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Hi Mark, By detrimental, I mean that sometimes a Grade 8, L9, SHCS, or higher, is the wrong bolt choice for a particular application, and that it can cause a joint related failure. In a simple case, lets say a Grade 5 bolt is used to hold a casting together. Standard torque for a 1/2-13 is 55 lb-ft (let's say it is oiled). If a SHCS is used in place of the Grade 5, the proper torque for the bolt would be 90 lb-ft (again, oiled). The difference in clamping force between the two bolts would be in the vicinity of 3,500 lbs (about 64% more force than originally designed for). With engineering being what it is, and castings being made to use the least amount of material possible (money savings), it is very conceivable that certain castings could fail. If the bolt is simply not torqued as much, then it could come loose prematurely. I have oversimplified this, as some of the other variables are platings, initial preloads, joint stiffness, and external loads including fatigue. An example that I have run into many times is when a person replaces a lower grade bolt with a higher grade bolt when using a gasket. Gaskets are designed with two main types of design forces in mind. The first is the minimum clamping force that is needed to make the gasket seat effectively into all of the surface irregularities. This design seating stress is called the yield factor, and is governed by the compressibility of each individual type of gasket material. These values are known by the gasket manufacturer and are crucial in the design of the gasket. The formula to calculate the clamping force required to satisfy the gasket seating requirement takes into account the gasket area, the yield factor of the gasket, and the number of bolts in the joint. The second type of clamping force is the force needed to keep the sealing pressure on the gasket after the pressure is applied inside the gasketed area. As the internal pressure rises, this additional clamping force ensures that the gasket remains leak free. This is called the maintenance factor, and takes into account the end force on the joint, internal pressure, gasket maintenance factor, gasket area, and number of bolts in the joint. For all these joints, the number of bolts is important, the bolt spacing, and the clamping forces are critical. So, if a gasket fails, why not simply tighten the bolts more, or use a higher grade bolt? This will work, but only to a point. Another known design value for gaskets is the crush strength. This too is unique to each individual type of gasket material. If clamped too much, the crush strength will be exceeded, and the gasket will have permanent deformation, and will fail prematurely. This is bad, and this is real. There are other gasketed issues as well, such as tightening patterns, creep, blind holes and threaded mating strength, and so on, but the point I am trying to make is that some bolted joints are sensitive to the types of bolts and torques that are used to hold them together. The reason for mentioning some of the things above, is to show that when parts were designed, there usually is a reason for the bolt grade that was selected. Greg
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