The Difference Between Bolts vs. Screws

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 This document has been superseded a few times but the changes have been to add many more extra definitions rather than change this one.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.

Externally threaded fasteners with a head that cannot be used to fasten it in place is a bolt. Credit (1)

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.

An externally threaded fastener that which has a thread that cannot be used with a nut is a screw. Credit: (1)

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.

A hex Structural bolt is a great example of a bolt that needs a nut to function as the lack of thread near the head cannot be used to fasten. Credit: (1)

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.


  1. Distinguishing Bolts from Screws – U.S. Customs and Border Protection – July 2012
  2. I you can get access – Specification for Identification of Bolts and Screws, ANSI – ASME B18.2.1 1981
  3. 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
  4. 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)

LM3909 – An IC Just to Flash an LED

So during my placement year I was getting really into old electronics, and old IC’s, especially those no longer in production. We were also on a project where we were trying to design a circuit that would flash an LED for a short period of time from the charge on a small super capacitor. The big issue we had was how to minimise current flow, and power an LED at really low voltages, less than 2V. This on the face of it seems like a simple problem, until you start to think about it.

Million Mile Light
Products like the Million Mile Light, a flashing low powered, high brightness LED indicator need to flash for long periods of time on very little charge, much like the problem I faced. Credit: Million Mile Light

There are two go to ways that most engineers would go with to make a flashing LED with a constant flash rate. First is to use a small microcontroller, such as an ATTiny, or a Pic12F series, and use software to flash the LED. This seems good on the surface (and it is what we used in the end product) but it has a big drawback, it can only output a voltage less than the power rail. some versions of the PIC12LF’s can function down to 1.8V, perfect for our power supply needs, but LED’s need upwards of 2.7V (usually) before they start to light, so although our micro will work the LED wont. The second go to way to make an LED flash would be to use the classic 555 timer, one of the most manufactured chips of all time. There is a good reason it is famous, it is extremely versatile. You can decide the frequency based purely on the capacitor and resistor choices. We still have a similar drawback though, a 555 timer needs at least 4.5v as a power supply. So with our potential sub 2V power supply, neither the IC or the LED will turn on. That is one way to conserve energy!

A cut out from the datasheet, with a basic view of the circuit inside the IC, which I will go through in another post, and a pinout diagram, very useful for prototyping. Credit: National Semiconductor.

This is where the LM3909 came in to play. You have to remember that this chip was developed prior to 1995 (so it is older than me) when the electronics market was very different. Battery technology was not the same, and nowhere near as cheap. It was much more common for people to want to use off the shelf single use batteries such as AA, C and D batteries, or even coin cells in most projects. If you wanted something with a little flashing light on it there were plenty of applications for it. There are buoys in the ocean, store signs and displays, and Christmas lights, all of which would benefit from minimising weight of batteries, but lasting for serious amounts of time. Just as a reference, you could get to 4.5V (to power a 555) by using 3 AA batteries, but the voltage across them would soon dip below this, so you would need at least 4 in most applications. 4 AA batteries take up a lot of space, and weight, not great for many of these applications. Plus most of the chips we have discussed use a fair amount of power, the 555 uses at least 3mA while running, not including the dissipation in the resistors, and all of the power charging the capacitor wasted.

1.5v schematic
A snippet of the datasheet, showing the simplest connection diagram, and a graph of typical current consumption with relation to the battery voltage. It also has a great table describing how long standard batteries tend to last in this configuration, up to 2 years! Credit: National Semiconductor.

So how does the LM3909 solve these issues? well it makes use of a clever concept similar to the 555 of charging up a capacitor. The difference is that the 3909 uses that charge in the capacitor to flash the LED. Although it is slightly more complex than the below schematic, you can think of it as there being a switch inside that oscillates between two states. We will go through how it actually works in a future post. To start with the capacitor is in series with the battery, and in parallel with the LED. The LED wont light, but the capacitor charges up to near the power supply voltage. Once charged, the switch inside flips, and now the power supply, charged capacitor, and LED are in series with each other. To the LED it now sees the capacitor (charged to 1.5V) plus the 1.5V power supply, equivilent to 3V, more than the forward voltage it needs to turn on. As there is a very small resistance, the LED will be on as long as the capacitor has some charge, which isn’t very long as it will discharge fairly quickly. This is the “flash”, as once the cap is discharged the LED will turn off, and the switch will flip back. The capacitor starts charging again, and the whole process restarts. This goes on for as long as the battery has power to give.

Rob Paisley
A great description of the basic principle of how the LM3909 works, charging the capacitor up, and then releasing all that energy through the LED, with increased voltage. Credit: Rob Paisley.

A couple of points to note, the timing and the brightness of the flashing is based upon the capacitor you use, which is quite clever. There are two settings, depending in the pin you put the capacitor in will also double (or halve) the time the cap will take to charge. This means slower flashing, but longer lifetime. Having a smaller capacitor will mean faster, but less bright flashing, and a bigger cap will therefore be slower and much brighter flashing. The design of the chip also means that only two external components are needed for it to work, a capacitor and the LED, compared to the many resistors and extra cap needed on things like a 555 timer. The fact it can use less than 1.5V power source means we can use a single AA battery to power this device, and according to the data sheet it can last up to 6 months on one battery! I have one on my desk that has lasted longer than this.

my LM3909 circuit
My version of this circuit fit into a AA battery box, with it being powered by a single AA battery. It has a switch meaning I can turn it on and off. Poundland LED lights are a good source of these!

All in all I can see why National Semiconductor decided to make this chip, it filled a gap, and was used widely for a long time. Developments in battery technology, and more complex designs needed for the applications this was for has meant that they no longer make the LM3909, but they are still available on Ebay and some Chinese manufactures make them. There is also a design out there to make a discrete version of the LM3909, and I may try that for a future post, as it looks interesting.

Thank you for reading, take a look at my other posts if you are interested in space, electronics, or military history. If you are interested, follow me on Twitter to get updates on projects I am currently working on.

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