what is fastener preload

What is Fastener Preload?

Fastener preload is a term that is commonly used in the construction industry. It refers to the tension that is applied to a bolt or screw when it is tightened. This tension is an important factor in ensuring that the fastener is securely fastened and can withstand the load that it will be subject to.

In this article, we will explore the concept of fastener preload in more detail. We will discuss why it is important, how it is calculated, and some of the factors that can affect it.

Why is Fastener Preload Important?

When a fastener is tightened, it is stretched like a spring. This stretching is what creates the tension that is required to hold the joint together. If the preload is insufficient, the joint may come apart under load or even vibrate loose over time.

On the other hand, if the preload is too high, the fastener can be overstressed and may fracture. The goal is to achieve the optimum preload for the particular application, which will provide a secure connection without overstressing the fastener.

How is Fastener Preload Calculated?

Fastener preload is calculated using the formula:

Preload = Friction Factor x Torque

The friction factor is a value that takes into account the type of thread, the lubrication, and the surface finish of the mating parts. The torque is the amount of force that is applied to the bolt or screw to tighten it.

It is important to note that torque is not a direct measure of preload. It is merely a way to apply a force to the fastener, which will create the desired preload. To achieve the correct preload, the torque must be adjusted based on the friction factor and other factors that influence the tightening process.

Factors That Affect Fastener Preload

There are several factors that can affect the preload of a fastener. Some of the most important are:

1. Surface Finish: The surface finish of the components that are being fastened can have a significant impact on the friction factor. Rough or uneven surfaces will create more friction, which will require more torque to achieve the desired preload.

2. Lubrication: The use of lubricants on the threads can reduce the friction and make it easier to achieve the desired preload with less torque.

3. Thread Type: The type of thread used on the fastener can also affect the friction factor. Coarse threads will generally have a higher friction factor than fine threads.

4. Fastener Material: The material that the fastener is made of can also influence the preload. Different materials have different elastic properties, which can affect how much they stretch when tightened.

5. Temperature: Extreme temperatures can cause materials to expand or contract, which can affect the preload of the fastener. In some cases, it may be necessary to adjust the torque based on the temperature at the time of installation.

Conclusion

Fastener preload is an essential concept to understand when working with bolts and screws. Achieving the correct preload is critical to ensuring a secure joint that will withstand the load that it will be subject to. By considering factors such as surface finish, lubrication, thread type, fastener material, and temperature, it is possible to calculate and adjust the preload to the optimum level for each application.