Recommended Tightening Torques For Machine Screws

What Is Torque?

Used by countless engineers across the world, ‘Torque’ is a rotational or twisting force resulting in movement around an axis. It can often be understood as an object’s ability to overcome resistance to turning.

Torque is also a crucial factor in fastener installations, defining the maximum rotational force that can be applied to a fixing. 

The term ‘Torque’ can be used in various ways throughout the industry but is most commonly used in the automotive and electrical industry in relation to the force that an engine can apply. In essence, the more torque, the higher the power of the engine and the faster the vehicle can accelerate.

Maximum Tightening Torques define the utmost limits of fastener applications, where further force applied to the fastener will often result in stress and lead to malfunction. 

Achieving this force is almost never advised, as it is the point just before failure, and as such would be near impossible to achieve without damage to the components.

It should be noted that Maximum Tightening Torques are not fixed values and can change in accordance with the application and to tighten a fastener to its very limits will often exceed the recommended threshold.

What Are Recommended Tightening Torques?

By understanding the maximum rotational torque that a component can tolerate, its ‘Recommended Tightening Torque’ can be determined.

Unlike the Maximum Tightening Torque, the Recommended Tightening Torque acts to ensure that fasteners are tightened to the correct value for assembly, and not over-tightened to the point where there is a potential risk of failure. 

The Relationship Between Preload And Torque 

Similar to Torque, Preload is a force that acts upon a component during installation. 

Rather than being a rotational force, Preload is the axial force that is applied to a fastener when it is tightened. 

In essence, when we apply torque to install a fastener, we begin to generate a tightening force known as ‘Preload’.

During installation, the component begins to stretch by a small degree and this, by consequence, creates tension in the assembly. In these instances, the components in the assembly are undergoing elastic deformation and when the tightening force is removed, will revert back to their original size.

In some use cases elastic deformation can be exceeded with specialist components often known as Torque-To-Yield Fasteners.

The tightening force of Preload is essential to maintain joint integrity, evenly distributing the load supported by a fastener in a bolted joint across the joined materials. 

Once a fastener is ‘tight’ it will compress the material between the fastening components, often the nut and bolt. This compressive force acts on both the fastener and the assembly to lock them together and create a secure joint.

This transfer of loading allows a fastener in a joint to resist a greater loading force than a fastener which is under no tensile load.

What Are Torque-To-Yield Fasteners?

Often used in automotive and other mechanical systems, Torque-To-Yield fasteners are specially engineered to be tightened beyond their elastic limit. 

When the component surpasses its Yield Strength, it begins to deform plastically, causing the component to permanently stretch.

This unique design allows the fastener to maintain tension above what is normally possible for a fastener of a given size. This - in turn - enables the use of a smaller fastener than is traditionally specified, which results in a reducing cost and weight, allowing miniaturisation of an assembly.

As a result, Torque-To-Yield components are often used in critical areas where these features are crucial for proper performance, reliability, and safety. For example, they may be used in critical engine applications like cylinder head bolts, main bearing cap bolts and other high-stress areas. 

As would be expected with components that are subject to permanent stretch, Torque-To-Yield fasteners are generally designed to be one time use and should be replaced when they are removed. 

Nevertheless, as they distribute the load more evenly, opting for Torque-To-Yield components allows for a higher clamping load with a smaller sized fastener.

Is The Recommended Tightening Torque The Same For All Materials?

The Recommended Tightening Torque for a fastener will vary depending on several factors, including the material of the fastener and the material being fastened, the size and type of the fastener, and the specific requirements of the application.

Different materials have different mechanical properties, such as strength, hardness, and elasticity, which impact how they respond to tightening forces. 

For example, when fastening two metal components, the recommended torque may differ depending on whether the materials in question are made from aluminium or steel. For fasteners with a lower tensile strength, such as a Polycarbonate Screw, the Recommended Tightening Torque would be significantly lower than that of a Hardened Steel Screw of the same size. 

