Automatic transmission with lubricating structure

An automatic transmission, comprising:

• a speed change mechanism for shifting between a plurality of speeds by engaging and disengaging a plurality of clutches and brakes, the speed change mechanism comprising a first clutch having a first drum, first friction engagement elements disposed in the first drum, and a first piston which is slidably mounted on a boss portion of the first drum, the first piston and the first drum together forming a first cylinder chamber, the first piston engaging the first friction engagement elements of the first clutch responsive to oil pressure supplied to the first cylinder chamber, and a second clutch including a second drum accommodated within the first drum, second friction engagement elements disposed in the second drum, and a second piston which is slidably mounted on a boss portion of the second drum, the second piston and the second drum together forming a second cylinder chamber, the second piston engaging the second clutch responsive to oil pressure supplied to the second cylinder chamber;
• a drain hole formed in the second drum for draining lubricating oil supplied to the second friction engagement elements, the drain hole being sized to limit the amount of drainage of the lubricating oil when the second clutch is engaged;
• and
• an oil reservoir, formed as an annular groove in an inner cylindrical surface of a skirt portion of the first piston, for receiving and storing the lubricating oil that is drained through the drain hole after lubricating the second clutch, the oil reservoir having a cylindrical section extending axially between a first barrier, at an axial side of the reservoir adjacent an open end of the skirt of the first piston, the first barrier having an inner diameter which is smaller than the inner diameter of the cylindrical section of the oil reservoir, and a second barrier axially spaced from the first barrier and having a larger inner diameter than the first barrier and smaller than the inner diameter of the cylindrical section of the oil reservoir;
• and
• wherein the first piston engages the first friction engagement elements at a location between an outer cylindrical portion of the first drum and an out cylindrical portion of the second drum, and
• wherein, when the second clutch is disengaged a larger amount of oil flows to the oil reservoir than when the second clutch is engaged, which larger amount causes lubricating oil to overflow the first barrier and flow to the first friction engagement elements.