Caution in the event of preload force losses!

Creating a permanent and secure connection – this goal is common to all bolted connections. To achieve it, a specific mounting preload force must be obtained when tightening the screw. In this article you can find out why the loss of preload force should be taken into account and what the type and condition of the screw have to do with it!

What is mounting preload force?

First, let’s take a closer look at the mounting preload force. It is the sum of three force actions on the screw:

1. Firstly, from the so-called clamping force, i.e. the force required to join the parts to be joined.
2. Secondly, from the operating force, which includes all static or dynamic forces that act on the connected parts in everyday use and try to drive them apart.
3. And last but not least, the loss of preload force: This describes the loss of force during or after the tightening process, which occurs, for example, when the screw is set.

Loss of preload force: The nature of the screw plays a role

The setting behavior of a screw made of a specific material with a specific geometry can be predicted and compensated for during the tightening process. Therefore, it is immensely important that the screw you use complies with the recommendations for bolting and that you don’t use just any screw.

But keep in mind: The condition and handling of the screw also influence the loss of preload force! For example, it makes a big difference whether the screw is uncoated or coated, whether it is lubricated or has rust on it. Model tests can thus demonstrate the following: When tightening screws with the same torque, the mounting preload forces achieved vary considerably, as shown in the following diagram:

Always adhere to manufacturer’s specifications!

Four clear tips for users can be derived from this:

1. Only use screws that are intended for the respective screwing process. For example, never use coated screws instead of uncoated screws (and vice versa).
2. Do not lubricate a screw unless it is specified for the screw case, or use the specified/recommended lubricant.
3. Pay attention to the condition of the screw. In particular, do not use rusty screws; when reusing screws, this is a common source of failure.
4. If a rotation angle is specified for the bolted joint in addition to the tightening torque, be sure to use the angle of rotation method as a tightening method, as this highlights possible interference factors with the help of this second measured variable.

Do you want to know more about the angle of rotation method? Here you can find more info: Tightening with angle of rotation – One decisive step ahead.

Missed our webinar on loss of preload force with live demonstration and real-time measurement of the forces acting during screw tightening? Then you can watch our webinar recording for free: 

LOSS OF PRELOAD FORCE WEBINAR