To a similar effect, when using different types of materials together, i.e attaching a metal to plastic or to wood, the torque requirements may vary once again.

While there are various factors influencing the required tightening torque for a given application, it is worth noting that general guidelines for torque specifications do exist. 

For the most part, Recommended Tightening Torque will be assessed based upon the size and material of the fastener.

These guidelines are based on engineering calculations, testing, and experience to ensure the proper clamping force without risk of damage to the materials or compromising the integrity of the joint.

Recommended Tightening Torques For Fasteners

Here we have the Recommended Tightening Torques for Accu’s metal fasteners including Stainless Steel and High Tensile Steel. 

The following data is based on a dry installation and should not be used for a lubricated installation. This data is based on a Metric Coarse Thread. The values stated are taken from a controlled environment and are not specific to a particular screw type - different fasteners may have different attributes which can affect the values given within this table. Accu cannot guarantee the figures given and they should be used as a guide only.

Recommended Tightening Torque For Stainless Steel Fasteners

 

 

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Recommended Tightening Torque For High Tensile Steel

 

 

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Factors That May Impact The Recommended Tightening Torques Of Fasteners

Just as a component’s Recommended Tightening Torque can be impacted by factors such as size and material, there are also several external factors that can change how the fastener behaves when it is being tightened. 

It is important to address, however, that there are certain misconceptions regarding which factors do and which factors do not have an effect on the recommended torque for an application. These are outlined below.

Selecting The Correct Tools

As the tightening torque of a fastener is related to the size of the thread, the tool that you are using will not have an impact on the tightening torque required - assuming you have the correct tool for the component you are fastening.

However, there are some drive types - such as Torx - which are specially designed to improve torque transfer, resist a higher installation torque and avoid slipping and stripping. 

Different Head And Drive Types

Just like tooling, the head and drive type have minimal impact on the tightening torque of a fastener.

That being said, some components are engineered for specialised use cases. For example, Captive Screws have a waist shank and therefore may require a reduced fastening torque.

Similarly, due to their reduced head and weak point, a Shear Bolt will often have a lower driving torque. Though, this is a design feature that is essential to its use and is an atypical scenario, not the standard. 

Dry vs Lubricated Installation

Unlike the choice of tool, head or drive type, lubrication does have an impact on the torque needed to tighten a fastener. 

Lubrication lowers the friction between the mating surfaces of a thread and thus reduces the necessary driving torque. 

This can lead to a significantly higher tensile stress in a joint for the same torque setting and may cause the fastener or connection to fail.

For the majority of torque charts it will determine that the figures provided are for a ‘dry installation’ i.e. the threads are clean with no lubrication.

Lubricant can significantly lower the amount of driving torque that is required and therefore it is rarely specified within installation guidelines.

If a lubricated installation is recommended, in the case of Stainless Steel components, an anti-seize compound will often be required; this reduces the risk of galling. 

If galling does occur, it may cause false readings on tightening torque and can also cause fasteners to fail prior to the joint being tightened to its full capacity.

Further information about thread galling can be found in the article ‘What is Thread Galling, And How Can It Be Prevented?’

Surface Finish

Just like lubrication, the surface finish of each mating component can have an effect on the driving torque of a fastener. 

For instance, if one of the mating components has a rough finish, it can lead to an increase in friction between the two parts. This can cause ‘false set’, a phenomenon where the user prematurely assumes the fastener is tight and secure when it remains to be loose. 

If ‘false set’ does occur, it can have severe consequences for the assembly where the fastener gives a high torque reading without the required preload. 

Thread Locking & Other Resin Patches

Adding Thread Locking and other resin patches should not affect the Recommended Tightening Torque of a fastener. Nevertheless, there are some exceptions that should be taken into account.

Liquid threadlockers, for example, can lower the tightening torque of a fastener by acting as a lubricant.

By comparison, a threadlocker that is pre-applied to a fastener may interfere with the threads creating more resistance and, in turn, raising the driving torque that is required for installation.

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