A clutch for a rotary shaft includes an annular pressure chamber with a sleeve surface engaging a surface to drivingly connect the shaft to the clutch, and further includes a trigger device and a channel system activatable by the trigger device upon a relative angular movement between the shaft and the chamber to release the shaft from the housing. The trigger device has a cam wheel having a constant first radius across a first angular interval and a second, smaller, radius across a second angular interval. The cam wheel is supported on an envelope surface within the first angular interval and rotating in relation to the envelope surface when the shaft rotates in relation to the housing. When a contact point reaches the second angular interval, the cam wheel is displaced to activate said channel system.