Why does the torque of a tightened bolt gradually decay?

Many mechanics, sheet metal workers, and equipment assembly technicians have encountered the same problem: the bolts were clearly tightened to the required torque during factory commissioning, yet after a few days, weeks, or even months, the torque quietly decreases, resulting in loosening, abnormal noise, and clearance shift. Many people’s first reaction is “the screws weren’t tightened” or “the worker didn’t operate properly.” But in fact, most bolt torque decay is not an assembly error; it is a normal phenomenon caused by material properties, structural loading, and environmental changes.

Today, let’s explain in plain language: Why does the torque of a tightened bolt gradually decay? What are the core reasons for this decay? How can it be effectively prevented on site?

I. First, understand: What exactly is bolt torque decay?

The essence of bolt tightening is to stretch the bolt shank through torque, generating preload. The preload compresses the workpieces to achieve the fastening effect. Torque decay means that after the bolt is tightened, the preload gradually decreases over time, and the corresponding locking torque continues to decrease, eventually leading to loosening, thread stripping, and connection failure.

This phenomenon is independent of material and strength grade. Whether ordinary carbon steel bolts or 304 stainless steel screws, all will exhibit varying degrees of torque decay; only the rate and magnitude of decay differ.

II. The 4 core reasons why bolt torque gradually decays

1. Material creep: the main natural cause of decay. All metal materials have creep characteristics. Simply put, under continuous tensile force, even if the force remains unchanged, the metal will slowly undergo slight plastic deformation.
   After a bolt is tightened, the shank remains under tensile load for a long time. The internal metal grains slowly slip and relax, the stretched length of the shank slightly retracts, the preload naturally decreases, and the torque decays accordingly.
   

This is especially evident in stainless steel and soft carbon steel bolts, where creep is more pronounced. This is also the core reason why 304 stainless steel screws are more prone to insufficient torque after long-term use.

2. Contact surface indentation and plastic settlement
When a bolt is tightened, the nut face, washer, and workpiece contact surfaces are subjected to enormous pressure. Metal contact surfaces that appear flat are microscopically full of peaks and valleys.
Under long-term pressure, the microscopic peaks on the contact surfaces are flattened and compacted, causing plastic settlement and slightly increasing the overall joint clearance. The bolt stretch is consequently reduced, preload is rapidly lost, and torque decay is directly caused.
In working conditions involving thin sheets, soft workpieces, or assembly without washers, this decay rate will increase significantly.

3. Vibration and alternating loads: accelerating torque loss

Continuous alternating loads generated by equipment operation, mechanical vibration, vehicle travel, and fan rotation are important causes of bolt loosening.

Continuous micro-vibration causes the thread flanks to undergo repeated tiny relative displacements. The friction between the threads continuously decreases, and the originally locked engagement gradually relaxes. The torque will decay continuously and in a stepwise manner, eventually leading to obvious loosening over time.

4. Temperature and environmental stress changes

Repeated thermal expansion and contraction are caused by equipment startup and shutdown as well as seasonal temperature differences. Bolts, washers, and workpieces are made of different materials, so their coefficients of thermal expansion differ.

When the temperature rises, the connected parts expand; when the temperature falls, they contract. Under repeated cycles, the thread fit clearance continuously changes, the internal preload is continuously released, and torque decay is caused. In working conditions involving moisture, oxidation, and slight corrosion, thread engagement loosening is also aggravated, accelerating torque loss.

III. Torque decay ≠ a product quality problem

Many procurement and O&M personnel mistakenly assume that torque decay means the bolt precision is insufficient or the material does not meet standards. In fact, even for legitimate GB- and DIN-standard bolts, normal torque decay can still occur provided that factory precision and materials are qualified.

Take 304 stainless steel screws and high-strength carbon steel bolts as examples. Even when products are manufactured strictly in accordance with GB and DIN standards, and thread precision, material composition, and strength grade all meet requirements, physical characteristics such as metal creep and contact surface settlement still cannot be completely avoided. The decay rate can only be slowed through processes and assembly methods.

IV. Practical on-site methods to effectively slow bolt torque decay

To control torque decay and avoid equipment loosening failures, the key is not frequent retightening, but standardized assembly:

1. Use standard spring washers and flat washers together: evenly distribute pressure, reduce contact surface indentation, and lower preload loss caused by plastic settlement.
2. Control the tightening torque precisely: do not use excessive torque or forceful tightening, so as to avoid excessive plastic deformation of threads, which would aggravate later creep-related decay.
3. Use anti-loosening solutions in critical working conditions: for vibrating equipment and outdoor equipment, lock nuts, anti-seize thread paste, and threadlocker can be used to lock the thread engagement state.
4. Recheck and retighten after initial startup of new machines: 24 hours after the equipment first runs, uniformly recheck the torque to compensate for the rapid decay caused by initial contact surface settlement.

Summary

In short, the gradual decay of bolt torque is a physical phenomenon resulting from the combined effects of metal creep, contact surface settlement, vibration loads, and temperature changes. It is a common industry problem, not simply a product quality defect. In daily assembly, as long as qualified bolts are selected, assembly procedures are standardized, and anti-loosening measures are taken, the risks of loosening and failure caused by torque decay can be greatly reduced, ensuring long-term stable operation of equipment.