Dart for downhole device and downhole device

A dart for a downhole device, comprising:

• a ball;
• a cylindrical mandrel having a ball seat on which the ball is seated;
• and
• a sleeve attached to an outer periphery of the mandrel,
• wherein the mandrel has a plurality of circumferential grooves on the outer periphery,
• the sleeve includes an upper collet and a lower collet that are inclined radially inwardly of the mandrel,
• one of the upper collet or the lower collet engages the groove of the mandrel by reducing a diameter, and the other is in contact with a part between the grooves of the mandrel by increasing the diameter,
• the ball is formed of a first degradable material having a first degradation rate,
• the mandrel is formed of a second degradable material having a second degradation rate that is less than the first degradation rate,
• the sleeve is formed of a third degradable material having a third degradation rate that is less than the second degradation rate, and
• the sleeve serves as an adjustment mechanism for engaging the dart at any frac valve position in a liner.