring method like the ‘on-off-on’ technique can therefore be the perfect solution for those who want to check the tightness of a fastener in a reliable manner.
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