The adhesives cure in the airless spaces between closely mating components.
Edited by Jessica Shapiro
One type of fastening component can boost equipment lifetime and reliability while cutting manufacturing, service, and warranty costs. Sound like a pipe dream? For designs that have threaded joints, mating flanges, or close tolerance shaft fits, anaerobic adhesives working as threadlockers, thread sealants, retaining compounds, and gaskets might do the trick.
Threaded fasteners are the most common class of detachable hardware in the world. More than 300 billion get used annually in the U. S. But stresses such as vibrations, shocks, thermal expansions and contractions, and micromovements can all reduce clamping force in threaded joints. Tiny gaps between threads’ mating surfaces — typical nuts and bolts may have as little as 15% metal-to-metal contact — allow side-to-side movement when joints are exposed to vibration and thermal changes. This lateral movement loosens the mated parts and ultimately causes fasteners to fail.
Mechanical locking devices such as split rings, spring washers, ribbed flange bolts, and tab washers were invented to solve this loosening problem. However, many of these devices loosen over time, and none seal fastener threads, leaving assemblies susceptible to corrosion. And stocking devices that fit all fastener shapes and sizes increases inventory cost.
In contrast, one-size-fits-all adhesive threadlockers keep assemblies secure and leakproof throughout service life. They completely fill voids between interfacing threads with tough resin, providing 100% contact between surfaces and preventing side-to-side movement, loosening, corrosion, leakage, and galling.
Anaerobic threadlockers flow between metal surfaces, like the threads on nuts and bolts. The materials harden without volatile solvents that can leave voids and weaken joints over the long term. Once isolated from oxygen, the single-component adhesives react by free-radical polymerization to form tough thermoset plastics.
The materials commonly start with an initiator such as cumyl hydroperoxide, which reacts with a metal ion via reduction-oxidation to become a free radical. The radicalized initiator then reacts with a monomer, radicalizing it. The radicalized monomer goes on to react with other monomers to increase polymer chain length.
Oxygen inhibits polymerization because it preferentially reacts with radicalized monomers, but the resulting radical is too weak to continue the polymerization chain. Two radicalized units — either chains or initiators — reacting with each other terminates the reaction by eliminating free radicals.
The latest generation of anaerobic materials cure within minutes on all types of metals, even those that are contaminated with oil and uncleaned or unprimed. The materials also disassemble easily and resist higher temperatures.
Designers can choose threadlocking materials that work with their assembly process, service temperatures and pressures, vibration and fluid exposure, and disassembly requirements.
Sealing it up
Thread sealants are frequently used on tapered threads and hydraulic fittings due to their fluid compatibility and sealing ability. They block naturally occurring leak paths where thread crests and roots meet. During assembly, they also promote tightening by lubricating threads. Lubricant lets applied torque convert into clamp load instead of dissipating it as friction or heat.
These materials quickly form seals that resist pressures up to 500 psi. Upon cure, many formulations withstand pressures of 10,000 psi.
Gel-type sealants can seal mating surfaces of flare-style fittings used in hydraulic and pneumatic applications by filling in scratches and surface imperfections. They are also used on dry-seal fittings to prevent rotation that leads to leakage. These gels work on any size NPT, O-ring boss, or JIC fitting — even those with large gaps or threads. High-temperature thread sealants survive continuous exposure to 530°F temperatures and are ideal for use on equipment exposed to steam.
Flange sealants also seal out fluids and gases. They function like gaskets to create and maintain a seal, remain impervious to fluid or gas flow, and withstand operating temperatures, pressures, and vibrations.
Unlike gaskets, flange sealants flow into microscopic surface irregularities on metal substrates and fill voids between flange faces. Anaerobic flange sealants cure after the flanges are assembled, ensuring flange faces are fully seated and have full metal-to-metal contact.
Flange-face contact eliminates a common cause of seal failure: hard-gasket compression set. This occurs when a gasket is permanently compressed and cannot return to its original shape. For example, thermal cycling can cause parts to expand and contract, continually squeezing and releasing the gasket. If this permanently compresses the gasket, compression forces diminish and leaks can form.
Flange sealants do not seal via compression so there is no compression set. The materials also create bonds that unite the assembly, withstand thermal expansion of mated parts and micromovements from high levels of torque, and resist a broad range of chemicals.
Flange sealants work with any size flange. There’s no need to stock custom gaskets for flanges in gearboxes, pumps, compressors, and drives in heavy equipment, wind turbines, and centrifuges.
