Threadlockers Unveiled: Expert Guide to Threadlocking Adhesives

Invented more than 2,000 years ago, threaded fasteners remain the backbone of modern machinery. They are integral to everything from automobiles and aircraft to household appliances, where they secure the most critical components.

In the United States alone, over 300 billion fasteners are consumed each year. This staggering volume underscores the necessity for fasteners that deliver predictable, uniform performance and stay secure once installed.

Despite continuous refinement, fasteners still fail. Two primary failure modes exist: tension relaxation, caused by temperature‑induced expansion and contraction of bolts and their substrates; and self‑loosening, which occurs when lateral sliding between contact surfaces reduces clamping force.

Mechanical solutions—spring washers, wire retainers, locking bolts—add cost and cannot fully prevent the sliding motion that leads to loosening. Chemical threadlocking adhesives, however, offer a cost‑effective, permanent fix.

Introduced nearly 50 years ago by Loctite Corporation, anaerobic threadlockers are single‑component liquid resins that cure to a tough solid when exposed to metal ions in an oxygen‑free environment. This anaerobic cure mechanism allows the adhesive to flow, fill thread grooves, and then harden without premature setting.

During cure, polymer chains penetrate every microscopic imperfection in the threads, increasing friction and creating a robust mechanical lock. The adhesive also seals the joint, protecting it from corrosion caused by moisture, gases, and fluids.

Just a few drops of threadlocker can maintain a torqued fastener until it is intentionally disassembled. While a typical nut‑and‑bolt assembly achieves only about 15 % metal‑to‑metal contact, threadlockers provide 100 % contact, enhancing torque control, reducing galling, and delivering lubricity for precise assembly.

Available in low‑, medium‑, and high‑strength grades, threadlockers can withstand even the harshest environments. They offer high shear strength, temperature resistance, rapid cure, easy dispensing, and excellent vibration resistance.

Factors to Consider When Selecting Threadlockers

Choosing the right threadlocker requires understanding your application’s requirements. In many cases, bolts must remain removable for maintenance. Fortunately, any bolt that has been locked can be reused—simply remove the old adhesive before reapplying.

Low‑strength formulations are ideal for fasteners up to ¼‑inch in diameter—such as adjustment screws, meters, and gauges—where frequent adjustment is needed. Medium‑strength grades suit fasteners up to ¾‑inch, common in machine tools, presses, pumps, and compressors. High‑strength lockants are reserved for fasteners up to 1‑inch, found in permanent assemblies on heavy equipment and large mounts.

Low‑viscosity, penetrating threadlockers are also available for fasteners up to ½‑inch that may benefit from easy wicking into pre‑assembled threads.

The operating environment dictates formulation choice. Modern threadlockers are surface‑insensitive, high‑temperature, chemically resistant, and vibration‑tolerant—features that were unavailable in earlier generations.

Metal type also matters. Fasteners made from inactive metals—stainless steel, zinc, magnesium, black oxide, cadmium, anodized aluminum, or passivated titanium—may require a primer to facilitate cure. If only one inactive metal is present, primer is unnecessary.

Applying Threadlockers

Proper application ensures the adhesive wets the entire thread engagement area. The required wetting depends on thread size, adhesive viscosity, and part geometry. For large parts, wetting both mating faces guarantees reliability.

For through‑hole nut‑and‑bolt assemblies, apply threadlocker only where the nut and bolt meet when fully tightened—only the adhesive between threads will cure. In blind‑hole assemblies such as cap screws, apply the lockant to both the bolt and the mating threads; otherwise, air pressure can force the adhesive out, leading to incomplete cure and potential failure.

Threadlockers can be dispensed manually, semi‑automatically, or automatically. Pre‑applied fastener coatings—microcapsules containing adhesive—are becoming popular. When the fastener is assembled, the capsules break, releasing the lockant. This method offers consistency, cost savings, and quality assurance.

Anaerobic threadlockers set in minutes at room temperature and fully cure within 24 hours. Exposure to heat (e.g., 30 minutes at 120 °C) can accelerate cure. If a primer or activator is needed, apply it to one or both surfaces before the adhesive; no mixing of components is required.

As a rule, avoid anaerobic threadlockers on plastic threaded assemblies, as substrate softening or stress cracking can occur.

New Developments

Traditional anaerobic adhesives faced three main challenges: curing on a wide range of metallic and non‑metallic substrates that may be greasy or oily; enduring long‑term exposure to temperatures above 300 °F; and withstanding prolonged mechanical stress.

Oils and other contaminants can prevent curing by inhibiting the anaerobic reaction, necessitating thorough cleaning before application. Inactive substrates—zinc‑dichromate/phosphate plated steel, magnesium, cadmium, stainless steel, aluminum, and thermoset plastics—formerly required solvent‑based primers before anaerobic adhesives could bond.

Modern surface‑insensitive formulations cure successfully on oily or contaminated surfaces and on inactive metals, reducing or eliminating primer requirements. New proprietary chemistries also extend operating temperatures up to 400 °F, ideal for automotive and aerospace applications that experience extreme thermal cycling.

To address vibration, tougher formulations have been engineered. These high‑strength, high‑temperature lockants are less impact‑sensitive and can withstand prolonged exposure to intense vibration, rotational, shear, and tensile forces.

A High‑Performance Application

Ryobi Outdoor Products—renowned for gas‑powered string trimmers—was developing a high‑powered blower capable of speeds up to 210 mph and engine speeds above 8,000 rpm. Under these conditions, components reached 500–600 °F and experienced severe vibration.

Conventional mechanical solutions were inadequate. Engineers turned to Loctite 266, a new surface‑insensitive, high‑strength, high‑temperature threadlocker. It cures in 40 minutes and reaches full strength in three hours, tolerating oil and other mild contaminants without surface preparation.

On the assembly line, the adhesive was applied to zinc‑dichromated steel bolts joining the muffler to the cylinder head and the crankcase to the cylinder. Three hours later, each unit was tested at wide‑open throttle for six minutes at 6,000 rpm before approval.

“The rapid cure of this product allowed us to perform engine tests at the end of the assembly line,” says Bill Gest, Vice President of New Products at Ryobi. “This accelerated our time‑to‑market, reduced in‑process inventories, and helped us maintain a competitive edge.”

About the author:
Mike Shannahan is the Manager of OEM Marketing at Loctite Corporation, a Henkel company. Henkel operates worldwide with leading brands and technologies in three business areas: Laundry & Home Care, Cosmetics/Toiletries, and Adhesive Technologies. Founded in 1876, Henkel holds globally leading market positions in both consumer and industrial sectors, with well‑known brands such as Persil, Schwarzkopf, and Loctite. Henkel employed approximately 50,000 people and reported sales of $18.86 billion and an adjusted operating profit of $1.84 billion in fiscal 2009. For more information, visit www.loctite.com.