SAG mill increases productivity, reduces unscheduled shutdowns via proper liner bolt installation, lubrication
Industrial milling of mined ore – from large rocks and boulders to fine particles – is a demanding process. Massive semi- autogenous mills – commonly known as SAG mills – use milling balls, as well as the weight of the ore itself, to break down the ore into smaller particles – a process known as comminution. The turning and tumbling of tons of ore within these mills creates a harsh environment with high heat and constant vibration. Downtime for these mills is costly, and repairs can be dangerous – which makes preventive maintenance and proper lubrication extremely critical.
Relining Minerals – a distributor of specialty industrial bolts for mining operations, with locations in Peru and Panama – was in search of a lubricant to protect SAG mill liner bolts from extreme environmental conditions, including high temperatures and harsh vibrations. With the help of Mining Products and Service (MPS) – an Australia-based company specializing in the design, supply and optimization of wear components
for the mining industry – Relining Minerals launched studies to determine the solution that would best protect the bolts from elongation beyond the stress yield threshold that was causing bolt deformation and failure.
In these trials, the companies sought to achieve consistent bolt tensions
by standardizing the type of lubricant used, how the lubricant was applied
to the fastener assembly and correct operation of the torque tools. They tested several different types of lubricants, including MOLYKOTE® P-74 Assembly Paste. Because Relining Minerals provides not only the bolts, but also the lubricant and the technical support, the distributor wanted a solution it could rely on – not simply as a recommendation to customers, but as its own preferred solution.
To meet mill MRO expectations and the heavy industrial demands of these large-scale ore-processing plants, the lubrication solution needed to help control maintenance costs by reducing unscheduled
stops due to loosened bolts and bolt failure. The optimal solution also needed to protect the bolts from the dusty and abrasive by-products of the comminuting process.
Mill liner bolts secure the protective liner to the inside of the mill. Proper lubrication is essential in ensuring the applied torque is converted to elongation and clamping force, minimizing the risk of tension loss and fatigue failure.
During the research and testing process, Relining Minerals and MPS put MOLYKOTE® P-74 Assembly Paste – selected for its low coefficient of friction – to the test by comparing it to other leading lubrication solutions.
During bolt tensioning trials, MPS brought in a pulp lifter liner, configured with M48 liner bolts, from a 36-foot SAG mill. The test jig was used to calibrate actual torque/bolt load and, through proper lubrication, get it closer to the calculated torque/bolt loads to reduce the variation in the bolt loads achieved.
In the trials, which compared MOLYKOTE® P-74 Assembly Paste with the most commonly used lubricants on mine sites, MOLYKOTE® P-74 Paste consistently outperformed the best of the competitive lubricants by at least 10%. The trials also demonstrated that the method of lubricant application to the assembly also was important. If the liner bolts aren’t correctly installed and are under-tensioned, they become fatigued and fail. MOLYKOTE® P-74 Paste helps reduce friction so a higher percentage of the applied torque is converted to elongation and clamping force.
In the graph below, MOLYKOTE® P-74 Paste was compared to the most commonly used anti-seize lubricant. This trial was done using a RAD 50 torque multiplier and best practices established from the tensioning trials, including:
- Hardened cup washer
- Lubricant applied to bolt thread and washer face
- Taking the torque multiplier to stall, releasing the trigger, then reapplying the trigger until the gun stalls again
MOLYKOTE® P-74 Assembly Paste was able to increase the proof strength of the bolts and more reliably prevent deformation of the bolts under stress. In tests that averaged the results of seven different five-bolt sample groups, MOLYKOTE® P-74 Paste was able to maintain a yield strength much closer to 70% than the competitive lubricant.
A clear choice for demanding environments
After seeing the results of the testing, Relining Minerals began recommending MOLYKOTE® P-74 Assembly Paste as its preferred specialty lubricant for mill liner applications. The company found that the paste kept bolt elongation in the optimal range with minimal tension loss, resulting in reduced bolt failures and downtime.
By preventing the bolt and liner from loosening, the distributor found that the lubricant helped reduce unscheduled stops – which increased productivity at the plants, which often are the entry point in the ore-refining process. It also resulted in reduced risk of accidents, as the calibration and adjustment work required to address loosened bolts or bolt failure commonly is a source of physical danger.
Another added benefit to the distributor and its customers was that the paste’s lack of metal content helped minimize environmental impact.
Get the reliability you need to avoid costly unscheduled maintenance
Not only is MOLYKOTE® P-74 Assembly Paste able to prevent bolts from loosening and hold the mill liners in place, but the specialty lubricant also is suitable for the assembly and fitting of a wide variety of threaded connections across heavy industrial applications, such as:
|Bolted connections||Exhaust bolts|
|Sliding contact bearings||Spark plug threads|
|Linear sliding guides||Flanges and flange seals|
|Splined shafts||Door hinges|
|Press-fit joints||Brake mechanisms|
MOLYKOTE® P-74 Paste offers high load-carrying capacity, good corrosion protection, a wide service- temperature range, and a lowand constant coefficient of friction. The paste can help:
- Facilitate disassembly, even after extended periods and after demanding and unfavorable conditions of service
- Avoid fastener brittleness or environmental contamination, courtesy of its metal-free formulation
With a coefficient of friction similar to oiled bolts, tightening torques can be calculated and applied uniformly – even upon repeated loosening and tightening – meaning the paste can aid in:
- Uniform clamping force on flanges, valves, housings,
- Increased protection against bolt fractures
- Increased protection against loosened screws due to vibration
Typical properties of MOLYKOTE® P-74 Assembly Paste
|CTM 191||Unworked penetration||280-310|
|Density at 68°F (20°C)||1.21 g/mL|
|Service temperature range
As dry lubricant(2)
-40 to 392°F
(-40 to 200°C)
|DIN 51 350 pt. 4||4-ball weld load||4,800 N|
|SRV oscillating wear tester, 300 N load, 1,000 µm
elongation, 20 Hz, 194°F (90°C), duration
|Coefficient of friction|
|M12, 8.8, blackened surface|
|CTM 1153||Thread/head||0.14 µ/0.08 µ|
|CTM 394||Press-fit test||0.12 µ|
|ISO/R 1456||Salt spray test at 50 µm||140 h|
|DIN 51 802||SKF-Emcor method (degree of corrosion)||2|
(1)CTM: Corporate Test Method; copies of CTMs are available on request. DIN: Deutsche Industrie Norm. ISO: International Organization for Standardization.
(2)Some of the raw materials still act as separating agents at 2,732°F (1,500°C).