Why is there so many different kinds of fasteners?

Answer 1

Fastener developement is a work in progress. Allen head bolts can be counter sunk into a smaller hole than a hex head bolt. Then can also be spaced closer together. However, Allen head bolts have a tendency to strip so someone invented star ( 6 point and 12 point ) headed bolts.

Different size bolts are used because of fit and strength requirements. A cv joint attached to a transmission output flange is an example. The force between the two parts requires a certain amount of steel to hold them together. Engineers use the strength of the bolt ( the grade ) to figure out what cross section of steel is necessary. The size of the bolts they can use by how much room is on the mating area. By taking the sum of the cross sections of each of the bolts they can come up with the correct number of bolts required. The reason it's necessary to torque all the bolts evenly is because one bolt tighter than the rest will take all of the load, and will fail in shear.

The threads on a bolt take all the load that is pulling on it. That's an interesting thing to think about. A fine thread bolt therefore has more metal holding it in place.

Reverse thread nuts and bolts are sometimes used on things that spin like pulleys and shafts. Spinning direction that tends to tighten a fastener is preferable to one that tends to loosen it for obvious reasons.