It is almost an age old question, and by many out there these two words are almost interchangeable. Ask one engineer and they will give you an answer, ask another you will likely get a slightly different answer. Over the last few hundred years engineers have developed fasteners, sometimes for very specific applications, so the line can sometimes become blurred. Some fasteners can only be defined when it has been put into an assembly, being dependent on the design. There are a few standards out there that make an effort to define this, with varying degrees of vagueness, but I am going to try and make sense of it in this post and give you a few examples to make it more obvious. So lets put a few definitions into the mix. The definition I think is the best I can find is from the Specification for Identification of Bolts and Screws, ANSI-ASME B18.2.1 1981. This document has been superseded a few times but the changes have been to add many more extra definitions rather than change this one. Their definition is:
A bolt is an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.
I like these definitions as they put it in super simple terms that you can get your head around. Think of how you can tighten the fastener, if you have to use the head then it much be a screw, if you have a nut on the other end it mush be a bolt. From there it gets a bit more complicated, as you can also often tighten a bolt by the head as well, but the point is that you can use either, whereas a screw can only be tightened or loosened by turning the head. The other key point is that a bolt should not be tapping its own thread in the part it fastens to. A screw doesn’t have to form its own thread in the material, but if it does it can only be a screw. Basically if it is pointy it is likely to be a screw, if it is flat ended it is more likely a bolt (but not always true as we will see. There is also one other way to loosely define a bolt and that is the way it drives. A screw drives from the center (like an Alan head or flat head screw) but a bolt tends to need to be fastened with a wrench, so away from the center. Some companies like Accu group use this as a definition but it is not defined in most standards I have read, but still a good rule of thumb. Now lets look at a few examples to get a better idea:
Bolts That Cannot be Unfastened by the Head
A definite subset of bolts, the round head, oval head and plow bolt have not way to be undone via the head. The round head and oval head both protrude above the surface, but are completely smooth and rounded on the edges, so there is no surface for a wrench to lever against, so they have to be fasted by a nut on the other end. These bolts usually have a non circular area near the head to stop it from turning when the nut is being attached.
Screws That Cannot Use a Nut
The classic screw is something that we are all familiar with, with it tapering to a point, often with a straight thread with multiple pitch length, and cannot use a nut. The tapering prohibits the use of adding a nut, this describes a classic wood screw. Other screws such as tapping and grub screws with points or shanks are also definitely screws by the fact they often make their own thread and have a point in the end to make some sort of non screwed connection with another part.
Bolts That Need a Nut to Function
Some bolts such as a hex structural bolt that have a shaft the same diameter as the thread (no shoulder) and therefore go through a part and needs to be attached into a nut on the other side to be fastened. The bolt has a smooth shaft near the head which cannot be fastened on its own, therefore needing a nut. By the fact it needs a nut it has to be a bolt. Most classic bolts often need a nut to work in an assembly, and it is the best way to recognise a bolt over a screw.
Screws That Look Like Bolts
This is where things can get a bit iffy, fasteners that on the face of it look like bolts but act a bit more like screws. Set screws for instance are a screw as they never use a nut, and they are usually used to secure an object within or against another object. Things like attaching a gear or pulley to a shaft is a common example. The other is a shoulder screw which looks much like a normal bolt but is different by the fact that the non-threaded shaft is bigger than the threads, hence the shoulder. The threaded part does not tend to be screwed into a nut, leading to the definition of it being a screw rather than a bolt. They tend to be used as a shaft for rotating things like pulleys or gears.
- Distinguishing Bolts from Screws – U.S. Customs and Border Protection – July 2012
- I you can get access – Specification for Identification of Bolts and Screws, ANSI – ASME B18.2.1 1981
- If you can get access – Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws (Inch Series), ANSI – ASME B18.2.1 2012
- For interesting reading about a court case about this: Rocknel Fastener, Inc. v. United States, 24 C.I.T. 900, 118 F.Supp. 2d 1238 (Ct. Int’l. Trade 2000)