Parts of a Gear – Labeled Spur Gear Diagram

All 17 parts on the diagram

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The whole gear 7

Teeth They mesh with the teeth of another gear. One gear then turns the other without slipping. A tooth is also called a cog.
Rim The teeth are cut in its outside. On a small gear there is no separate rim and the gear is one solid disk.
Web It holds the rim to the hub. Holes are often left in it to save metal. A large gear may have arms there in place of a web.
Hub It fits on the shaft. Its outside is usually about twice as wide as its bore.
Bore The shaft goes through it.
Keyway A small metal key sits half in this slot. The other half is in a slot in the shaft, so the gear and the shaft have to turn together.
Face width It is measured along the shaft. The drawing at the right is the same gear seen edge on.

The teeth, close up 10

Face The working surface from the pitch circle out to the tip.
Flank Runs from the pitch circle down to the fillet at the root.
Fillet It joins the flank to the bottom of the space. A generous curve here makes the tooth stronger.
Addendum circle Its diameter is the outside diameter of the gear.
Pitch circle Two meshing gears act like two smooth wheels of this size. The wheels roll on each other. A gear's size is given by this circle's diameter.
Root circle Its diameter is the root diameter.
Addendum Measured from the pitch circle out to the addendum circle. Measured along a radius.
Dedendum It is a little more than the addendum. That leaves a clearance for the tip of the mating tooth.
Circular pitch It is measured along the pitch circle. One circular pitch is one tooth and one space.
Tooth thickness On a standard gear it is half the circular pitch. The tooth and the space beside it are equally wide.

How this list was compiled: Part names checked on 2026-09-30; a name is kept only when two independent sources use it. (1) Wikipedia, text only: List of gear nomenclature (addendum, addendum circle, circular pitch, face width, pitch circle, root circle, tooth surface or flank, fillet, top land and bottom land, outside diameter), Gear, Involute gear, Key (engineering) (keyway, keyseat). (2) American Machinist Gear Book, McGraw-Hill, about 1910, full text at https://archive.org/details/americanmachini03logugoog (tooth parts: pitch circle, addendum circle, addendum, dedendum, clearance, whole depth, circular pitch, thickness of tooth, face and flank of the tooth, fillet; gear blanks: rim, web, arms, hub, bore, keyway and keyseat, width of face). (3) Machinery's Handbook, The Industrial Press, 1924, full text at https://archive.org/details/machineryshandbo00indu (pitch circle, addendum, dedendum, circular pitch, thickness of tooth, root diameter and root circle, width of face, rim, web, hub, bore of the hub, keyway, fillet at the base of each tooth, face and flank). (4) G. B. Grant, A Treatise on Gear Wheels, full text at https://archive.org/details/treatiseongearwh00gran (addendum, dedendum, flank, fillet, clearance). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work

How it works

The parts of a gear come in two sets. The wheel itself has teeth, a rim, a web, a hub and a bore. Each tooth then has its own names, measured from the pitch circle.

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The wheel

Start with the whole gear. The teeth stand on the rim, the web joins the rim to the hub, and the shaft goes through the bore, locked by a key in the keyway. The edge-on drawing at the right shows the face width.

One tooth

Three circles cross the teeth: the addendum circle at the tips, the pitch circle part way up and the root circle at the bottom. The addendum is a tooth's height above the pitch circle and the dedendum its depth below. Above the pitch circle the side of a tooth is its face; below, its flank.

Cog can mean a whole gear or one of its teeth. Either way, the parts of a cog are the ones named here.
The pitch circle is not drawn on a real gear. It is the size of smooth wheel the gear behaves like.

Questions and answers

What are the main parts of a gear?

The teeth round the outside, the rim that carries them, the web or arms between the rim and the middle, the hub, and the bore for the shaft, which often has a keyway.

What is the pitch circle of a gear?

An imaginary circle part way up the teeth. Two gears in mesh act like two smooth wheels of their pitch circle sizes rolling together, so it is the circle a gear's size and tooth spacing are measured on.

What is the difference between addendum and dedendum?

The addendum is how far a tooth stands above the pitch circle. The dedendum is how far the space goes below it. The dedendum is slightly larger, which leaves clearance for the tip of the other gear's tooth.

What is the hole in the middle of a gear called?

The bore. The thick part round it is the hub. The small square slot in the side of the bore is the keyway, which takes a key that locks the gear to its shaft.