Freeport-McMoran Copper & Gold Inc. (FCX) is a leading international mining company with headquarters in Phoenix, Arizona. It operates the Morenci copper mine in southeast Arizona. Morenci is North America’s largest producer of copper and one of the largest open-pit mines in the world. Its operations pioneered the use of Global Positioning Satellite system technology and computer-controlled dispatching in its haul trucks, shovels and other heavy equipment.
Another technology Morenci has pioneered has been the use of HR SmartBolts for bolt tension set-up and assurance during operations in its Crush and Convey systems. HR SmartBolts is the flagship product of Stress Indicators, Inc. HRs provide a very distinctive “grass green” indication when Design Tension is reached and are specified to provide tension accuracy of plus or minus 5%.
The HR SmartBolt is “yellow” when loose and remains yellow until 80-90% of Design Tension is reached. At that point, the indicator rapidly changes to “grass green”. If tightened beyond that point, the indicator will darken towards blue-black. This distinct color change combination allows the user to “dial-in” the proper tension and not go beyond to an over-tension state. If the fastener loses 20% of tension, it changes back to yellow, providing visual feedback that it needs to be re-tightened.
The HR SmartBolts for this application were custom-designed to meet application needs. Stress Indicators engineers worked in cooperation with Morenci Mine’s operations experts to build HR SmartBolts technology into 1-1/4” hex-bolts with a Design Tension of 58 kilopounds-force.
Morenci’s application involves a rotating drum for copper ore processing that weighs 88 tons when empty. Operators haven’t even estimated how much additional weight the ore provides. Furthermore, the load produces unequal weighting on the drum and supporting frame as it rotates. To add to the forces, the drum sits inclined by about 10 degrees fore and aft.
In this picture, one can see that the drum rotates on two shiny metal “tires”. These tires depend on trunion wheels (base of tire) which must bear all of the dynamic forces. These trunions are then bolted to the frame through the bearing housings. As Field Manager Bill Mitchell tells Stress Indicators, “It is here that we suspect the slipping of these bearing housings on the frame. As you can imagine, the outward force placed on these bearing housing bolts is tremendous, hence the need for the SmartBolts. They are working great and the 1,100 ft/lbs of torque seems to be doing the trick.”
Mr. Mitchell went on to discuss the results of the bearing slipping outward on the frame. He said that the drum would become misaligned, stripping gears and causing other expensive damage. The worst part is that the cost of the downtime to do repairs is extremely high, as Morenci depends on this mixing drum to provide continuous throughput.
This picture shows the SmartBolts going through the bearing housing. The trunion wheel is also visible against the drum “tire”. These bolts are 1-1/4” shank diameter bolts with a Design Tension of 58 kilopounds. Installers use 1100ft-lbs of torque to tighten the bolt sufficiently to produce the proper tension. Once the indicator reaches the “grass green” color, they know that proper tension has been achieved.
This picture shows the distinctive “Grass Green” color of the HR SmartBolt. This color shows the bolt is tensioned to meet design requirements. In the past, inspectors had to go to great effort to verify that the 1100ft-lbs of torque was still present, and even then they could not be certain that the torque was supplying the necessary tension to hold the bearing in place against the rotation of the massive 88-ton drum. Now all they need to do is visually verify the color at each maintenance interval.
