What Is Torque Tightening?
When it comes to bolt tightening, it’s not as simple as "righty tighty, lefty loosey." We have to understand the physics of industrial bolting to ensure that these bolts can stand up to incredible weight loads. It starts with torque.
What Is Torque?
Torque is the measurement of rotational force applied to an object.
How Torque Applies to Bolt Tension
In simple terms, you need torque to create tension in a bolt. You use torque to turn a nut onto a bolt and then to stretch the bolt, making it a solid spring that clamps the two materials together. The bolted materials cannot come apart if the clamp load remains more than the tensile load (the ability of a material to withstand a pulling force).
The Importance of Proper Torque
Torque applies to nearly all aspects of bolting, including tightening. When we tighten nuts and fasteners, we have to make sure we’re using the right amount of torque.
What Is Torque Tightening?
Torque tightening is the accurate application of torque to a nut so that a bolt can hold its load securely without breaking. Too little and our materials aren’t properly tightened – making it structurally unsafe. Too much torque can break the bolt.
When you apply the right amount of torque, the bolt is properly stretched so that it can act as a solid spring to clamp the two materials together.
Torque tightening begins with preload, which is the amount of tension that needs to be applied to a bolt before its load, or full weight it needs to support, is added. Preload can also be defined as the applied torque minus its frictional losses.
To figure out what the preload should be, you need to account for the bolt’s material and grade, how much torque will be lost to friction and the ultimate load the bolt will be required to carry. Ultimately, your preload contains other variables and you’ll use an equation or an online bolt torque calculator to determine the proper torque.
Friction Points and the Importance of Lubrication
Friction points are where the torque is lost due to friction. This is more easily understood as the touching threads between a nut and a bolt and whatever material the bolt is clamping. Without knowing your friction points, you won’t know how much torque should be applied and how much will be lost from friction.
Anywhere from 80 to 90% of your torque will be lost to friction, but we can reduce friction through lubrication. This provides more torque to the preload and removes stress from the bolt through the process. You’ll decrease your chances of bolt failure during installation and extend its life.
To make sure you apply the right amount of lubrication to reach your desired preload, manufacturers provide the value for the friction coefficient.
Torque wrench tightening sequence
When you’re torque-tightening multiple bolts on flanges or surfaces, you need to evenly spread the load across the bolts throughout the process.
The point load is the amount of load in a specific spot in the structure, as opposed to a uniform load, which is spread evenly throughout the structure. Since we typically tighten bolts one at a time, you want to be certain that the bolts will all carry an even load. Otherwise, a point load can cause structural failure.
Torque Pattern and Load Scattering
Torque pattern is the proper tightening sequence so the bolts are properly stretched and can evenly carry the load.
Whether there are four bolts or 12, torque pattern allows you to scatter the load. Instead of tightening bolts clockwise – which would put more load on one side – torque pattern typically dictates that you tighten bolts on opposite sides of the flanges.
It's more than just the sequence that you tighten in though. You must also pattern the amount of torque throughout the sequences, steadily increasing it in each round. For instance, in a four-bolt pattern, you would apply one-third the final torque to each bolt, then two-thirds on the second round and finally full torque on the third round. If we were to apply the full torque in the first round, then bolt one would have too much point load regardless of the following sequence.
The opposite of torque tightening is breakout – or breakaway – torque. This is the torque required to loosen bolts and is much higher, typically at least twice as high due to corrosion and possible expansion.
Advantages of Torque Tightening
Torque tightening allows you to control and evenly distribute torque to all bolts so that they’re stretched properly, can carry their load and aren’t over-tightened.
By following precise calculations and using calibrated torque wrenches, you have peace of mind that bolts are tight and the load is even.
Disadvantages of Torque Tightening
If you don’t follow proper torque tightening techniques, you risk under or over-tightening. If your torque pattern doesn’t apply load evenly through the process, you will have poor sealing, which can result in leaking flanges. As always, do not cut corners.
Why Aztec Bolting Is the Right Choice for You
We’ve been bolting since 1987 and can provide all of your torque tightening needs through product purchases or rentals. Plus, we back up our equipment with on-site training and full-service maintenance. Discover the Aztec difference today.