A bolt and nut are used together as a compressive force to keep materials connected. Nut threads work with screw threads to apply force to both sides of the fastened material. This tension is called Bolt Preload, which is the compression created when 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 withstand before failing. However, when a screw is tightened against a material, it allows the screw to distribute the force through the material , so the screw itself only supports part of the load. This means that a bolt can support a significantly greater load when the right amount of tension is applied. This tension is known as preload.
Load: The amount of force acting on a set of fasteners
Preload: The amount of tension (compression) required to distribute the force of a load across a set of fasteners
Workload: The load placed on the assembly when it is ready to run
Bolt Preload – The tension created when the nut is tightened on a bolt to hold two materials together. When tension reaches the ideal preload, the working load (load added after the assembly is created) placed on a bolt will be distributed across the installation materials so that the bolt does not absorb all of the load.
The results of screw preload:
- If the assembly is loose (preload is not applied correctly), the external load will only increase the load on the bolt. This will result in the screw breaking.
- If the assembly is tight (correct preload has been applied), the load will cause the bolt to deform just by distributing the load through the bolt and nut.
How Bolt Preload Works
Screws are amazing tools, but they're actually not as strong as we think. When you first look at a group of bolts with a workload attached, it appears as if the bolt is supporting the entire load by itself. This is not the case.
When a bolt has preload, it is able to distribute the working load across the plate near the bolt head. We'll call this the backing plate. This means that a properly installed bolt group can support a much heavier load because it distributes the force away from itself. When a working load is applied to a fastener assembly that has not been preloaded, the full force is applied only to the bolt, which makes failure much more likely.
Why is bolt preload so important?
As discussed previously, without bolt preload, the entire structure would be completely dependent on the bolt to support the weight. When preload is applied, far fewer bolts are required as the material (backplate) will play a significantly greater role in supporting the workload. However, this is not a panacea; a working load can still exceed the screw preload, which can cause screw failure, backing plate failure, or both.
Simple Ways to Determine Bolt Preload
- Use a torque wrench to obtain ideal torque
- Although this is not a true measurement of bolt preload, if a bolt is adjusted to its ideal torque, it can be assumed to be close to the correct preload. The reason this method is not easily accepted is because the torque will be directly affected by the material it is turned on. A rougher material will produce more friction, which will increase the torque value, reducing the preload tension. The opposite is also true. You can achieve ideal torque without achieving ideal preload (and vice versa). Ultimately it depends on the material, which is why this method isn't exact, but it's a decent guess.
- Use preload indicator washers
- Washers that indicate preload are washers designed to rotate until a certain amount of load is applied. This way, when the washing machine is no longer spinning freely, the preload is satisfied. This is a much easier way to determine whether the correct compression has been achieved.
- Use front tension indicator washers
- These washers have small projections that flatten when preload is reached. Once flattened, a feeler gauge is used to ensure the screw shaft is accessible no more than 50% below the washer (preferably less).
- Use silicone direct tension indicator washers
- These washers function similarly to the direct tension indicator washers above. These washers have small recesses into which silicone paste is filled. As the nut tightens against them, the silicone begins to leak down the sides of the washer. To identify when the ideal preload is achieved in this type of washer, the number of recesses in the washer minus one must be exposed on the sides. For example: If there are 6 recesses, at least 5 must be exposed. 6 recesses – 1 recess = 5 (preload has been reached)
- Tighten until comfortable and then tighten “X” degrees.
- A less reliable approach is to tighten the nut until tight and then tighten the nut by “X” degrees. For example, a 90 degree recommendation would look like this: Rotate the nut on the assembly until it is snug against the material. Attach the wrench to the nut and turn it a quarter turn (90 degrees). This method will provide almost accurate preload.
So what does bolt preload do?
Ultimately, bolt preload allows a fully tightened bolt to survive in an application where an untorqued or loose assembly would fail too quickly. When tightened, the joint provides a channel for force to flow through the mounting materials themselves. This means that the bolt assembly itself is only under part of the working load force. While you can't look at a bolt and see this transfer occur, preload allows bolts to survive much more rigorous applications.
