Thread locking technologies
Proactively treating threaded pipes and fasteners with thread treatment products is critical to efficient and cost-effective manufacturing. One of the major causes of industrial equipment failure, threaded fastener loosening, results in millions of dollars of unscheduled downtime costs each year.
More than a billion gallons of industrial fluids are wasted through leakage each year. 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 ensure reliable performance of these pieces of machinery, it is imperative that specified tolerances be held during the entire service life of the equipment.
To lubricate, protect, seal, and hold threaded fasteners to their original tolerances, various forms of thread treatments have been used to increase an assembly’s reliability. There are three common thread treatments; thread lockers, thread sealants, and antiseize compounds.
Various types of differential stresses are continuously exerted on threaded fasteners. Stresses such as vibration and shock, thermal expansion and contraction, and micromovement of the fastened parts, can reduce clamping force and ultimately cause machine failure.
Many mechanical devices, such as spring washers, wire retainers, and locking bolts, have been developed in an effort to prevent uncontrolled loosening of fasteners. These mechanical locking methods add substantial cost to the fastener assembly, yet they cannot reliably prevent loosening due to the side sliding motion that causes self-loosening. They also do not seal or prevent corrosion within the fastener assembly and must be sized for the specific fastener.
Liquid thread locking adhesives have become one of the most reliable and inexpensive ways to ensure that a threaded assembly will remain locked and leak proof for its entire service life. Applied in drops 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 (Fig. 1).
Locking the threads together prevents unwanted movement or loosening of the fastener and seals the threads, preventing leakage or corrosion. Any excess thread locker that overflows the threads remains liquid and is easily wiped away.
Although a properly prepared surface ensures the most consistent thread locker performance, advancements in anaerobic technology have delivered many unique products that limit the need for thorough cleaning and surface preparation.
The latest thread locker formulations now cure on inactive metal surfaces, tolerate oily surfaces, and allow cure speed to be specified. Surface primers are only required when using conventional liquid products on challenging applications and substrates.
Thread lockers can help prevent common failures, such as misalignment, when there is a loss of clamp load. When shafts for gearboxes and motors are aligned, the mounting bolts need to be properly torqued to maintain proper alignment.
Over time, these mounting bolts can loosen due to vibration, thermal expansion and contraction, or shock, which results in a loss of clamp load and ultimately misalignment. The proactive use of liquid thread lockers helps to maintain clamp load in this example and prevent misalignment.
Available in different strengths for different applications, thread lockers 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.
Anaerobic formulations are available including surface-insensitive varieties, high-temperature formulations for exposures up to 450 F, chemically resistant materials, and formulations engineered to withstand extreme vibration.
Several factors are key to selecting the right thread locking adhesive for an application. Contrary to common belief, any bolt that has been previously locked with thread locking adhesive may be reused simply by removing the old adhesive before applying new thread locking material and reassembling.
Thread lockers are available in low-strength formulations for easy removal, medium-strength grades that can be removed using common hand tools, and high-strength formulations that offer the highest holding abilities.
No thread lockers are completely permanent; even the highest strength thread lockers can be removed using standard hand tools following direct exposure to 450-500 F temperatures for about five minutes.
Significant amounts of old thread locker residue can typically be removed using a stiff brush. Light levels of thread locker residue are compatible with the next application of thread locker, as long as they do not restrict the reinstallation of the fastener.
The potential for costly, dangerous leaks exists in all fluid systems — gas, vapor, or liquid. The degree to which a leak can be tolerated varies significantly with each application.
For example, a few drops of water per hour leaking from a pipe fitting in a drainage hose would often be ignored. However, a similar amount of water leaking into an electrical switchboard could be disastrous.
Most leaks can be traced to pipe joints. Threaded joints in piping are necessary. Pipe system designers seek to reduce the number of joints wherever possible, but without joints that can be dismantled, each repair would require the removal of massive pipe links.
Despite the standards created to maintain uniform fittings, tapered pipe threads are imprecise and during the course of use and repair the threads can become damaged and even more vulnerable to leakage. The area where the crest and the root of the thread meet forms a spiral leak path no amount of tightening will eliminate. Thread sealing is an attempt to block this leak path that occurs naturally in pipe connections (Fig. 2).
Thread sealants include a variety of products such as sealing tapes, dopes, pastes, O-rings, and cone fittings. While effective, these pipe sealing methods create problems over time.
Tapes act as a lubricant only and can shred, clog pipes, or cause over-tightening, which can damage threads. Tapes must be manually applied and offer poor vibration resistance due to their slick surface.
Most pastes contain solvents and exhibitcreeping and shrinking problems as they dry, which limits their chemical and vibration resistance.
O-rings require large inventories and a special joint design, and may be easily damaged during handing or assembly. Machined cone fittings require costly machining, and are also easily damaged. Thread sealant adhesives are liquid-to-pastelike products that can be easily applied, and cure through an anaerobic reaction. Like anaerobic thread lockers, thread sealant formulations do not contain any volatile solvents that evaporate out of the threads over time and affect the long-term performance of a sealed joint. These materials incorporate plasticizers and formulation modifiers to help lubricate and instantly seal the joint while the cure is in process.
Once cured, anaerobic thread sealants will not 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. Any uncured thread sealant dissolves, eliminating the potential for contamination.
Once applied, these materials provide an instant, low-pressure (500 psi) seal. After curing, many formulations are rated to seal to pressures of 10,000 psi. Thread sealants can also seal pipe unions and compression fittings, and provide exceptional fluid compatibility and sealing ability.
Threaded hydraulic fittings are a common place for leakage. Alternative sealing products fail over time either because of shrinkage, which results in a leak path, or loosening because the alternative sealant is really just a lubricant and does nothing to seal voids.
Antiseize 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 the most severe operating environments.
High-performance greases, formulated with or without specific types of metal flakes, are antiseize products that help ensure fasteners are easy to assemble and disassemble. At the same torque, a consistent bolt tension is always achieved using antiseize materials, even in extremes of performance.
Specific formulations cater to the type of metal used for the threads, as well as the temperature extremes the fastener may see. Many products perform well in excess of 1000 F and some perform in temperature extremes up to 2400 F. Antiseize compounds can be used on a furnace door hinge, for example, preventing the hinge from seizing in the harsh environment.
Typically, these products are 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 as it results in over application, contamination, and an inconsistent, messy assembly.
Semisolid stick technology facilitates dispensing directly onto a part while at the same time minimizing over-application. Liquids previously may not have been chosen because the liquid was considered too messy or could potentially migrate into areas where a cured adhesive could create problems. Thread sealant sticks are excellent alternatives in overhead or in difficult-to-see areas where fluids and tapes are cumbersome to use (Fig. 3).
Semisolid formulations are easy to store and transport. They may be exactly applied without running or migration concerns. The semisolid material remains on the threaded part as it is positioned and assembled.
Thread locker sticks are particularly useful when an assembly operation is time sensitive or must be staged. In this situation the thread locking material can be applied in advance without having to worry about it running off the part. These treated parts can then be assembled all at once without time-consuming matching of mechanical locking devices or drop-wise application of liquid thread treatment.
Author Andy Bardon can be reached at 860-571-2707. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com .