Thread analysis

Examining your thread treatment options for MRO applications

by Andy Bardon

Proactively treating threaded pipes and fasteners with thread treatment products is critical to efficient and cost-effective manufacturing. A major cause of industrial equipment failure, threaded fastener loosening results in millions of dollars in unscheduled downtime each year. Similarly, more than a billion gallons of industrial fluids are wasted each year through leakage. Beyond the value of lost fluids and gases, leaks affect toxicity, emissions, safety, contamination and personnel.

Threaded fasteners set and hold tolerances on assemblies ranging from light-duty equipment to heavy machinery. To lubricate, protect, seal and hold threaded fasteners to their original tolerances, and to increase assembly reliability, various thread treatments are used. These products are commonly broken into three categories: thread sealants, threadlockers and anti-seize materials.

Threadlockers
Threaded fasteners continually receive various types of differential stress. Stresses such as vibration and shock, thermal expansion and contraction, and micro-movement of the fastened parts all can reduce clamping force and ultimately cause machine failure. Many mechanical devices such as spring washers, wire retainers and locking bolts were developed to prevent uncontrolled loosening of fasteners. These locking methods add substantial cost to the fastener assembly, yet cannot reliably prevent loosening due to the side-sliding motion that causes self-loosening. They also don’t seal or prevent corrosion within the fastener assembly and must be sized appropriately for the specific fastener.

Liquid threadlocking adhesives have become a reliable and inexpensive way to ensure that a threaded assembly remains locked and leak-proof for its service life. Applied drop-wise to fastener threads, liquid anaerobic products fill the grooves of the threads and cure to a hard thermoset plastic when exposed to active metal ions in the absence of air. Locking the threads together prevents unwanted movement or loosening of the fastener and seals the threads, preventing leakage or corrosion. Excess threadlocker that overflows the threads remains liquid and is easily wiped away.

Available in various strengths for different applications, threadlockers allow threaded fasteners to maintain critical clamp load pressures even in the most severe environments. These adhesives offer high shear strength, very good temperature resistance, rapid cure, easy dispensing and excellent vibration resistance. Unique new anaerobic formulations are available, including surface-insensitive varieties, high-heat formulas for exposures up to 450 degrees F, chemically resistant materials, and ones engineered to withstand extreme vibration.

Consider several factors to select the right threadlocking adhesive for an application. Contrary to common belief, any bolt previously locked with threadlocking adhesive is reusable simply by removing old adhesive before applying new threadlocking material and reassembling. Threadlockers are available in low-strength formulations for easy removal, medium-strength grades that are removed using common hand tools and high-strength grades that offer strong holding abilities. However, no threadlocker is completely permanent; standard hand tools remove even the highest-strength threadlockers following direct exposure to heat (450 to 500 F) for about five minutes.

Thread sealants
The potential for costly, dangerous leaks (gas, vapor or liquid) exists in all fluid systems. The degree to which a leak is tolerated varies significantly with each application. A few drops of water per hour leaking from a pipe fitting in a drainage hose is often ignored. However, a similar amount of water leaking into an electrical switchboard can be disastrous.

Mechanical thread sealants include products such as sealing tapes, dopes, pastes, O-rings and cone fittings. While effective, these pipe-sealing methods create problems over time. Tapes can shred and clog pipes or cause over-tightening that damages threads. Tapes are manually applied, limiting automation of the application, and offer poor vibration resistance due to their slick surface. Most pastes contain solvents and exhibit creeping and shrinking problems as they dry, thus limiting their chemical and vibration resistance. O-rings require large inventories and a special joint design, and are easily damaged during handling or assembly. Machined cone fittings require costly machining and are also easily damaged.

Thread-sealant adhesives are liquid to paste-like products that are easily applied, and cure through an anaerobic reaction. Like anaerobic threadlockers, thread-sealant formulations don’t contain added solvents that can volatilize out of the threads and affect long-term performance. These materials use plasticizers and formulation modifiers to help lubricate and instantly seal the joint while the cure is in process.

Once cured, anaerobic thread sealants won’t melt, eliminating leak paths. These materials seal and lock threads at the same time and act as a lubricant during assembly to promote tightening while ensuring consistent assembly torque. Uncured thread sealant dissolves, eliminating the potential for contamination. Once applied, the materials provide instant low-pressure (500 psi) seals. After cure, many formulations are rated to seal pressures of 10,000 psi. These products also seal pipe unions and compression fittings, and provide great fluid compatibility and sealing ability.

Anti-seize
Anti-seize materials protect threaded and slip-fitted metal parts from rust, corrosion, galling and seizing at high temperatures. They also reduce friction, wear and breakage on critical parts in severe operating environments.

These are high-performance greases formulated with or without specific types of metal flake. They help ensure that fasteners are easy to assemble and disassemble. At the same torque, a consistent bolt tension is always achieved using anti-seize materials, even in performance extremes. Specific formulations cater to the type of metal used for the threads, as well as temperature extremes. Many products perform well in excess of 1,000 F. Some perform in extremes up to 2,400 F.

These products are typically brush-applied to parts, which requires a reservoir and a separate applicator to spread the material. Dipping a fastener into the material is not recommended since it results in over-application, contamination and an inconsistent, messy assembly.

New technology
Recent advances in the stability and reactivity of threadlocking and thread-sealing materials have allowed the development of semi-solid “stick” formulations that complement their liquid counterparts. Semi-solid threadlocking, thread-sealing and anti-seize products work well in applications not previously considered because the liquid was considered too messy or could potentially migrate into areas where a cured adhesive could create problems. For example, sealant sticks are excellent alternatives in overhead or difficult-to-see areas where fluids and tapes are cumbersome to use.  Anti-seize stick technology facilitates dispensing directly onto a part. At the same time, it minimizes over-application.

These semi-solid formulations are easy to store and transport. Apply them without running or migration concerns. The semi-solid material remains on the threaded part as it is positioned and assembled. The sticks are very useful when an assembly operation is time-sensitive or must be staged. Here, the threadlocking material is applied in advance without having to worry about it running off the part. Treated parts are then assembled all at once without time-consuming matching of mechanical locking devices or drop-wise application of liquid thread treatment. s

Andy Bardon is an application engineer for Henkel Loctite Corporation, a manufacturer of high-performance adhesives, sealants and coatings. To learn more, call and visit www.loctite.com.

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