When it comes to fastening materials, engineers and fabricators often face a critical choice: rivnuts or screws? While both serve to join components, their fundamental designs and applications lead to distinct performance characteristics, particularly concerning strength.

Understanding the Fasteners

Before comparing their strength, let's briefly define each fastener:

• Screws: These are threaded fasteners designed to be driven into a material, creating their own mating threads (like wood screws or self-tapping screws) or engaging with pre-tapped threads or a nut. Screws rely on the integrity of the material they're threaded into or the strength of the nut for their holding power. They are known for their removability and reusability.

• Rivnuts (Rivet Nuts/Nutserts): Rivnuts are internally threaded tubular rivets that are designed to be installed from one side of a workpiece (blind installation). When installed, the rivnut deforms and bulges on the blind side, creating a permanent, strong, load-bearing thread in thin or hollow materials where tapping threads directly is impractical or impossible. A bolt or screw can then be threaded into the rivnut.

Strength Comparison: Where Each Excels

The "strength" of a fastener isn't a single metric; it encompasses various aspects like tensile strength, shear strength, pull-out strength, and resistance to vibration and loosening.

Tensile Strength (Pull-Out Resistance)

Screws: The tensile strength of a screw (its resistance to being pulled straight out) is highly dependent on several factors:

• Material properties: The strength of the screw material itself (e.g., steel grades like Grade 8 are much stronger than Grade 2) and the material it's fastening into are crucial.

• Thread engagement: The more threads engaged, and the stronger the threads in the base material, the higher the pull-out strength. In soft materials like wood or thin sheet metal, screws can strip or pull out relatively easily, especially under repeated stress or vibration.

• Hole size: An incorrectly sized pilot hole can significantly reduce a screw's holding power.

• With a nut: When a screw is used with a nut and washer (i.e., a bolt connection), its tensile strength is often superior to a rivnut of comparable size because the load is distributed over a larger backing area, and the full strength of the bolt's cross-section is utilized.

Rivnuts: Rivnuts are designed to create a strong, permanent thread in thin materials. Their pull-out strength is generally superior to a self-tapping screw in thin sheet metal or composite panels for a few reasons:

• Blind-side expansion: The key to a rivnut's strength is its expansion on the blind side, which creates a larger bearing surface against the material, significantly increasing resistance to pull-out.

• Load distribution: The deformed rivnut distributes the load over a wider area of the base material, reducing localized stress that can lead to tearing or deformation.

• Material thickness: Rivnuts have specific grip ranges and require a minimum material thickness to perform effectively. If the material is too thin or the rivnut isn't properly installed, its strength can be compromised.

• Type of rivnut: Different rivnut designs (e.g., pre-bulbed plusnuts) offer even greater pull-out strength by creating a larger, more robust flange on the blind side.

Shear Strength

Screws: The shear strength of a screw (its resistance to forces acting perpendicular to its axis, trying to cut it) is primarily determined by:

• Screw material and diameter: A thicker screw made of high-strength steel will generally have higher shear strength.

• Joint design: How the components are clamped together and the fit of the screw in the hole impact shear performance.

Rivnuts: While rivnuts create a strong threaded anchor, their primary function isn't necessarily to provide high shear strength as a standalone fastener. When a bolt or screw is threaded into a rivnut, the shear strength of the connection is largely dependent on the strength of the bolt or screw itself, and how well the rivnut is seated in the material. In applications where very high shear loads are present, alternative fastening methods like solid rivets or structural bolts might be preferred. However, for many typical applications, a properly installed rivnut with an appropriate bolt can offer sufficient shear resistance.

Resistance to Vibration and Loosening

Screws: Self-tapping screws, especially in applications subjected to constant vibration or dynamic loads, can loosen over time. This is because the threads can wear down the material, or the screw can simply back out. While locking washers or thread-locking compounds can mitigate this, it remains a concern.

Rivnuts: Rivnuts provide a more secure and vibration-resistant connection. Once installed, the expanded body of the rivnut creates a tight, immovable joint that is less susceptible to loosening under vibration. This makes them particularly advantageous in automotive, aerospace, and other industries where structures are exposed to dynamic forces.

When to Choose Which

The choice between rivnuts and screws ultimately depends on the specific application requirements.

Choose Screws When:

• Removability is essential: Screws allow for easy assembly and disassembly without damaging the components.

• Direct threading is feasible: If the material is thick enough to tap strong threads directly, or if a nut can be accessed and used on the backside.

• Cost is a primary concern: Screws are often more economical than rivnuts.

• Shear strength is paramount (with a nut): A properly sized and torqued bolt-and-nut combination typically offers superior shear strength.

• Material thickness allows: In thicker materials, self-tapping screws can provide adequate holding power.

Choose Rivnuts When:

• Blind access is required: Rivnuts are ideal for situations where you can only access one side of the workpiece.

• Adding strong threads to thin materials: They excel at creating robust, reusable threads in sheet metal, plastics, composites, or tubing where traditional tapping is not possible.

• Vibration resistance is critical: The expanded body of a rivnut offers excellent resistance to loosening from vibration.

• A permanent, secure anchor is needed: Rivnuts create a fixed, durable anchor point.

• Repeated assembly/disassembly is anticipated in thin materials: The added thread strength prevents wear and tear on the base material.

Conclusion

While a simple comparison might suggest that a large, high-grade screw (especially with a nut) could have higher ultimate tensile or shear strength than a smaller rivnut, this oversimplifies the true professional decision-making process. Rivnuts are not necessarily "stronger" than all screws in all scenarios, but they offer distinct advantages that make them the stronger choice for specific applications.