An Overview Of Nut And Bolt Compatibility

The importance of finding the nut material that will work well with your bolt cannot be stressed enough. Considering that these fasteners can be found in the critical parts of major appliances, car engines and building designs, utilizing compatible bolts and nuts represents the best way to make sure they stay together for an extensive period of time.


The IFI standards
The strength of the nuts and bolts – or proof stress as it is known in the industry – is measured in pounds per square inch (psi). For average fastening needs, the IFI guidelines suggest you utilize a nut with the same or higher strength than the screw or the bolt. For example, if you have a fastener made of medium carbon that underwent a heat treatment, it means the bolt has approximately 90,000 psi.
Therefore, you should look for a nut with at least 90,000-psi strength, if you want it to hold your components tightly together. In the event that safety is of utmost importance in your industry, then it is recommended to select a nut with a 20% higher strength than the bolt or screw.
Take note that for non-ferrous materials and most stainless steel alloys, you don’t need the extra strength and nuts with similar psi will perform well in all settings. On the other hand, nickel-based or stronger stainless steel alloys usually require a greater proof stress to do their job properly.

Make sure they share the same strength
Because the bolt and the nut are fastened together via their threads, the feeblest shear plane in the thread profile will be the weakest link and the precise area where the breakage starts. To put it simply, the overall strength of the connection will be given by the thread made of the weakest material. Consequentially, it is advisable to choose nuts and bolts made from the same alloys and with similar strength to avoid problems in the future.
The exception to this rule is when the galvanic corrosion is irrelevant for the expected lifespan of the connection. This type of corrosion refers to an electrochemical process where one metal corrodes preferably to another, a process commonly used in primary batteries to generate electrical voltage.
Then again, even in this case manufacturers manage galvanic compatibility via finishes; while the dissimilar plating allows the metals to be in contact, the base materials are well-protected from corrosion. The process of coating fasteners and nuts is called overtapping. Despite the fact that overtapped fasteners have a higher resistance to corrosion, the process reduces their proof stress to around 60,000 psi. Because the process will clog up the threads of the fastener, it is advisable to choose nuts that have been hot dipped galvanized as well.
Balance is the word of the day
If you’re working with a low budget, then choosing a nut with specs that are near the minimum proof stress of the fastener will end up costing you a lot more cash in the future. While the connection will usually hold well in stress situations, it’s just a matter of time before the weaker nut will give into tension and start stripping completely.