In bolted joints, as long as the preload is always higher, the simple approach should be adequate. When it was all said and done, our numbers were less than 0.50% different. Their approach was vastly different and took four pages to solve. My calculation took about 10 minutes to do and I went to the answer section. I thought this would be an easy problem, well…yes and no. I had two plates that were bolted together, and the force applied never exceeded the preload. I had to determine the calculation of the force on the bolts which is force applied with a particular tension on the bolt. The bolts were preloaded and then pressure was applied to the vessel. When I was studying for the PE exam there was a practice question which included a pressure vessel with a bolted end cap on it. The initial tension load is set above the applied load so the fatigue cycles are reduced to ½ a cycle, (a cycle is defined as one min and one max) and the load never changes. The blue line above is the desired state. However, while this is a vast improvement, it is still not how we would want an engineered joint to behave. The fatigue calculation still has the same number of cycles, but the intensity of the cycle is greatly diminished. Now, only the peak of the load shows itself in the cycle. To get to specifics, if the bolt had a tension load of 2000lbs, we would be able to pull these two plates apart up to 1999lbs without changing the load on the bolt at all! Therefore, we would say they are magic and act like big rubber bands which allow us to put different stresses on the bolt without causing failure. As we pull, the tension load in the bolt does not change until we pull more than the bolt’s pre-load. Now, let’s start pulling those plates apart slowly. We are now in a static condition with constant stresses for all eternity. This in turn puts compression on the interface and tension in the bolt. When we tighten the screw, the two surfaces will be pulled together. For example, let’s say I have two plates bolted together using a hex-head screw and nut. Here is how bolts are magic: Essentially, bolts act as rubber bands holding together plates through applied tension. Installing the washer with the smooth side toward the bolt head is also necessary for a joint that lasts. Using corrosion resistant platings like yellow zinc and ovulated lock nuts will keep the joint strong and tight. Understanding the load and selecting the correct grade of bolt are essential for a strong bolted joint. They can be used for a variety of applications ranging from easily removable components to semi-permanent structures. They allow us to complete many projects and serve us in a variety of ways, which are vital in everyday applications. Marvin Franklin, “Bolts are Magic.” This is true! In the world of engineering, bolts simply are magic. Or The Nuts and Bolts of…well…Nuts and Bolts

Eldevin nuts and bolts