To ensure safety and reliability, it is very important to choose the right grade and the right installation to ensure your safety on the machine.
The main brands of bolts and screws made of inch and metric steel are shown in table 1 (see below).
Mild steel bolts (also known as grade 2 or metric 5.8) have a tensile strength of about 385 MPa. The proof strength is the load that a fastener can withstand without any permanent deformation (irreversible damage). The guaranteed strength of class 5 bolts is 600 MPa, and the guaranteed strength of class 8 bolts is 840 MPa. Therefore, the better the class of the bolt, the greater the load it can withstand, while remaining elastic. When the load is removed, high quality bolts return to their original length and remain elastic, while poor quality bolts can plasticize and permanently stretch.
Most engineers use a factor of 60% to calculate the shear strength of a bolt, so this is a useful rule of thumb for choosing the correct size and shear strength class. I think it’s fair to say that most modified vehicles are redesigned in terms of fastener size, and most of the time it’s not safe, but it can make things look like a truck in some cases. Smaller fasteners can make a machine sharper, but as one of my lecturers once said, no farmer would buy a tractor if it was built to its limit because it looked too weak to lift. However, in modern cars we are now seeing the use of smaller mounts – so it’s up to you! Many modifiers use socket head fasteners that are grade 8 quality.
Once an appropriate fastener size has been selected, grade selection should be based on a balance between tensile strength, ductility and toughness. Tensile strength is the ultimate strength of the material from which the fastener is made. Ductility is the degree to which a fastener is stretched before it fails. Strength refers to fatigue resistance and the ability to withstand cyclic loads.
Thus, knowing these facts about physical metallurgy, it becomes easier to choose the right bolt grade.
For the seatbelt bolt, we need a pliable buckle that can stretch a lot before it fails, at which point the shock is over and the passenger is saved. As such, seat belt anchorages are in the soft class and should not be replaced by high-strength bolts that reach peak loads and suddenly fail. Similarly, for fenders, footpegs, firewall mounts, and other less stressed non-critical applications, Class 2 or unclassified fasteners can be used.
I find class 5 fasteners (8.8 metric) to fit just about everything else on a modified car. They have class 8 bolt tensile strength but no brittleness. In suspension they will withstand cyclic loads, in other applications they will provide shear and clamp strength without weakening due to ductility or over tightening.
For critical engine applications such as cylinder head bolts and differential kits, class 8 bolts with the proper torque to preload clamp joints with minimal bolt stretch are the best choice.
Nuts are usually made from mild or mild steel, similar to class 2 bolts. Strange as it may seem, the length and thread design of the nut is determined so that the bolt achieves full strength under load. This means that if you overtighten a fastener, you will often remove the nut first, leaving the bolt intact.
When you tighten a fastener to specification, you apply a preload that opposes the load in the opposite direction and also loads the fastener so that the additional load does not stretch the fastener and cause it to come loose.
Proper tightening of all fasteners on a car is very important, but in less critical places, experience and ordinary wrenches can be used for this. Wrenches are designed for specific fastener sizes; the larger the fastener, the longer the wrench will be to provide the correct torque from “standard” human strength. You all remember that the student always breaks the quarter-inch screws until he knows that the smaller screws need to be loosened.
Nylon nuts are so common in tram designs that they have largely replaced the old-fashioned spring washers. They work particularly well in applications where vibration or rotation are important factors and also where maintenance is difficult. Make sure at least one thread goes through the nut for proper engagement. Do not use nylon plastic if the heat will melt the nylon fasteners, and do not overuse it after loosening it more than twice.
Post time: Aug-03-2023