Flange sealants designed for easy removal from soft metal flanges permit frequent disassembly and reassembly with a plastic putty knife that doesn’t damage flange surfaces.
The perfect fit
Anaerobic adhesive liquids and pastes also serve as retaining compounds for rigid cylindrical assemblies such as bearings mounted onto shafts. In such applications, retaining compounds on the inside or outside diameter of joining parts augment the strength of press and shrink-fit assemblies.
When a liquid retaining compound is applied, it conforms to the surfaces. Once cured, its strength is due to a combination of adhesive bonding to the metal surfaces and mechanical interlocking with rough metal surfaces on the shaft and bearing or gear ID.
A bonded slip fit can typically have twice the strength of an interference fit. Pins and collars held in place with adhesive typically withstand 4,500 to 7,000 psi of shear stress.
These strengths are consistent over a wide range of fits. The adhesives can fill gaps up to 0.015 in., so diametrical tolerances can be relaxed to 0.003 in. to 0.005 in. The adhesives also work better on rougher surfaces, 32 to 125 rms, not the fine surface finishes generally specified for interference fits.
Specifying slip-fit tolerances and rougher surfaces can cut machining costs. Assembly may cost less, too, because manufacturers bond slip-fit parts without presses or heating equipment. The compounds can also simplify the task of assembling interference-fit parts by lubricating surfaces and preventing galling.
Until recently, anaerobic adhesives have been available only as thin liquids. Advances in stability and reactivity of the adhesives’ chemistries have opened the door to gel and stick formulations. And manufacturing innovations have brought anaerobics to market on dry-to-the-touch tape as well.
Both semisolid sticks and tape work well in vertical and overhead applications where liquids would be too messy or would not stay where they were applied. Dry-to-the-touch tape threadlockers come on a roll and can be preapplied to threaded fasteners for future assembly or applied during maintenance.
The strength and viscosity required of an adhesive are directly related to the size of the fastener it is used on. Low-strength threadlockers — those with 50 lb-in. shear strength (breakaway torque measured on a M10 steel nut and bolt) — are recommended for fasteners up to ¼-in. diameter. Medium-strength materials with shear strengths of 150 lb-in. are for fasteners up to ¾-in. diameter. High-strength threadlockers are typically used on larger fasteners designed for permanent assembly applications such as heavy equipment and have a shear strengths over 200 lb-in.
Low-viscosity penetrating threadlocking adhesives are also available that wick into preassembled fasteners up to ½ in. in diameter. These are water-thin, about 12 cP, out of the package.
Although most threadlockers cure in minutes, slow-cure products give operators time to sequentially torque a number of fasteners. Cure is slower at cold temperatures and on larger fasteners with larger thread gaps.
Liquid threadlocking adhesives should fill threads the length of the engagement area, usually the three or four threads on the bolt where the nut seats once tightened. The amount needed depends on the thread size, the adhesive’s viscosity, and the part’s geometry. Manual, semiautomatic, or automatic dispensing machines are available.
For through-hole nut-and-bolt assemblies, only apply threadlocker where the nut and bolt would meet in a fully tightened assembly. For blind-hole assemblies, apply the compound to both the bolt and mating threads. As the bolt is tightened, liquid threadlocker will push into the joint, forcing out air trapped in the blind hole.
Assembly and service
While first-generation threadlockers were effective at continuous operating temperatures up to 300°F, newer versions generally withstand 20% higher temperatures. And certain formulations withstand temperatures up to 650°F without breaking down.
New high-lubricity anaerobics cut friction and reliably convert torque into increased clamp loads. The relationship between assembly torque and clamp load is:
T = K × D × F
where T = torque, D = bolt diameter, F = clamp load, and K is an experimentally determined constant related to the threaded fastener’s coefficient of friction. Oiled metals have Ks ranging from 0.13 to 0.22. Metals treated with lubricating threadlocker have Ks ranging from 0.09 to 0.29.
Friction matters because in unlubricated, coarse-thread fasteners only about 15% of torque generates bolt clamp load; the other 85% is used to overcome thread and head friction. In fine-thread fasteners, 90% of torque works to overcome friction in the system.
A nut and bolt assembled with a liquid threadlocker can be repeatedly reused by brushing off cured threadlocker, reapplying several drops of new threadlocker, and reassembling the fastener. Low-strength versions permit easy removal, and medium-strength grades can be removed using common hand tools.
Contrary to common belief, no threadlocker is permanent. Even joints using the strongest threadlocker on the market can be taken apart after heating it to at least 450°F with a torch and using tools to loosen the fastener.