Words for axle, axis and related things in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*akslā, *aksilā, *axsilā = axis
Old Irish (Goídelc)	ais = back, hinder part
Middle Irish (Gaoidhealg)	ais = back, hinder part
Irish (Gaeilge)	ais [aʃ/æʃ/ɛʃ] = back
Scottish Gaelic (Gàidhlig)	aiseal [aʃəl̪ˠ] = axle(-tree), axis
Manx (Gaelg)	essyl = axle, axis
Proto-Brythonic	*axl, *ėxɨl = axle, axis (?)
Middle Welsh (Kymraec)	echel, achel = axle-tree, axle, axis, pivot
Welsh (Cymraeg)	echel [ˈɛχɛl/ˈeːχɛl] = axle-tree, axle, axis, pivot echelu = to furnish with an axle, place on an axle echelbin = linch-pin echelin = axis, axial, axile
Cornish (Kernewek)	aghel = axle
Middle Breton (Brezonec)	ahel, aël = axle
Breton (Brezhoneg)	ahel [ˈɑ(h)ː.ɛl] = axle ahelan = to centre sth on, to pin (together) ahelek = axial

Etymology: from Proto-Celtic *axsi- (back), from Proto-Indo-European *h₂eḱs- (axis), possibly from *h₂eǵ- (to drive) [source]. Words from the same PIE roots include: axis and axle in English, as (axis) in Dutch, Achse (axis, axle) in German, ais (board, plank) in French, and eje (axis, axle, shaft, spindle, core, heart, hub) in Spanish [source].

Proto-Celtic	*u̯ert-iti-, *u̯er-t- = to turn
Old Irish (Goídelc)	fertas = axle, spindle, shaft, rung, ridge, bank
Middle Irish (Gaoidhealg)	fertas, fersat = shaft, spoke, axle, shaft, axletree, swingletree
Irish (Gaeilge)	fearsaid [ˈfʲaɾˠsˠədʲ] = spindle, shaft, axle(-tree), passage, piece of poetry, verse, ditty fearsaideach = shafted, spindly
Scottish Gaelic (Gàidhlig)	fearsad, feairisid [fjar̪ˠsəd, fɛr̪ʃɪdʲ] = spindle, shaft, axle, rung, spoke, track, rut, sandbar, ripple mark (in sand at low tide)
Manx (Gaelg)	fess = spindle
Proto-Brythonic	*axl, *ėxɨl = axle, axis (?)
Middle Welsh (Kymraec)	guerthyt, gwerthyt = spindle, axle, shaft
Welsh (Cymraeg)	gwerthyd = spindle, axle, shaft, rung, stave gwerthydu = to furnish with a spindle or axle, to fashion like a spindle gwerthydaid = spindleful of yarn gwerthydwr = spindle-maker
Old Cornish	gurhthit = spindle
Middle Cornish (Cernewec)	gurthid, gurhthit, gurthyd = spindle
Cornish (Kernewek)	gwerthys = bobbin, cage, spindle
Old Breton (Brethonoc)	guirtit, guirtitou = spindle, axis
Middle Breton (Brezonec)	guerzit = spindle, axis
Breton (Brezhoneg)	gwerzhid [ˈɡwɛrzit] = spindle, axis

Etymology: possibly from Proto-Indo-European *wértti (to be turning around), from *wert- (to turn, rotate) [source]. Words from the same PIE roots include: verse, version, versus, vertebra, vertical, vortex, wierd and worth in English, worden (to become, get, grow, turn) in Dutch, and werden (will, be going to) in German [source].

Proto-Celtic	*molos = grinder
Old Irish (Goídelc)	mol = axle (for a mill-wheel), shaft
Middle Irish (Gaoidhealg)	mol, moil, muil = axle, shaft (of a mill-wheel)
Irish (Gaeilge)	mol [mˠɔl̪ˠ / mˠʌlˠ] = hub, shaft, pivot, nave, Pole, top, crown Mol Thuaid = North Pole Mol Theas = South Pole
Scottish Gaelic (Gàidhlig)	mul [mul̪ˠ] = axis, axle-tree
Manx (Gaelg)	moll = nave

Etymology: from Proto-Indo-European *molh₂ós, from *melh₂- (to grind, crush) [source]. Words from the same PIE roots include: Mjollnir (Thor’s hammer), mallet, meal, mill, millet, molar, and mull in English, Mühle (mill, grinder) in German, milho (maize, corncob) in Portuguese, and mijo (millet) in Spanish [source].

Sources: Wiktionary, Etymological Dictionary Of Proto Celtic, In Dúil Bélrai English – Old Irish glossary, eDIL – Electronic Dictionary of the Irish Language, Teanglann.ie, Am Faclair Beag, An etymological dictionary of the Gaelic language, Fockleyreen: Manx – English Dictionary, Online Manx Dictionary, Gaelg Corpus, Geiriadur Prifysgol Cymru, Lexicon cornu-britannicum : a dictionary of the ancient Celtic language of Cornwall, Gerlyver Kernewek, Devri : Le dictionaire diachronique du breton, Dictionnaires bilingues de Francis Favereau / Edition Skol Vreizh, TermOfis