SYN] Magazine
#2 | 2019

Efficient Tightening

Introduction to Tightening Technology

Just tighten it? We all know the situation from our own homes. We only need to tighten a few screws and a piece of furniture is ready for use. Often, things are not so easy in industrial production. Even if a bolted joint is not safety-critical, loose components almost always cause higher recall, guarantee and reworking expenses. The joint may loosen or the screw may break as it is tightened – this is anything but efficient. 

In the worst case, a faulty joint may even cause injury or death. You only need to think of what can happen if the suspension of a car becomes loose at high speed. So what does “efficient tightening” mean? The answer is provided by physics and we will give you a brief explanation. >>

Now Available!

You will find an overview of our tightening technology seminars and workshops and all our product training events in our current training catalogue. 

Interested? You can request our digital catalogue by email.

1. Torque

To tighten a screw or bolt on the thread, force needs to be applied in the form of torque. The screw is tightened by applying the correct torque until a durable, safe joint is formed. The torque to be applied depends on the various forces to which a screw or bolt is exposed.

The clamping force on the joint is the result of the torque applied to press the components together. The tensile force is applied parallel to and in the opposite direction to the clamping force. It tends to pull the joint apart. In addition, shear forces are transmitted by the various parts. If the parts slip in relation to each other, they pull the joint sideways, exposing the bolt to stress. A secure joint should be designed so that the clamping force between the components is higher than the tensile and shear forces to which it is exposed. The pre-load corresponds to the load on the bolt following tightening. It must be sufficiently high to absorb the entire load for which the joint is designed. Otherwise, the joint may loosen under this load and the bolt will be ruptured.

2. Rotation angle

A higher stage in quality assurance can be reached by measuring the rotation angle in addition to the torque. In more complex assembly processes, the angle is defined to ensure that a bolted joint is tightened up to its yield strength. The yield strength is the point of transition to plastic deformation of the bolt. When this point has been passed, the deformation of the bolt can no longer be reversed.

3. Friction

In addition to the various forces, friction also plays a key role. The force applied in the form of torque must overcome friction under the bolt head and friction between the nut and bolt threads. This friction is the result of a number of factors including material and surface properties (e.g. coating or lubrication). In the case of a non-lubricated bolted joint, only 10 percent of the tightening torque is transformed into clamping force; the remaining 90 percent is required to overcome friction.

4. Relaxation behaviour

After it has been tightened, any bolted joint is subject to relaxation. Initially, this is a very fast process. After 30 ms, the clamping force in the joint is already lower than immediately after tightening. In the case of joints with elastic components (“soft” tightening operation), this loosening may be very severe. Relaxation is less apparent in the case of “hard” joints. In both cases, a special tightening sequence or subsequent re-tightening may be necessary.

The answer? Tightening analysis! 

A tightening analysis is the optimum way to identify the cause of defective joints and to determine the appropriate tightening strategy for safety-critical or process-critical joints.

For analysis of this type, Atlas Copco Tools has a specially equipped tightening laboratory where we use a wide range of test procedures to determine the behaviour of a tightened joint under realistic conditions. Following the analysis, customers are informed of the results and we provide recommendations for the optimization of joints. We are also pleased to provide support for the implementation of a production standard. For example, it may be beneficial to develop and document individual test specifications. 

The “human factor”

In this context, the importance of the human factor should not be underestimated. It is important not only to know what makes up a secure bolted joint but also how the joint should be tightened. Our use case oriented tightening technology seminars and courses support customers to develop the competence of their employees in a sustainable way. Tool failures and tightening problems caused by human error can be avoided and awareness of quality issues will be raised.

Just take a look at our new training catalogue and select the appropriate training for you and your company.

Know-how by Atlas Copco Tools 

Industrial tightening technology is a complex topic. We have the right people for you. The Atlas Copco Tools team of tightening specialists covers optimization potential in the field of tightening systems and provides you with the support you need to optimize the quality and efficiency of your production operations. If you are interested in further information, please contact us.