Valley Forge Total Solutions Approach Saves Mine Millions
Mills are a crucial part of mining operations, but also a source of concentrated vibration that can wreak havoc with bolted joints. Most mills are expected to run continuously for extended periods—often months at a time—until scheduled downtime for maintenance. Depending on the mine, some mills can even require a total liner changeout every three months. It’s an accepted fact of life for a punishing environment, in which hardened material is rotated and smashed by hundreds of steel grinding balls within the mill shell, which works much like a large cement mixer. Material is constantly falling 40 feet within the shell, crashing into the charge and mill liners fastened to the inside of the mill. It is here, in the bolted joints that fasten each mill liner to the mill shell, where bolt loading can vary widely. It is here, at the bolted joints that fasten each rib to the tumbler, where bolt life can vary widely.
Because of this, it is vital that the fasteners used in the mills’ bolted joints maintain proper tension throughout these extended in-service periods. Improper bolt tensioning that requires mill stoppage can easily add tens of thousands of dollars to a mine’s operating costs. In mining for precious metals such as gold, these costs can quickly balloon into the millions.
In such circumstances, it is critical to have a bolt manufacturer with both an innate understanding of joint tension and the ability to offer analysis, advice, and support. Valley Forge & Bolt, which engineers and manufactures fasteners with Load Indicating Technology, offers its capabilities and expertise to mining customers through a multifaceted solution:
- Providing torque-to-tensioning verification equipment for better bolt-load accuracy
- Providing on-the-ground support
- Creating an application-specific blend of bolt materials
- Using exclusive Wi-Fi technology to hone bolt performance
Understanding the mill reline
A grasp of the relining process is crucial to understanding the total solution approach provided by Valley Forge.
Each mill liner is attached to the inside of the mill with multiple bolts. Each bolt is secured in place by means of a two-part process. First, impact wrenches are used to set a bolt partially in the joint. Then, torque multipliers are used for final tightening. Each reline requires hundreds of bolts.
After all liners and bolts are installed and initially torqued, the mill is turned on to run for a predetermined period of time, after which the bolts are retorqued to accommodate any shift during usage. The amount of time to wait between initial install and retorquing is an important factor in prolonging fastener life. In some instances, mine operators wait several days before retorquing. Valley Forge products and equipment can reveal what period of time is optimal.
Valley Forge solutions
Mines can use the Valley Forge Clarkester™ Tester to calibrate their torque wrenches to bolt tension, vastly improving tension accuracy at installation. Purchase of the tester includes training from Valley Forge on the machine’s proper use.
By request, Valley Forge technical personnel are available to travel to the site to oversee liner replacement and help ensure it is performed properly.
Valley Forge Load Indicating Fasteners measure and display tension from within the bolted joint, accurate to +/-5% of true clamp load (compliant with ASTM 52 standards). Valley Forge can increase ductility by creating an application-specific formula for manufacturing bolts needed for individual applications.
Valley Forge SPC4™ fasteners and SPC4 702 SERIES wireless sensors relay tension information back to a data-logging station, allowing tension to be read in situ.
The result
The ability to read tension in situ means mine operators can determine when bolts are losing tension, even if it’s just hours after the initial installation and preliminary start-up period. This information can revolutionize a mine’s approach to retorquing. Rather than three to four days after installation, bolts can be retorqued far sooner—astonishingly, some need to be retorqued within two to four hours of installation.
When mine operators adjust their retorquing schedule according to information gleaned from Valley Forge products, the result can be incredible. One mine that had been experiencing $1.6 million in downtime each month related to improper tensioning has now gone more than two years without any tensioning issues (saving a staggering $38 million over that time). Savings continue to grow each month.
Obviously, the amount that individual mines will save depends on how much improper bolt tensioning is currently costing them. What is clear, however, is that Valley Forge Load Indicating Fasteners and related products, along with the company’s technical support, can help mine operators save time and money.
