This piece of equipment was the first successful mining device to replace human labor in removing the rubble from underground hard rock-blasting. The Model 12B, which weighs 4,200 pounds and requires 60 to 125 pounds-per-square-inch air pressure, was first introduced in 1938. When the bucket is loaded with rubble, the operator actuates the bucket drive motor to exert force on the rocker pull chain, which is attached near the outer end of the rocker arms. The rocker arms roll the bucket upward toward the rear of the machine so that it maximizes the initial lift force on the bucket. As the bucket is thrown rearward, its vertical velocity decreases, and it's horizontal velocity increases until the rocker arms strike shock-absorbing stops on the frame. When the bucket stops, the contents are flung into an attached mine car.