The invention relates to a motor 
assembly method, in particular to a 
servo motor 
assembly system and a rear end cap sealing ring 
assembly device. The 
servo motor rear end cap sealing ring assembly device comprises an indexing rotary table device and a sealing ring assembly device. The indexing rotary table device is sequentially provided with a loading and unloading 
station, a rear end cap detection 
station, a bearing loading 
station, a pressing station, a pressing detection station and a sealing ring assembly station. Carriers are respectively arranged on each station, and the sealing ring assembly device corresponds to the sealing ring assembly station. The sealing ring assembling device comprises a fixed bracket, a sixth cylinder placing table, a sealing ring spreading mechanism, a fifth cylinder, a linear moving assembly, a 
photoelectric sensor assembly, a sixth cylinder, a sealing ring 
ferrule and a transverse moving guide plate, wherein the sealing ring assembling device comprises a fixed bracket, a sixth cylinder placing table, a sealing ring spreading mechanism, a fifth cylinder, a linear moving assembly, a 
photoelectric sensor assembly, a sixth cylinder, a sealing ring 
ferrule and a transverse moving guide plate; the fixing bracket is fixedly arranged on the mounting rack for transversely mounting the linear moving component; the elastic sealing ring is stretched and pushed off, and the device is assembled on the 
servo motor 
tail cap by 
internal stress thereof, and the action is in order, so the automatic assembly of the sealing ring is convenient.