Thread-locking fluid or threadlocker is a thin, single-component adhesive, applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion.
Most thread-locking formulas are methacrylate-based and rely on the electrochemical activity of a metal substrate to cause polymerization of the fluid. Thread-locking fluid is thixotropic, which allows it to flow well over time, yet still resist shocks and vibrations. It can be permanent or removable; in the latter case, it can be removed with heat, for example. Typically, brands are color-coded to indicate strength and whether they can be removed easily or if they require heat for removal.
Thread-locking fluid was developed by American professor Vernon K. Krieble in 1953. His company, American Sealants, founded the Loctite brand.An early version of the compound was patented in 1960.
Typically, thread-locking fluids are methacrylate-based, and cure anaerobically. Thread-locking fluid is a thixotropic fluid: under shear stress, it exhibits a time-dependent decrease in viscosity. This allows it to flow well over time, yet still resist short-duration shearing, as in vibration or shock.
Thread-locking fluid is typically sold in small containers, in amounts from 5 millilitres (about one teaspoon) to 250 millilitres (8.5 US fl oz). Threadlocker is also sold in sticks and in tape form, similar to Teflon tape.
|Type||Typical color code||Torque to break free(3⁄8-16 size bolt)||Torque to continue turning||Temperature range|
|Low strength||■ Purple||62 in-lb (7 N⋅m)||27 in-lb (3 N⋅m)||−54 to 149 °C|
|Medium strength||■ Blue||115 in-lb (12 N⋅m)||53 in-lb (6 N⋅m)||−54 to 149 °C|
|Medium strength surface insensitive||■ Blue||180 in-lb (20 N⋅m)||62 in-lb (7 N⋅m)||−54 to 149 °C|
|High strength||■ Red||230 in-lb (25 N⋅m)||225 in-lb (25 N⋅m)||−54 to 149 °C|
|High temperature||■ Red||180 in-lb (20 N⋅m)||270 in-lb (30 N⋅m)||−54 to 232 °C|
|Penetrating||■ Green||90 in-lb (10 N⋅m)||310 in-lb (35 N⋅m)||−54 to 149 °C|
Thread-locking fluid may be applied before or after assembly, depending on the type. Threadlockers are available in varieties of "permanent", "removable", and "low-strength" formulas. Many brands color-code the container and the fluid itself to indicate the degree of permanency. The low-strength types prevent loosening under vibration, but may still be readily disassembled. Removable types resist higher amounts of vibration, but may still be disassembled with hand or power tools. The strongest permanent threadlockers are rated at 3,000 psi (21 MPa) in shear strength. The applied torque required to loosen a permanently threadlocked fastener may exceed the yield strength of the fastener itself, such that attempting disassembly by force may twist off the stem of the fastener. However, high-strength permanent threadlockers become potentially removable by heating the assembly, typically to 230 °C (450 °F).
Working temperatures for threadlocked fasteners are typically limited to 150 °C (300 °F), which is below the softening point of the methacrylate polymer. Above this temperature, the material softens and strength reduces.
Because thread locking adhesives typically rely on the electrochemical activity of a metal substrate to form a bond, surfaces must be clean to develop the full bonding strength. In the case of less electrochemically active metals such as the normally oxidised surface of aluminium, an additional step of priming is required for full strength results.
Lock washers, locknuts, jam nuts, and safety wire may be used in conjunction with thread-locking fluid to prevent loosening of bolted joints.
Because electrochemical activity is one of the two triggers that cause polymerization of the threadlocker fluid, care must be taken to avoid contaminating the entire container of threadlocker with threadlocker that has had contact with metal, otherwise the material in the container may polymerize.
Adhesive, also known as glue, cement, mucilage, or paste, is any non metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
Cyanoacrylates are a family of strong fast-acting adhesives with industrial, medical, and household uses. They are various esters of cyanoacrylic acid. The acryl groups in the resin rapidly polymerize in the presence of water to form long, strong chains. They have some minor toxicity.
A rivet is a permanent mechanical fastener. Before being installed, a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite to the head is called the tail. On installation, the rivet is placed in a punched or drilled hole, and the tail is upset, or bucked, so that it expands to about 1.5 times the original shaft diameter, holding the rivet in place. In other words, pounding creates a new "head" on the other end by smashing the "tail" material flatter, resulting in a rivet that is roughly a dumbbell shape. To distinguish between the two ends of the rivet, the original head is called the factory head and the deformed end is called the shop head or buck-tail.
Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow over time when shaken, agitated, shear-stressed, or otherwise stressed. They then take a fixed time to return to a more viscous state. Some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a steep change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated. Thixotropy arises because particles or structured solutes require time to organize. An excellent overview of thixotropy has been provided by Mewis and Wagner.
A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring, wear pad, preload indicating device, locking device, and to reduce vibration. Washers often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.
Bolted joints are one of the most common elements in construction and machine design. They consist of fasteners that capture and join other parts, and are secured with the mating of screw threads.
Safety wire or locking-wire is a type of positive locking device that prevents fasteners from loosening or falling out due to vibration and other forces. The presence of safety wiring may also serve to indicate that the fasteners have been properly tightened.
A set screw or grub screw is a type of screw generally used to secure an object within or against another object, normally without using a nut. The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless, meaning that the screw is fully threaded and has no head projecting past the major diameter of the screw thread. If a set screw has a head, the thread will extend all the way to the head. A grub set screw is almost always driven with an internal-wrenching drive, such as a hex socket (Allen), star (Torx), square socket (Robertson), or slot. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts compressional or clamping force through the bottom tip that projects through the hole.
Loctite is a German-owned American brand of adhesives, sealants and surface treatments that include acrylic, anaerobic, cyanoacrylate, epoxy, hot melt, silicone, urethane and UV/light curing technologies. Loctite products are sold globally and are used in a variety of industrial and hobbyist applications.
Sealant is a substance used to block the passage of fluids through the surface or joints or openings in materials, a type of mechanical seal. In building construction sealant is sometimes synonymous with caulking and also serve the purposes of blocking dust, sound and heat transmission. Sealants may be weak or strong, flexible or rigid, permanent or temporary. Sealants are not adhesives but some have adhesive qualities and are called adhesive-sealants or structural sealants.
A locknut, also known as a lock nut, locking nut, self-locking nut, prevailing torque nut, stiff nut or elastic stop nut, is a nut that resists loosening under vibrations and torque. Elastic stop nuts and prevailing torque nuts are of the particular type where some portion of the nut deforms elastically to provide a locking action. The first type used fiber instead of nylon and was invented in 1931.
A threaded insert, also known as a threaded bushing, is a fastener element that is inserted into an object to add a threaded hole. They may be used to repair a stripped threaded hole, provide a durable threaded hole in a soft material, place a thread on a material too thin to accept it, mold or cast threads into a work piece thereby eliminating a machining operation, or simplify changeover from unified to metric threads or vice versa.
A jam nut is a low profile type of nut, typically half as tall as a standard nut. It is commonly used as a type of locknut, where it is "jammed" up against a standard nut to lock the two in place. It is also used in situations where a standard nut would not fit.
A distorted thread locknut, is a type of locknut that uses a deformed section of thread to keep the nut from loosening from vibrations or rotation of the clamped item. They are broken down into four types: elliptical offset nuts, centerlock nuts, toplock nuts and partially depitched (Philidas) nuts.
A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads' friction, a slight stretching of the bolt, and compression of the parts to be held together.
A bolt is a form of threaded fastener with an external male thread. Bolts are very closely related to screws.
A Junker test is a mechanical test to determine the point at which a bolted joint loses its preload when subjected to shear loading caused by transverse vibration.
A Palnut® Fastener is a variation of the lock nut device for bolts which are intended to fasten securely without welding or any other permanent fastening. The Palnut is a registered trademark of Tinnerman Palnut Engineered Products LLC. The device is screwed on the bolt on top of an underneath the nut, and has a series of protruding barbs that locks the nut in place when the nut is tightened then twisting the Palnut to lock the bolt in place. The Palnut itself is a reusable means of bolt stability. While there are some better means of permanent fastening methods. Palnut's are a good solution to low maintenance and fast means of securing two items together and ensuring that they stay together.
Adhesive bonding is a process by which two members of equal or dissimilar composition are joined. It is used in place of, or to complement other joining methods such mechanical fasting by the use nails, rivets, screws or bolts and many welding processes. The use of adhesives provides many advantages over welding and mechanical fastening in steel construction; however, many challenges still exist that have made the use of adhesives in structural steel components very limited.