An employee was putting a chain back on a sprocket. The gearbox and sprocket contained two drive chains connected to the lowerator (vertical conveyor) and counterweight assembly. The employee received an alarm indicating that one of the chains had gone slack. While the employee was putting the chain back on the drive sprocket with tongue and groove pliers, the weight of the chain and counterweight fell back in place on the sprocket, pulling the employee's left hand into a pinch point. The employee's ring fingertip was amputated up to the fingernail.