Threaded inserts are one of the most versatile types of fasteners available, partly because they are available in so many different types and materials. When you need to create a lasting bond in a thin, soft, or brittle material, threaded inserts are your best option.
Threaded inserts were originally designed in the 1930s by BF Goodrich to affix deicers to airplane wings. The challenge was that because the wings were made of thin, light aluminum, they couldn’t accommodate conventional tap holes—when builders tried to insert a screw or bolt, the material was too thin and soft to hold the thread. The solution was to insert a threaded sleeve that could be securely fashioned to the aluminum and serve as a secure anchor for a bolt or screw.
The concept behind the threaded insert is simple but ingenious. If you have thin material, such as sheet metal, or soft material, such as wood or plastic, then you can install a threaded insert into a predrilled hole and secure it in one of two ways.
For thin materials, the insert or rivet nut is pushed through the workpiece and secured on the other side by either expanding the insert to create a flange or pulling the insert back into itself to hold it in place in the hole. For thicker materials, such as wood, you can use an insert with an exterior pattern or shape that fits securely in the material and will not turn when a bolt is inserted.
Threaded inserts are used in a wide range of applications. For example, in addition to airplane manufacturing, they are used in automobile manufacturing to fasten body parts together or attach molding and for body repairs. They are used in consumer electronics to fasten together plastic components or to secure circuit boards, and in boat building to secure fiberglass and to create waterproof joins. For furniture making, threaded inserts are ideal to secure soft wood parts. Any application for which you need a strong join can benefit from a threaded insert. They also are commonly used to repair stripped holes.
The Four Primary Types of Inserts
Although threaded inserts come in all materials and sizes, there are four basic types of insert design:
- Thick-wall inserts – As the name suggests, thick-wall inserts are used in thicker materials. They are typically tubular with a threaded interior, although they also come in hexagonal configurations to prevent turning in the hole. These are also blind inserts because they can be installed from one side of the parent material without having to access the other side. Thick-wall inserts are available with either flat or countersunk heads, and they come in open- and closed-end designs. Closed-end inserts are useful for applications where leaking or corrosion could be an issue. Additionally, these inserts come in various materials, including steel, stainless steel, aluminum, and alloy-hardened steel, either with keyed or non-keyed heads.
- Thin-wall inserts – Similar to thick-wall inserts, thin-wall inserts come in round and hexagonal shapes, as well as semi-hex and knurled body types. They also have closed-end designs with weather-resistant plating for special applications. Thin-wall inserts also tend to have a wide-bearing head to create a stronger bond. Nutserts are a specific type of round thin-wall insert with a low-profile flange for near-flush applications. To install a nutsert, you place it in a predrilled hole, and as the bolt is tightened, the outer body is pulled into the inner body to create a firm bond.
- Slotted-body, wide-tail formation inserts – Slotted-body inserts have a different type of design with cuts in the body of the insert that collapse on the blind side to form a strong connection. They come in a straight shank version, which is ideal for soft materials such as wood and composites, and in a pre-bilged version, which works well with thin materials such as plastics and metals.
- Euro-style inserts – Some applications require metric threading, which is why there are Euro-style threaded inserts. Euro-style inserts are similar to other types of inserts and available in round, hexagonal, semi-hex, and square body types. They are often used for repairs or to convert from standard to metric threading.
Specialty Threaded Inserts for Unique Applications
Although the four standard types of threaded inserts are the most commonly used in manufacturing and assembly work, there are other types of insert designs that have proven useful for specific types of applications:
- Wellnut inserts – Wellnut inserts are rubber or neoprene inserts with a brass nut embedded in one end. They are ideal for creating a waterproof join or for applications where you want to reduce vibration. Once they are inserted, tightening the machine screw causes the insert to expand to create a watertight seal. Wellnut inserts are often used in boat building and automotive applications.
- Molly Jack inserts – Molly Jack or jack nut inserts are specifically designed for use with thin or brittle materials. They have four ribs instead of a solid body and are designed to collapse on the blind side to form a permanent, reusable threaded insert. They are available in steel, brass, and coated steel and can be installed by hand.
- Riv-float inserts – Riv-Float® is a registered trademark of Sherex Fastening Solutions, and riv-float inserts are designed for applications where two holes do not alight for a clean join. The riv-float insert has a diagonal insert so a bolt can be inserted and firmly connected through misaligned holes.
These are the most common types of threaded inserts, but they come in various sizes and materials. You can find inserts for jobs of any size and holes of any diameter. Additionally, there are materials for special applications, such as brass and aluminum that won’t corrode, or non-conductive inserts for electronic applications. If you’re looking for guidance in choosing the right threaded insert, contact one of our industry-experienced experts who can help you find the perfect parts for your next job.