. ▁Finally , ▁the ▁ring ▁g ear ▁also ▁cou ples ▁and ▁co - rot ates ▁with ▁the ▁outer ▁plan ets ▁in ▁a ▁fixed ▁g ear ▁ratio ▁with ▁respect ▁to ▁them . ▁ ▁Planet ary ▁g ear ▁trains ▁ ▁In ▁contrast ▁to ▁a ▁R avig ne aux , ▁a ▁ ▁simple ▁planet ary ▁g ear ▁train ▁has ▁two ▁concent ric ▁g ears , ▁one ▁or ▁more ▁plan ets ▁brid ging ▁the ▁gap ▁between ▁those ▁g ears , ▁and ▁one ▁arm ▁carrying ▁the ▁plan ets . ▁ ▁Usually , ▁the ▁smaller ▁diameter ▁concent ric ▁g ear ▁has ▁external ▁teeth ▁and ▁is ▁called ▁a ▁sun ▁g ear , ▁and ▁the ▁larger ▁concent ric ▁g ear ▁has ▁internal ▁teeth ▁and ▁is ▁called ▁a ▁ring ▁g ear . ▁ ▁The ▁carrying ▁arm ▁is ▁usually ▁called ▁the ▁planet ▁car rier . ▁ ▁Planet ▁g ears ▁are ▁usually ▁not ▁directly ▁couple d , ▁leaving ▁three ▁components ▁that ▁are ▁directly ▁couple d : ▁ ▁Sun ▁g ear ▁Ring ▁G ear ▁Planet ▁car rier ▁ ▁This ▁provides ▁for ▁two ▁inputs ▁and ▁one ▁output . ▁ ▁Usually , ▁in ▁an ▁automatic ▁transmission , ▁one ▁component ▁is ▁held ▁fixed , ▁while ▁another ▁component ▁is ▁input , ▁and ▁the ▁remaining ▁component ▁is ▁used ▁for ▁output . ▁ ▁By ▁choosing ▁a ▁set ▁of ▁components ▁for ▁held , ▁input , ▁and ▁output , ▁the ▁planet ary ▁g ear ▁will ▁increase ▁or ▁decrease ▁speed ▁and ▁change ▁direction . ▁ ▁In vention ▁of ▁the ▁R avig ne aux ▁The ▁R avig ne aux ▁impro ves ▁upon ▁earlier ▁planet ary