*gembh-

1777, "a projecting part of a rotating machinery used to impart motion to another part," from Dutch cam "cog of a wheel," originally "comb," from Proto-Germanic *kambaz "comb," from PIE root *gembh- "tooth, nail." It is thus a cognate of English comb (n.). This might have combined with English camber "having a slight arch;" or the whole thing could be from camber.

It converts regular rotary motion into irregular, fast-and-slow rotary or reciprocal motion. "The original method was by cogs or teeth fixed or cut at certain points in the circumference or disc of a wheel ..." [OED]. Cam-shaft attested from 1850.

Old English camb (later Anglian comb) "thin strip of toothed, stiff material" (for dressing the hair), also "fleshy crest growing on the head of the domestic fowl" (so called for its serrations), hence "crest of a hat, helmet, etc.;" also "honeycomb" (for which see honeycomb (n.)) , from Proto-Germanic *kambaz (source also of Old Saxon and Old High German camb, German Kamm, Middle Dutch cam, Dutch kam, Old Norse kambr), literally "toothed object," from PIE *gombhos, from root *gembh- "tooth, nail."

From c. 1300 as a tool for carding wool (probably earlier; Comber as a surname is from c. 1200). Comb-paper (1866) is marbled paper in which the design is produced mostly by use of a comb.