A bolt and nut are used together as a compressive force to keep materials connected. The threads of the nut work with the threads of the screw to apply force to either side of the fastened material. This tension is called Bolt Preload, which is the compression created as the nut is tightened against the bolt (or vice-versa).
When a load (weight) is placed on a bolt, it is limited to the amount of load the bolt can handle before failing. However, when a bolt is tightened against a material, it allows the bolt to distribute the force through the material, so the bolt itself only holds a portion of the load. This means that a bolt can hold a significantly higher load when the correct amount of tension is applied. That tension is known as preload.
Load – The amount of force acting on a fastener assembly
Preload – The amount of tension (compression) needed to distribute a load’s force throughout a fastener assembly
Working Load – The load placed on the assembly once ready to perform
Bolt Preload – The tension created when the nut is screwed onto a bolt to hold two materials together. When the tension reaches the optimal preload, the working load (load added after creating the assembly) placed on a bolt will be distributed into the installation materials, so the bolt does not take the entire load.
The Outcomes of Bolt Preload:
- If the assembly is loose (the preload is not correctly applied), the external load increases the load on the bolt only. This will result in bolt failure.
- If the assembly is tight (the correct preload has been applied), the load will only cause bolt deformation by distributing the load through the bolt and the nut.
How Bolt Preload Works
Bolts are incredible tools, but they are actually not as strong as we’d think. When first glancing at a bolt assembly with a working load attached to it, it appears as though the bolt is holding that entire load on its own. This is not the case.
When a bolt has preload, it is able to distribute the working load out across the plate near the head of the bolt. We will refer to this as the support plate. This means a properly installed bolt assembly can withstand a much heavier load as it distributes the force out away from itself. When a working load is applied to a fastener assembly that has not been preloaded, the entire force is placed on the bolt alone, which makes it much more likely to fail.
Why Is Bolt Preload So Important?
As discussed above, without bolt preload, the entire structure would be totally reliant on the bolt to hold the weight. When preload is applied, significantly less bolts are needed, as the material (Support Plate) will play a significantly larger role in holding the working load. This is not a cure-all however; a working load may still exceed the preload of the bolts which can result in the bolts failing, the support plate failing, or both.
Easy Ways to Determine Bolt Preload
- Use a Torque Wrench to Reach Optimal Torque
- While this is not a true measure of bolt preload, if a bolt is fastened to its optimal torque it can be assumed it is close to the correct preload. The reason this method is not readily accepted is because the torque will be directly affected by the material it is being spun against. A rougher material will produce more friction, which will make the torque value higher while reducing the preload tension. The opposite is also true. You can hit optimal torque without hitting the optimal preload (and vice versa). In the end it depends on the material, which is why this method isn’t exact, but it is a decent guess.
- Use Preload Indicating Washers
- Preload indicating washers are washers that are designed to spin until a certain amount of load is applied to them. This way, once the washer is no longer free-spinning, the preload has been met. This is a much simpler way of determining if the correct compression has been met.
- Use Direct Tension Indicating Washers
- These washers have little bumps that flatten when preload is achieved. Once flattened, a feeler gauge is used to make sure the bolt shaft is no more than 50% accessible under the washer (ideally less).
- Use Silicon Direct Tension Indicating Washers
- These washers operate in a similar way to the direct tension indicating washers above. These washers have small recesses where a silicon paste is filled in. As the nut tightens against them, the silicon begins to come out of the sides of the washer. To identify when optimal preload is achieved on this type of washer, the amount of recesses on the washer minus one need to be exposed out the sides. For example: If there are 6 recesses, then a minimum of 5 need to be exposed. 6 recesses – 1 recess = 5 (Preload has been reached)
- Tighten Until Snug, and Then Tighten “X” Degrees
- A less reliable approach is to tighten the nut until snug, and then to torque the nut “X” degrees. For example, a recommendation of 90 degrees would look like the following: Spin the nut onto the assembly until snug against the material. Attach the wrench to the nut and spin it a quarter of a turn (90 degrees). This method will give you a close to accurate preload.
So what does bolt preload do?
Bolt preload ultimately allows a fully tightened bolt to survive in an application where an untightened or loose assembly would fail very quickly. When tight, the joint provides a conduit for the force to flow through into the assembly materials themselves. This means the bolt assembly itself is only under a portion of the force of the working load. While you cannot look at a bolt and see this transfer occurring, the preload allows bolts to survive in much more rigorous applications.