Power train of automatic transmission

A power train of an automatic transmission, comprising:

• an input shaft;
• an output gear;
• a transmission case;
• a speed reduction unit comprising a fixed input member that is fixedly connected to the input shaft, a fixedly stopped member that is fixedly connected to the transmission case, and first and second reduced speed output members that respectively output first and second reduced rotational speeds;
• an output unit comprising a first variable input member that is variably connected to the first and second reduced speed output members and the input shaft, a second variable input member that is variably connected to the input shaft, first and second variably stopped members that are variably connected to the transmission case, and a shift speed output member that always acts as an output member by being fixedly connected to the output gear, wherein the output unit generates seven forward speeds and three reverse speeds at the shift speed output member by operations of the first and second variable input members and the first and second variably stopped members;
• a first clutch for variably connecting the first variable input member to the first reduced speed output member;
• a second clutch for variably connecting the first variable input member to the second reduced speed output member;
• a third clutch for variably connecting the first variable input member to the input shaft;
• a fourth clutch for variably connecting the second variable input member to the input shaft;
• a first brake for variably connecting the first variably stopped member to the transmission case;
• and
• a second brake for variably connecting the second variably stopped member to the transmission case;
• wherein the speed reduction unit comprises:
• a first planetary gear set having a first sun gear, a first planet carrier, and a first ring gear as operational members thereof;
• and
• a second planetary gear set having a second sun gear, a second planet carrier, and a second ring gear as operational members thereof,
• wherein the second ring gear is operated as the fixed input member,
• the first sun gear is operated as the fixedly stopped member,
• the first planet carrier is operated as the first reduced speed output member,
• the first ring gear is operated as the second reduced speed output member,
• the first ring gear is fixedly connected to the second planet carrier, and
• the second sun gear is fixedly connected to the first sun gear;
• wherein the output unit comprises:
• a third planetary gear set having a third sun gear, a third planet carrier, and a third ring gear as operational members thereof;
• and
• a fourth planetary gear set having a fourth sun gear, a fourth planet carrier, and a fourth ring gear as operational members thereof,
• wherein the third sun gear is operated as the first variable input member,
• the third planet carrier is operated as the second variable input member,
• the fourth sun gear is operated as the first variably stopped member,
• at least one of the fixedly connected third planet carrier and the fourth ring gear is operated as the second variably stopped member, and
• at least one of the fixedly connected fourth planet carrier and the third ring gear is operated as the shift speed output member.