After tightening, is the torsional energy in the bolt shank released?

During the tightening process of a long bolt, torsion occurs in its shank. Since torsion exists, deformation must occur, and this torsional deformation inevitably stores a certain amount of energy.

So, after the tightening operation is completed, will the energy stored in the shank due to torsion be released, thereby causing loosening (back-off) of the bolt head? If the bolt head does loosen, will the torque or preload be reduced as a result?

Energy Distribution and Storage Mechanism During Tightening

1. Torque Distribution and Energy Conversion Mechanism

When tightening a bolt, the applied torque is primarily converted into the following three forms of energy:

• Preload work (approximately 10%): This portion of energy causes axial stretching of the bolt shank, thereby storing elastic strain energy and generating the required preload effect.

• Thread friction energy dissipation (approximately 40%): This portion of energy is mainly consumed by friction at the thread contact surfaces. Friction generates heat, which is ultimately released in the form of thermal energy.

• Under-head friction energy dissipation (approximately 50%): This portion of energy is consumed by friction between the bolt head/nut and the surfaces of the joined components.

During the tightening process, the bolt shank undergoes torsion due to the thread torque. The energy stored from this torsion is also a form of elastic strain energy and accounts for only a small portion of the total energy (existing simultaneously with the tensile strain energy). Its magnitude can be expressed quantitatively as:

Among them, T represents torque, l represents the shank length, G represents the shear modulus, and Ip represents the polar moment of inertia of the cross-section.

Therefore, after the tightening operation is completed, this portion of energy remains stored in the bolt shank. This torque is transmitted via the thread torque, so it can be approximately regarded as the thread torque. Typically, the thread torque is less than or equal to the bolt under-head torque, so this torque does not dissipate immediately.

However, according to the VDI 2230 standard, after the tightening process is completed, when the bolt is subjected to external loads, especially shear loads, this torsional shear stress will decrease, and may even drop to zero. Nevertheless, for conservative calculation purposes, it is generally assumed that 50% of the tightening thread torque remains.

Analysis shows that this portion of energy will gradually dissipate at the contact interfaces between the bolt head and the threaded portion. However, from a microscopic perspective, there is essentially no detectable relative rotation between the bolt head and the threads.

2. Storage Form of Torsional Energy

During the tightening process, the shank simultaneously experiences normal stress in the tensile direction and shear stress in the torsional direction, resulting in a combined stress state. The torsional strain energy is stored within the material as elastic potential energy. Theoretically, after the tightening action ceases, there is a possibility of its release, similar to how the energy stored in a spring is released.

Here is the English translation of your text:

**Does the release of torsional energy cause bolt loosening?**

**1. Delay and Dispersion of Energy Release**

- **Friction-dominated energy dissipation:** The moment the tightening action stops, the static friction at the thread flanks and bearing surfaces quickly balances the residual torsional stress, effectively preventing rotational back-off caused by energy release. Experimental data indicates that if the friction coefficient is sufficiently large (it is generally believed that when the bearing surface friction torque is higher than the thread torque, significant rebound will essentially not occur. However, in practice, some bolt connections do exhibit a slight rebound of the socket wrench at the exact moment tightening stops—has anyone else made a similar observation?), the torsional rebound angle is negligible.

- **Limitations of the rebound effect:** Even if a tiny amount of elastic rebound occurs (e.g., less than 0.5°), the rebound angle is far smaller than the critical value required to cause a significant drop in axial preload (for example, tightening a bolt typically requires a rotation of 50° or more; therefore, such a tiny rebound is essentially negligible).

- **Preload does indeed decrease:** This phenomenon has been observed during ultrasonic measurement of bolt preload. After bolt tightening is completed, continuous preload measurements show a persistent decrease. Analysis suggests that, on one hand, this is due to the gradual dissipation of heat generated during tightening, with the resulting temperature drop causing a reduction in the measured preload. On the other hand, could it also be caused by the phenomenon described above? That is, immediately after tightening, the bolt gradually releases its torsional elastic deformation, thereby causing a drop in preload. However, under normal circumstances, this preload reduction is not substantial, typically within 2% of the preload at the moment of tightening.

**2. The Primary Cause of Loosening is Interface Slip, Not Energy Release**

Both theoretical and experimental studies have clearly shown that the main cause of bolt loosening is micro-slip at the contact interfaces (e.g., reciprocating fretting between thread flanks or between bearing surfaces), rather than energy release from the shank. Examples include:

- When transverse vibration causes slip at the bearing surfaces, the thread flanks gradually rotate and relax in a "stepwise" manner.
- Finite element simulation results indicate that if the friction torque at the bearing surface is lower than the friction torque at the thread flanks, the bolt is more prone to loosening under torsional loading.
- Thus, the torsional rebound caused by energy release from the shank does not have a significant impact on bolt preload.

**3. Reverse Torque Must Overcome Static Friction**

For the torsional energy stored in the shank to be converted into reverse torque, T_reverse ≈ T_tightening. However, this reverse torque must exceed the static friction torque at the contact surfaces to drive bolt rotation. In practice, static friction must be overcome after tightening, and the static friction torque is generally greater than the kinetic friction torque during tightening. Therefore, the condition M_friction > T_reverse is always satisfied, meaning bolts generally do not undergo spontaneous loosening.

**Engineering Measures and Experimental Research**

**1. Key Factors for Improving Anti-Loosening Performance**

- **Increase friction coefficient:** Increasing the friction coefficient at the threads or bearing surfaces (e.g., reducing lubrication) can significantly enhance anti-loosening torque. Experimental data shows that for unlubricated bolts, the residual preload decay rate (8%) is significantly lower than for lubricated bolts (31%).
- **Improve head design:** Hexagonal flange bolts, due to their increased bearing surface friction radius, exhibit the best anti-loosening performance (residual preload decay of only 6–12%).

▲ Table: Comparison of anti-loosening performance of bolts with various head configurations

**2. Controversy Regarding the Effect of Spring Washers**

Experimental results indicate that under low preload conditions, spring washers provide some degree of anti-loosening effect. However, for high-strength bolts (e.g., Grade 8.8), spring washers may reduce contact stability and thereby weaken anti-loosening effectiveness (residual preload decay is 10% with a spring washer versus 8% with a plain washer).

**3. Energy Release from Torsional Rebound**

Based on the above analysis, it can be concluded that the energy generated in the bolt shank due to torsion does not significantly affect preload.