To arrive at the correct torque value, several other values must be found first. Two principles influence the correct clamping pressure for each bolt, known as clamp load. The first is bolt diameter. Thankfully, standards organizations have assembled the standard tensile strengths for common bolts into easy-to-use standards. For imperial bolts, grades 5 and 8 are most common. SAE J conforming bolts will have radial markings machined on the head of the bolt that indicate bolt grade.
A grade 2 bolt has no markings, a grade 5 bolt will have three markings, while a grade 8 bolt will have six lines. Metric bolts are more simply identified: the class is explicitly stamped on the bolt head. Other standards regulate specific types or applications for bolts and they should be consulted as needed.
Examples include, but are not limited to those in the accompanying table. Standard specification for alloy-steel and stainless steel bolting for high temperature or high-pressure service and other special purpose applications.
Utilizing bolt class information, the clamp load of the bolt can be determined with the following equation. Preload can also be defined as the applied torque minus its frictional losses. 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. This provides more torque to the preload and removes stress from the bolt through the process. To make sure you apply the right amount of lubrication to reach your desired preload, manufacturers provide the value for the friction coefficient.
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 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. You also need to take into account friction on bearing surfaces, the bolt diameter, and other variables, which are best examined through experimentation. As mentioned above, bolt or stud tensioning produces axial load by pulling up on a fastener with what looks like a hydraulic load cell.
To achieve the targeted bolt load, you need to know the area of the tensioner and the amount of force on the fastener, and then adjust the amount of hydraulic pressure. Because tensioning does not place a twisting force on the fastener as applied torque does, we see tensioners used with long threaded fasteners and on rotating equipment such as reciprocating rods.
Another good use of tensioning is large bolt diameters. On large bolt diameters, tensioning will save you time compared to using hydraulic torque wrenches. That means your bolt scatter will be lower. The preload on the bolt creates an opposing compressive force between the head and the nut, clamping the joint together. This compression is referred to as the clamping force and is used to analyze the strength of a bolted joint.
However, not all the applied torque is converted into clamping force. The majority of the torque typically serves to overcome friction underneath the head of the bolt itself and the friction between the threads. Bolt Torque Specifications Any standard inch or metric bolt will have a recommended torque value from an industry standard or the manufacturer based on the size and grade.
These torque values are generally designed to stretch the bolts to their proof load. The proof load of a bolt is the maximum tension the bolt can handle without plastic deformation. Higher grade or larger diameter bolts will have a greater tensile strength, a higher proof load, and thus a higher recommended torque.
This is because a large portion of the applied torque is lost to thread friction, so lubricating threads reduces that friction and the torque required to reach a given preload value. Similarly, lubricated threads will reach a much higher preload with the dry torque value than dry threads potentially overloading the bolt or the joined members and causing failure. Even thread-locking compounds can provide significant lubrication to threads, reducing friction and increasing the preload for a given torque value.
Therefore, it is always important to use a torque value appropriate for the bolt size, grade, and condition of the threads. Lubricants or locking compounds should not be used on threaded fasteners unless you use wet torque valves.
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