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There’s many different ways manufacturers can secure components together: welding, crimping, riveting, glueing, soldering, etc. 

Yet, as is often the case, simpler is better, and the most common method is the use of screws and bolts to clamp different parts together; the automotive industry alone uses over 30 billion fasteners per year in the United States alone.

Why is that? 

Besides their simplicity of design (there’s a reason the concept has been around since Ancient Greece!), fasteners have four major other advantages

  • High level of clamp force: their effectiveness in joining parts is often underestimated; most are surprised to learn that a fastener with a diameter of only 1/4 inch can provide a significant amount of clamp force, up to 4,000 pounds. 
  • Ease of assembly & disassembly: fasteners follow UN and metric (M) standards and can be easily installed and removed, allowing for easier maintenance and repair. 
  • They can be used on dissimilar materials and joints, something that’s difficult to do with adhesives or welding.
  • They are very cost-effective.

Overall, fasteners provide a cheap, reliable and efficient method for joining critical components.

The importance of high-quality manufacturing practices

However, a fastener’s reliability depends on proper installation. Consider, for instance, Xiaomi’s high-profile recall of 10,000 electric scooters in 2019, after several reported cases of the handles breaking off while in use (fortunately with no fatal accidents).
The issue turned out to be due to a fastening-related defect that caused a screw in the folding apparatus to loosen, ultimately causing the vertical component of the scooter to break off.
While details on the exact cause of the loosening screw in this case were not made public, there are best practices that manufacturers can follow in order to ensure the proper tightening of every single bolt and screw installed in their production line.
But before we outline them, let’s take a look at some basic science behind fasteners and their properties.

Preload and torque

The clamping force that is generated in the fastener when it is tightened is known as prestress or preload.
When a screw is tightened, the force applied to the head of the screw is transferred to the threads, which then generate an equal and opposite force on the walls of the threaded hole or  mating component. This force results in a tension, or pre-stress, in the screw that holds the joint together, resisting external forces and maintaining its position. 
It is absolutely crucial to ensure that the clamping force is sufficient for the intended application, as insufficient clamping force can result in joint failure or loosening, while excessive clamping force can damage the joint or fastener.

Because there is no practical way to measure clamping force in production environments, tightening torque is typically used instead, as there is a direct relation between clamping force and tightening torque. As more torque is applied to the fastener, the higher the clamping force on the joint. Variance in friction between joints, as well as other environmental factors, means there will be variations in the clamp force produced at the same target torque between joints, but these variations are typically small enough that the tightening torque method is still the preferred one.

By specifying a particular torque value for a given fastener and application, fastener specialists can help ensure that the screw is tightened to a consistent level of tension, which, as we saw, is key to maintaining the clamping force and stability of the joint without causing any damage.

But how can we know that the joint has been properly tightened to the specified torque?

Measuring torque

Torque can be measured on a fastener using either static or dynamic measurement techniques.
Static torque measurement means the residual torque is checked after the tightening process has been completed. 
This can be accomplished with a click wrench that, having been set to a specific torque, will release with a click once that torque value is reached, or, for more accurate measurements, with an electronic torque wrench. For more information on measuring residual torque, you can read our article on the topic.
Dynamic torque measurement, on the other hand, involves continuously measuring torque while the fastener is in motion, and is the most common choice in high-volume production environments like assembly floors, where power tools are needed in order to increase productivity and reduce the risk of operator fatigue and error. 

Electric screwdrivers with torque and angle control like Kolver’s PLUTO and K-DUCER lines can be programmed to simultaneously handle different types of joints and tightening sequences and to measure the torque and the rotation angle of the screw in real time, stopping automatically at the desired torque. Furthermore, they can be programmed to prevent the operator from proceeding with the assembly process unless all tightening requirements have been met for each screw, avoiding costly defects and recalls, and resulting in increased efficiency and productivity.

The big picture

Overall, fasteners are widely used in the assembly world because they are flexible, easy to install & uninstall, modular, adjustable, and cost-effective. While measuring actual clamping force is impractical in a production environment, using torque control tools can still allow for a consistent and repeatable tightening process and ensure the integrity and safety of the final product.

For the most demanding and safety-critical applications, more advanced tightening strategies can be employed, which we will cover in the next articles of this series.