As a non-native English speaker, one of the most recurring confusions I've had, and which I can't seem to ever get straight, is how to refer to those mechanical "thingies" you often see in old machines and in cartoon depictions of machines.

Are they "cogs", "wheels", "cogwheels", "cog wheels", "sprockets", or something different entirely? I suspect that there are multiple synonyms in English and also that there are different variations of the same basic idea, given different words, which is probably what confuses me.

The one I imagine is like a metallic "circle" (although usually not very flat) with "teeth" around it, made to hook into another such object for the purpose of doing some kind of mechanical work in an efficient and predictable manner. Sometimes, they seem to not have teeth but instead a belt. I don't know if these are considered distinct objects or all the same.

I assume that there are still large, industrial machines that use these, but I primarily associate them with the 1800s and first half of the 1900s. They also exist as very flat and small versions in mechanical watches.

  • Then there are gears and gear wheels, pinions, toothed wheels, and yes, those flat wheels which drive belts which drive other flat wheels. Don't worry, native speakers would find the range of terminology confusing too if they were to think about it, bar those native speakers familiar with the terminology and its application. Which, I suspect, varies from one branch of mechanical engineering to another. Jan 15 '20 at 16:03
  • Sprockets, cogs, and gear wheels are the 19th-century equivalent of nuclear physics and rocket science. They were gee-whiz stuff then and they're sort of archaic comic items now; google "Rube Goldberg" to see their epitome. Jan 15 '20 at 16:13
  • One problem is that "cog" is used as another word for "cogwheel" (lexico.com/en/definition/cogwheel). More info: en.wikipedia.org/wiki/Gear. We laypeople usually call them "cogs" here (US, SE Region).
    – KannE
    Jan 15 '20 at 16:16
  • All of the above, and more. "Cog" is especially used by cyclists to refer to the chain sprockets on a bike. And "cog" is sometimes used to mean "cam", as seen on the camshaft of an auto engine.
    – Hot Licks
    Jan 15 '20 at 17:06

Cog traditionally referred to the individual teeth on the cogwheel. Here is the first definition in "cog, n. 2" in the Oxford English Dictionary:

One of a series of teeth or similar projections on the circumference of a wheel, or the side of a bar, etc., which, by engaging with corresponding projections on another wheel, etc., transmit or receive motion.

A wheel with cogs would be more specifically called a cog-wheel. The OED again, "cog-wheel, n.":

A wheel with cogs, used to transmit motion; more generally, a toothed wheel which engages with another similar wheel, or with a toothed bar or rack; a gear-wheel.

Now, you've posited several other options for what these items may be called. Languages often generate synonyms or hyponyms (more specific words) for the same item. Here are a few for cogwheel:

Cog: Yes, by the 18th century some English users had shortened cogwheel to cog and used that to refer to the entire wheel apparatus. From "cog, n. 2":

1712 J. Browne tr. P. Pomet et al. Compl. Hist. Druggs I. 54 The great Roller in the middle is surrounded with a Cog.

Sprocket: A term for a kind of cog-wheel that meshes with a chain; note that some cog-wheels work with other toothed parts like belts or other cog-wheels (OED, "sprocket, n."):

b. A wheel equipped with projections along its rim that are used to engage with the links of a chain, the perforations along the edge of a strip of film, etc.

Gear or gearwheel: cog-wheels that work upon each other (OED, "gear, n."):

b. Wheels working one upon another, by means of teeth, or otherwise. a train of gears: a set of such wheels

Cogwheel is virtually identical to cog-wheel. The OED uses the hyphen; Merriam-Webster omits it.

Finally, people mix these uses all the time. I couldn't have told you the exact difference between a sprocket, a cog, and a cogwheel without looking it up. Unless you're working in a highly technical context or enjoy linguistic trivia, I wouldn't worry.

  • There is an important difference between a sprocket and a gear (or cog wheel) which is the shape of the teeth. The teeth of a sprocket have concave curves to fit the chain whereas the teeth of a gear are straight so that they fit the rack, pinion or the gear with which they mesh. If you try to mesh two sprockets they might work for a while but will not work well.
    – BoldBen
    Jan 15 '20 at 20:41

When they have teeth, my experience is that they’re “gears” (definition 6a/2) or “sprockets” (definition 1), though the latter is contextually specific. A “cog” is one of the teeth on a gear, but “cogwheel” is synonymous with “gear”.

When it doesn’t have teeth, and carries a belt, it’s called a “sheave” or “pulley”.

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