Written by Simla Ay, Technical Marketing Writer
Threadlockers are recommended for use anytime a threaded fastener will be exposed to vibration, repeated impacts, or when it’s imperative to maintain a product’s structure and function. This is because nuts and bolts often loosen due to thermal expansion, vibration, or improper torque.
Although conventional mechanical fasteners, such as nuts or washers, are commonly used to keep the threads secure, mechanical devices are subject to loosening or failure over time.
A chemical threadlocker is a thin, mainly anaerobic adhesive that’s applied to the threads of fasteners to prevent them from loosening. It fills the gap between the threads of a nut and bolt, significantly increasing the area of contact, and forming a bonded thermoset plastic seal. This seal ensures unwelcome liquids stay out of the threads and prohibit any penetration of the bolt hole.
The use of threadlocker also increases the area of friction between the mated surfaces. Typically, there’s a range of about 15% metal-to-metal contact between the two (and this is where friction occurs), but the remaining 85% of the threads are not in contact.
In addition to holding the assembly in place, the cured thermoset plastic provides some flexibility to tolerate thermal and cycling while offering an inert barrier that resists galvanic corrosion.
Only a small drop of threadlocker is required for most applications, which is applied near the end of the male threads and away from the bolt head. As a bolt is screwed into a nut or retaining piece, the adhesive will coat the female threads. Once the fastener is fully tightened, the threads will bear down on one another, leaving a small amount of liquid threadlocker as oxygen is pushed out.
This creates an anaerobic environment, given the absence of free oxygen, which triggers threadlocker curing and the threaded assemblies then lock into place.
When choosing a threadlocker, the most important considerations are the:
1. Strength of the adhesive (which is denoted by the color)
2. Fastener size
3. Application methods
Threadlockers are available in a variety of strengths, colors, and grades. The strength is usually denoted by a color.
• Purple = low strength. Low-strength bonds can be disassembled using hand tools. This is important when disassembly of an application is a routine occurrence, such as for maintenance reasons.
• Blue = medium strength. Medium-strength assemblies require power tools for disassembly. They’re used for critical joints that only rarely need disassembly.
• Red = high strength. High-strength threadlocker is only used for permanent bonds. Disassembly is not easy and requires a combination of high heat and power tools.
A fastener’s size is an important consideration when choosing the ideal strength and viscosity of the threadlocker required.
• A low-strength threadlocker is ideal for fasteners that are less than one-quarter inch in diameter, such as calibration screws or gauges.
• Medium-strength formulations lock fasteners up to three-quarters of an inch in diameter, such as those used in pumps or compressors.
• High-strength threadlockers are most often used on fasteners between three-quarters of an inch, up to one inch in diameter. Typically, these are used for heavy-equipment applications.
If a threadlocker is applied during the assembly of an application, either low, medium, or high-strength grades are used. However, if a project requires a threadlocker for parts that are already assembled, wicking grades avoid the need to disassemble and reassemble all the fasteners.
A wicking threadlocker is a low-viscosity liquid for penetrating and locking pre-assembled parts. It moves between fastened threads by capillary action.
Wicking is associated with the color green to differentiate it from the other strengths available. Localized heating and hand tools are needed for disassembly. It is the go-to threadlocker when fastener disassembly is not an option.
Pre-applied threadlockers are also available. They incorporate a chemical activator through a process called microencapsulation. Microencapsulation is a method in which tiny amounts of an activating agent are stored within a shell or bead. The activating material is isolated during storage and released during the assembly process, as shearing the treads burst the beads.
In rare cases, off-the-shelf threadlockers are not ideal for an application. This is typical when temperature extremes are a concern, such as with cryogenic products. It can also occur if the nut and bolt substrates for an application are made of atypical materials or have an unusual shape or coating that will not bond to conventional threadlockers.
In such cases, it’s possible to have a custom formula designed for the application. An experienced adhesive manufacturer can recommend an ideal solution.