Load-indicating fastener technology senses grinding mill clamp load in real time


A large polymetallic underground mine in South Australia is using technology to improve safety and productivity. Because the mine is one of the largest global suppliers of the metals being mined, every aspect of the operation was put under the microscope—from the vehicles moving material to the kinds of bolts used in its autogenous grinding mills. Valley Forge & Bolt, a Phoenix-based maker of custom fasteners, used its unique Load Indicating Technology to assist the mine in this focus on efficiency.SPC4™ bolts

Valley Forge’s SPC4™ bolts have built-in load indicators that measure clamp load in real time and relay the information to a variety of meter options. When accompanying wireless probes are linked to the SPC4 bolts, mine engineers are able to incorporate the captured data into their SCADA system and track changes to clamp load as it happens, in situ.

“There was a feeling that this particular mill was nearing the end of its service life,” said Brandon Proebster, sales engineer for Valley Forge & Bolt. “Mine engineers wanted the ability to closely monitor its performance to strike that balance between getting maximum use out of the equipment, but not pushing it beyond its service life.”

Valley Forge’s bolts with Load Indicating Technology, such as SPC4, are accurate to within +/-5%, measuring full bolt stress from zero to 100% rather than a single load preset like some other fasteners that purport to indicate load.

Proebster and Valley Forge’s regional director, Barry Thomas, traveled to the mine to get a better understanding of conditions on the ground and what mine engineers hoped to accomplish. The mills begin their process with material the size of footballs that is then processed down to powder. It is in this violent transformation that vibration within the mill takes its toll on bolted joints.

To reduce expensive manual monitoring of the joints and to prolong the life of the equipment, Valley Forge custom manufactured a total of 376 SPC4 bolts, ranging in size from 2.5 to 3.5 inches in diameter and from 10 to 16 inches in length. The installation also included electronic and mechanical load meters. A set of SPC4-702 wireless sensors was then added, allowing the SPC4 bolts to deliver real-time data to the site’s mill monitoring dashboard.

“Engineers selected strategic locations for the SPC4 fasteners within the mill, focusing attention on areas deemed to be most critical,” said Proebster. “Using SPC4, we replaced the radial flange and trunnion bolts at both the feed and discharge ends.”

Due to the severe service conditions present in large grinding mill applications, the fasteners used to assemble the mill shell are considered “critical fasteners” and require regular testing and maintenance. Traditionally, ultrasonic elongation testing is used to ensure that proper clamp load is being maintained. However, this method requires a scheduled outage and trained personnel to accurately interpret the results. Ultrasonic measurement also fails to provide real-time bolt load information.

With the SPC4’s ability to tie into the mine’s SCADA system, engineers got their first-ever real-time look at how the mill’s performance affects clamp load. Doing so without requiring a planned outage saved precious time and money, a benefit that continues. Plus, the information gleaned informs maintenance schedules and provides early alerts to potential problems. The data also provides insight into the general health of equipment.

In the ensuing months, mine engineers transitioned other grinding mill shell fasteners to SPC4. “Alerting operators to the smallest loss of tension in real time allows them to immediately investigate, diagnose, and prevent equipment failure,” said Proebster. “The information provided by SPC4 bolts is extremely valuable because time is money.”

More so, according to mine personnel, the use of Valley Forge & Bolt’s wireless data-relaying load indicating fasteners was a huge gain for mine safety and asset integrity.

Valley Forge Total Solutions Approach Can Save Mining Operations 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. Valley Forge Clarkester™ Tester

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 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.

About Valley Forge & Bolt Co.

Valley Forge & Bolt Mfg. Co. is a manufacturer of standard and custom fasteners, proudly producing both for over 40 years. The company is a leader in controlled bolting and its load indicating fastener technologies offer unrivaled quality, extraordinary performance and enduring reliability. For more information on technologies to optimize the integrity of the bolted joint, call 800.832.6587 or visit vfbolts.com.

In response to recent public health concerns and multiple requests regarding our ability to fulfill customer orders, Valley Forge does not currently foresee any negative impact from the Coronavirus (COVID-19).  

 All parts manufactured at Valley Forge are made with U.S. domestic steel and our raw material supply for this and other materials / components has not been affected. We will be maintaining continuous communication with our suppliers and will maintain an ongoing dialogue to ensure there are no disruptions in our supply chain.

  • Customer Service, Operations, and Logistics are continuing without disruption and we have contingency plans in place should our teams experience disruption from employees reporting in sick.   Sick leave benefits have been extended so that employees will stay home if experiencing symptoms.
  • We are in a strong inventory position and do not anticipate any stockouts on materials in the coming weeks.  Our inventory levels are deliberately being kept at higher levels so that no shortages will occur if there is any disturbance in the supply chain.
  • We will discontinue providing onsite services to customers at this time.  If assistance is needed, we are equipped to videoconference through Zoom, Skype or any customer facilitated media.
  • We have discontinued sales travel by our sales force in the immediate future.
  • We are asking all customers, vendors, and inspectors to reschedule their visits to our facility or conduct by videoconference or document/picture exchange.
  • We are allowing only essential personal into our facility and will be checking temperatures before entrance.
  • At this time, we have not experienced any labor shortages.  If needed, our manufacturing work force is cross trained and can step into operations where needed.
  • Our backlog is strong, and we are working to keep to our manufacturing schedules and see no move outs on order ready dates at this time.

We thank you for your business and working with us to follow the guidelines set by WHO and the CDC at this time.  Should there be any changes or any future developments, our ongoing assessment of the situation will be communicated to our customers at that time.

Michele Clarke

Chief Executive Officer