a self - energizing , positive engagement safety mechanism for long stroke , well pumping units , whether powered mechanically or hydraulically , and which employ a lift belt and counterweight . upon failure of the sucker rod , polish rod or lift belt , a latching mechanism is actuated to engage a rack on both sides of the counterweight thereby to arrest and lock the counterweight against free fall . actuation of the latching means is controlled by sensing belt tension below the counterweight thereby to enhance the sensitivity and reliability of the safety mechanism .