Torque tightening is a process used in many industries and sectors, essential to maintaining safety to products and assemblies. However, how do you apply torque tightening correctly? In this post, we will discuss the basics of torque tightening and some of the theory behind the process.
What is Torque Tightening
Torque tightening is the application of torque to a nut allowing a bolt to be pre-loaded. Pre-load in a bolt is an axial force that creates tension. As every force has an equal and opposite reaction, bolt pre-load is equal and opposite to the clamping force witnessed on the materials in the joint. When we apply a greater torque, a greater bolt pre-load and compressive force are seen as a result.
Torque is the product of a force and a length. Therefore if we apply the same force with a larger wrench, we will be introducing more torque. It is common for large torques on a joint to be applied with large torque wrenches because it is extremely difficult to achieve it without them.
What Effects Torque Tightening?
Lubricants
Torque tightening works from the principle of friction. So anything that will affect the friction between parts of the system will affect the torque applied. It is common for fasteners to be lubricated or have a thread lock applied before they have torqued applied. The primary purpose of using a lubricant on the thread of a bolt is to make bolt tightening more efficient.
When applying a torque to a bolt, approximately only 10% of that force applied translates into preload/clamping force. The remaining 90% overcomes friction in the system, 40% to overcome the friction between the threads and 50% to overcome the friction under the fastener head. Applying a lubricant reduces the force required to overcome friction and allows more of the torque applied to the bolt to translate into preload.
Thread lock behaves the same during the installation, acting as a lubricant. However, it has an additional purpose. After its ‘curing time’, the thread lock will set and seal, acting as a secondary locking mechanism.
Surfaces
When applying torque to a bolt and nut, they will interact and generate friction when it’s tightened. Keeping these surface consistent, allowing a repeatable and consistent process, is crucial for torque tightening bolts. This is where washers come in. Using washers provides a consistent surface every time. Washers must be used under both nuts and bolt heads to give as much consistency as possible, helping apply torque tightening correctly. Washers are selected dependent on the application, the surface roughness and the grade of the material. The grades of washers should ideally match the grade of the fastener and the nut.
When torquing on painted surfaces, drips or lumps of paint will affect how the washer will sit on the surface. The washer will no longer sit flat against the surface, which will affect the preload in the bolt. Additionally, over time the paint drip will reduce and eventually be removed due to embedment. As a result, the load will lose preload, and the joint will experience detachment which can lead to detrimental failure.
Credit: ArnoldReinhold. License: CC BY-SA 3.0
How Much Torque Should Be applied?
How much torque is required for a fastener is dependent on several factors such as the bolt size, the bolt material, the washer used etc. In this section, we will focus on the material of the bolt and bolt size.
Bolts are commonly manufactured with stamping on top of the bolt head. There are commonly two stampings: 1) to signify the grade of the material used 2) The company who manufactured the fastener. The grades of steel signify the amount of stress (tensile strength) the bolt can handle. The most commonly used grades for bolts are 8.8, 10.9 and 12.9. Luckily, these have sensible nomenclature.
The grade classes can be split into two. For example, grade 8.8 into 8 and 0.8. If you times the first number by 100 (8 x 100 = 800) will give you the minimum tensile strength (within close range). A grade 8.8 bolt has a minimum tensile strength of approximately 800 MPa. Next, if we times this value by the second (0.8), it will give us the elastic limit, which is the stress necessary to cause plastic deformation. 800 x 0.8 = 640 MPa. This is important because it is essential to keep the bolt within its elastic region to prevent permanent deformation. Torque tightening a bolt to 90% of the elastic limit is commonly specified by manufacturers, as it allows the fastener to withstand any additional external load.
Bolt size is another factor that determines the suitable torque to be applied to a bolt. A larger bolt has more material to distribute stress, meaning it can withstand more load. Therefore an M10 bolt will have a recommended torque higher than an M8 bolt even if they are both grade 8.8.
It is essential to use a calibrated torque wrench when torque tightening fasteners. The torque wrench must be calibrated as this helps ensure that the actual torque applied matches the setting on the wrench. For those uncommon with torque wrenches: a torque wrench will click once the torque applied equals that set on the wrench’s gauge
Once operators have applied the required torque to a bolt, it is good practice to mark the bolt head to identify that it has been torqued. Marking bolts will prevent the operator from forgetting to torque tightening any fasteners. The marks should be drawn from the centre of the bolt head and onto the surrounding material. The benefit of this is providing a quick visual aid to check if the bolt has loosened. If the line on the bolt head and the surrounding material no longer align, loosening has occurred.
Benefits of Torque Tightening
The biggest benefit of torque tightening to bolts allows control and even distribution of torque to all bolts allowing sufficient pre-load to be applied. Additionally, it will prevent bolts from being overtightened.
Calculations will allow the designer of the system to ensure it is safe and the bolts are tight with a torque applied correctly.
Summary
Torque tightening is a critical process in the building stage of manufacturing, which will determine the safety of a system. There are many factors which can affect the torque tightening process such as the grade of bolt, the washers used, the size of the bolt etc. Without a torque tightening process, the torque applied to a bolt is unknown and may potentially lead to over torquing. Over torquing will take the fastener beyond its elastic limit and induce plastic deformation. A consequence of this will be rapid detrimental failure. You should now know how to apply torque tightening correctly.
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