Lubrication mechanism and balance piston for a high speed clutch

A transmission unit of a work vehicle comprising:

• a drive shaft having a clutch fluid supply passage and a balance fluid supply passage therethrough;
• a housing surrounding the drive shaft and rotatable therewith;
• a gear rotatably mounted around the drive shaft;
• a clutch assembly provided around the drive shaft and including a clutch piston and a clutch pack, the clutch pack provided in a clutch pack cavity provided between the housing, the clutch piston and the gear and being coupled to the gear and to the housing, wherein a clutch piston cavity is provided between the housing and the clutch piston and is configured to receive fluid under pressure through the clutch fluid supply passage to move the clutch piston into contact with the clutch pack;
• a balance piston mounted within a pocket of the clutch piston and encircling the drive shaft, the balance piston being fixed with respect to movement along a longitudinal axis of the drive shaft, wherein a balance piston cavity is provided between the clutch piston and the balance piston and the balance piston cavity receives fluid under pressure through the balance fluid supply passage;
• and
• a cooling fluid shutoff piston mounted in a passageway of the clutch piston, wherein the passageway is open to the balance piston cavity, wherein the cooling fluid shutoff piston is configured to be in a closed position which blocks a lubrication supply opening of the passageway through which fluid is supplied to the clutch pack cavity from the balance fluid supply passage when the clutch piston is disengaged from the clutch pack, and wherein the cooling fluid shutoff piston is configured to be in an open position which opens the lubrication supply opening when the clutch piston is engaged with the clutch pack and a first fluid pressure is within the clutch piston cavity, and wherein the cooling fluid shutoff piston is configured to be in the closed position when the clutch piston is engaged with the clutch pack and a second fluid pressure is within the clutch piston cavity, wherein the second fluid pressure is greater than the first fluid pressure.