Topic 2

Car Mechanics (UK) - - Automatic Gearboxes -

The epicyclic gear­box

While the ac­tual au­to­matic gear­box unit is quite daunt­ing, the prin­ci­ples upon which it op­er­ates are also fairly sim­ple. The heart of the unit is the epicyclic gear set, which cre­ates two in­puts and one out­put. At least sev­eral of these gear sets are in­stalled within a typ­i­cal au­to­matic trans­mis­sion. Named af­ter an­cient as­tron­omy the­ory, a so-called sun gear is mounted in the cen­tre and mesh­ing with its outer teeth is, usu­ally, a trio of plan­e­tary gears that are sup­ported by a plan­e­tary car­rier frame. The ring gear (or an­nu­lus) sits on the outer edge and its in­ter­nal teeth run on those of the plan­e­tary gears. Thus, the plan­e­tary car­rier acts as the out­put and in­ter­acts with both the sun and ring gears as the in­puts (see il­lus­tra­tion be­low). The­o­ret­i­cally, any one of these three gears can be locked, which al­lows the oth­ers to han­dle the out­put. By hav­ing sev­eral in­ter­linked epicyclic gear sets and a set of brakes, or clutches, a gear­box can be de­signed with a num­ber of dif­fer­ent ra­tios.

The clutch con­sists of a se­ries of fric­tion plates, each of which bears against metal plates (called ‘steels’) and this ar­range­ment pro­vides a large work­ing sur­face area. The fric­tion plates and their steels are housed within a clutch drum that con­tains a pis­ton, ac­ti­vated by oil pres­sure, which squeezes the clutch pack to­gether, lock­ing the rel­e­vant com­po­nent and per­mit­ting it to turn. Older trans­mis­sions tended to use ad­justable brake bands, which are wrapped around the ring gear, lit­er­ally hold­ing it in po­si­tion. In most cases, mod­ern wet clutch plates have re­placed them. The oil pres­sure is pro­vided by the valve body, which re­ceives com­mands from the trans­mis­sion con­trol mod­ule (TCM), based on feed­back from the var­i­ous sen­sors on the trans­mis­sion and ve­hi­cle.

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