▁autom obile ▁transm issions , ▁such ▁as ▁the ▁Sim pson , ▁which ▁has ▁two ▁complete ▁planet ary ▁g ear sets . ▁ ▁As ▁originally ▁invent ed , ▁the ▁R avig ne aux ▁puts ▁two ▁planet ary ▁g ear ▁trains ▁on ▁a ▁single ▁planet ▁car rier . ▁ ▁It ▁has ▁two ▁sun ▁g ears , ▁two ▁ring ▁g ears , ▁and ▁two ▁sets ▁of ▁planet ▁g ears ▁on ▁a ▁single ▁car rier . ▁ ▁Using ▁a ▁single ▁car rier ▁makes ▁the ▁R avig ne aux ▁smaller , ▁l ighter , ▁and ▁less ▁expensive ▁to ▁make , ▁because ▁the ▁car rier ▁is ▁usually ▁the ▁largest ▁and ▁most ▁expensive ▁part ▁of ▁a ▁planet ary ▁g ear set . ▁ ▁The ▁original ▁pat ent ▁application ▁proposed ▁that ▁elect romagnet ic ▁cl ut ches ▁be ▁used ▁in ▁the ▁transmission . ▁ ▁The ▁year ▁of ▁the ▁pat ent ▁was ▁ 1 9 4 9 , ▁a ▁time ▁when ▁pressure ▁operated ▁hyd ra ul ic ▁technology ▁was ▁less ▁well ▁developed . ▁ ▁Several ▁years ▁later , ▁when ▁the ▁R avig ne aux ▁went ▁into ▁production ▁at ▁Ford ▁Motor ▁Company , ▁hyd ra ul ically ▁operated ▁band ▁bra kes ▁and ▁cl ut ches ▁were ▁used . ▁ ▁Ford ▁FM X ▁R avig ne aux ▁example ▁ ▁The ▁photos ▁on ▁the ▁right ▁show ▁the ▁planet ▁car rier ▁from ▁a ▁Ford ▁FM X ▁automatic ▁transmission . ▁The ▁planet ▁car rier ▁is ▁the ▁central ▁part ▁of ▁a ▁planet ary ▁g ear ▁set . ▁Not ▁shown ▁are ▁the ▁two ▁sun ▁g ears , ▁called ▁primary