Why Do Bolts Need to Be Tightened?

The operation of tightening bolts, including the method and process used, is not decided at the whim of the assembly worker but is meticulously defined by designers according to standard specifications. This includes the selection of tools and the manner in which the operation is carried out.

One might wonder, why is there a need for operational specifications for something as seemingly simple as tightening a bolt? Is the designer just making trouble out of nothing?

In reality, the operation of tightening a bolt is far more complex than it appears. The purpose of tightening a bolt is to apply an initial preload to the bolt, but what exactly is the role of this preload?

Let's explore the function of bolted connections to understand why we need to apply an initial preload to the bolt.

Transmitting Lateral Force or Torque The preload in the bolt can create pressure on the contact surfaces of the connected parts. The greater the pressure on the contact surfaces, the stronger the ability to transmit lateral force. When the contact surface pressure is sufficient, the lateral force is entirely borne by friction, and the bolt only bears the preload, no longer subjected to shear stress and compressive stress, thus significantly enhancing the safety of the bolt.

Sealing Effect When bolted connections are used in pressure pipes or pressure vessels, the bolt must have sufficient preload to achieve a sealing effect. The total preload of the bolt needs to be greater than the fluid pressure between the contact surfaces.

Preventing Structural Gaps Bolted connections subjected to fatigue loads do not allow gaps to form between the connected parts. Once a gap appears, the bolt tension will rapidly increase with the addition of external loads. As shown in Figure 3, the bolt tension and external load curve indicates that in the first half of the curve, where there are no gaps between the parts, the bolt tension increases slowly; once a gap appears, the bolt tension increases quickly.

The appearance of gaps in the connected parts can cause a sharp rise in the amplitude of the alternating fatigue load on the bolt, significantly reducing the fatigue strength of the bolt. The analysis of bolt tightening mainly includes the following three aspects:

Preventing the separation of connected parts and improving the fatigue strength of the bolted connection;

Preventing the sliding of connected parts, resisting external loads through friction, and avoiding shear loads on the bolt;

Preventing internal pressure leakage in pressure vessels.

These three points are precisely the three elements for calculating the required preload for bolts according to the VDI2230 standard, which is the minimum preload required for the bolt. Based on this, designers can determine the appropriate bolt specifications and performance grades, and thus establish the preload for the bolt.