This preload ensures that the connected components are clamped together sufficiently and do not move relative to each other under load.
However, applying the same tightening torque does not automatically result in the same preload. The conditions within the bolted connection – and friction in particular – play an important role.
What is tightening torque?
Tightening torque is the rotational force applied when tightening a bolt or nut.
During tightening, the applied torque is used for several purposes, including:
- overcoming friction in the threads;
- overcoming friction beneath the bolt head or nut;
- stretching the bolt and thereby generating preload.
A significant part of the applied tightening torque is used to overcome friction. Only a limited part actually contributes to stretching the bolt and generating the required preload.
This makes friction an important factor in the final reliability and predictability of a bolted connection.
Why doesn’t the same torque always result in the same preload?
A common misconception is that applying the same tightening torque automatically results in the same preload.
This is not the case.
For example, two identical bolted connections can both be tightened to 100 Nm, while the preload ultimately achieved can still differ.
One of the main reasons for this is the coefficient of friction (μ). This can be influenced by factors including:
- coatings;
- corrosion;
- contamination;
- the surface condition of the contact surfaces;
- the presence and quality of lubrication.
When the coefficient of friction changes, the relationship between the applied tightening torque Mₐ (Nm) and the achieved preload Fᵥ (kN) changes as well.
As a result, the same torque wrench setting can produce a different preload under different conditions.
The influence of friction
For a reliable and repeatable bolted connection, it is therefore important to control not only the tightening torque, but also the friction within the connection.
When the coefficient of friction varies, the variation in the achieved preload increases as well. Stable and predictable friction helps to reduce this variation.
This is why the condition of the connection is an important part of a controlled assembly process.
The role of dry lubrication
At Lubo, we therefore look beyond the specified tightening torque.
Lubo’s dry lubrication provides a low and consistent coefficient of friction (μ). This makes the relationship between tightening torque and preload more controllable and helps reduce variation between individual connections.
This contributes to a repeatable assembly process and a predictable preload in the bolted connection.
In addition, Lubo Fasteners are ready for immediate use. No separate assembly paste or grease needs to be applied during installation, helping to keep lubrication conditions consistent throughout the assembly process.
Tightening torque is only part of the story
The specified tightening torque (Mₐ) remains an important assembly parameter, but it does not determine the quality of a bolted connection on its own.
The preload ultimately achieved (Fᵥ) is significantly influenced by friction within the connection. A controlled coefficient of friction (μ) and consistent assembly conditions are therefore essential for a reliable and repeatable result.
Only when both the tightening torque and the factors affecting friction are controlled can the required preload be achieved reliably and consistently.