Adhesives vs. Mechanical Fasteners Comparison

The two primary ways of joining components are adhesives such as glue and mechanical fasteners such as screws, nails, bolts, rivets, or pins. The primary difference between the two is in the way they transfer force. An adhesive distributes the load across much of the bonded area. A mechanical fastener distributes load through a limited number of attachment points where most of the stress is concentrated around the fasteners and their holes.

Adhesives are suitable when the configuration must be lightweight, smooth, sealed, or made from dissimilar materials. Mechanical fastening is suitable when the attachment needs to carry a load right away, remain easy to inspect, or to allow future disassembly.

Adhesives are not necessarily superior to mechanical fasteners, their choice depends on:

• The materials being connected
• If the joint must be permanent or removable
• The magnitude and direction of the loads.
• Operating temperature and environmental conditions
• The required production speed
• The available surface preparation techniques
• Inspection and maintenance requirements
• Manufacturing and lifecycle costs

It is also not vital that you use one or the other, both methods can be combined if your assembly permit it to.

Adhesives vs. Mechanical Fasteners Comparison:

What is Adhesive Bonding?

Adhesive bonding is a method of joining materials using a substance that may chemically react, possess intermolecular attraction, mechanically interlock, diffuse, or a combination of these methods. Once the adhesive is applied, the bond develops enough strength to hold the components together.

The materials that are bonded together are referred to as the substrates or the adherends. The layer of adhesive between the substrates is referred to as the bond line.

Generally, there are two substrates and an adhesive layer between them. The adhesive must be capable of transferring force between the substrates to hold them in place.

The two primary types of strength adhesive bonding must possess are adhesion and cohesion. Adhesion is the ability for the adhesive to bond with the adherend. Cohesion is the internal strength of the adhesive itself.

A durable adhesive joint requires sufficient adhesion strength between the adhesive and adherend and sufficient cohesion strength within the adhesive itself.

How Adhesives Join Surfaces

The purpose of an adhesive is to flow, spread, and create intimate contact with the substrate surface in a process called wetting. During this process, the adhesive and substrate may undergo a chemical reaction, along with intermolecular attraction and mechanical interlocking.

Typically, there are two categories of adhesives: structural and non-structural. Structural adhesives are made to carry significant loads and include adhesive families such as epoxies, acrylics, polyurethanes, anaerobic adhesives, cyanoacrylates, and phenolic adhesives. Non-structural adhesives are made for light-duty and intricate attachment where the joint does not carry a significant structural load. They include adhesives such as pressure-sensitive; craft, hot-melt, and light-duty contact adhesives.

Common Types of Adhesive Joints

Adhesives can be applied in many joint geometries for structural applications including:

• Lap joints
• Scarf joints
• Butt joints
• Stepped joints
• T-joints
• Corner joints
• Tubular joints
• Cylindrical joints
• Mortise and tenon joints
• Biscuit joints
• Dowel joints
• Dovetail joint
• Laminated joints
• Strap and patch joints

Joint geometry significantly affects adhesive performance, and adhesives perform best where the loads are distributed across large areas. Shear and compression stress types generally perform better while peel or cleavage concentrations are less effective.

What are Mechanical Fasteners?

Mechanical fasteners include components that physically connect two or more parts and include common types such as nails, bolts, screws, rivets, nuts, washers, clips, studs, pins, dowels, staples, retaining rings, and clamps.

Mechanical fasteners often puncture a hole into the substrate which is generally not reversible and will remain for the duration of its lifetime. Mechanical fastening can also be performed through threads, edges, groovers, or other features developed to physically connect two parts together.

How Mechanical Fasteners Join Components

There are multiple mechanisms that support the physical assembly of two components through mechanical fastening:

• Clamping: Is the use of a clamp to fasten components tightly together
• Shear resistance: The fastener prevents the components from sliding
• Bearing: The fastener presses against the wall of a hole
• Thread engagement: Fasteners such as screws mate threads that resist withdrawal
• Mechanical interlocking: The fastener’s shape prevents separation
• Friction: Clamping pressure creates friction between the joined surfaces.

While we do not go too far into mechanical fasteners themselves, we can offer insights into whether they will, or adhesives will make a preferable choice for your joining application.

Advantages of Adhesive Bonding

One of the primary advantages of adhesive bonding is that it distributes force more evenly across larger surface areas. A mechanical fastener transfers force at individual points where the screw or bolt is applied. On the contrary, an adhesive reduces the overall stress across the bonded region. However, the stress distribution is not always uniform, with the corners and sides being more susceptible to the load.

Furthermore, adhesive bonding does not require drilling, punching, or forming holes in the adherends. This is especially useful because fasteners can sometimes cause wood to crack or tear under pressure.  Some components may also be too small to accommodate holes and require a more seamless bond that an adhesive can provide.

Adhesives are particularly good at bonding dissimilar materials such as metal to plastic, glass to metal, rubber to steel, or aluminum. Mechanical fasteners are effective at joining these materials, too, but they often impose much greater pressure on the softer or brittle material. A suitable adhesive is generally more effective, distributes the load well, and accommodates differences in stiffness.

Adhesive bonding is generally lighter than mechanical fasteners and can weigh significantly less when measured in grams.

Another key consideration is that adhesives create a smooth, seamless appearance that is not visibly obstructed by screws, screw heads, rivets, nuts, or nails. This may improve the appearance, aerodynamics, and aesthetics of the finished product.

Moreover, adhesives often provide environmental protection against water, air, dust, chemicals, and other contaminants that fasteners do not. If you were to use mechanical fasteners instead of adhesive, you may require additional weatherproofing measures, such as a sealant.

Finally, adhesives can provide electrical or thermal insulation or isolation between adjoining materials. This may be important if you try to reduce the direct electrical contact between metals or limit galvanic corrosion. It can also reduce thermal bridging through an insulated configuration. If you need to improve conductivity, conductive adhesives are available.

Disadvantages of Adhesive Bonding

Like everything else, there are some disadvantages to using adhesive bonding compared to mechanical fasteners.

Firstly, adhesive bonding requires careful surface preparation from contaminants such as oil, moisture, dust, oxidation, and mold release agents. If these contaminants are not properly treated, they can interfere with the adhesion between adhesive and substrate. If time is limited, there may not be enough time to effectively treat the substrate surface, which could interfere with the curing process.

Time may also be a problem for adhesives that require time to set or cure. Additional clamping may be required during curing time to ensure effective bonding. Certain environmental conditions may also be required for the adhesive to cure, such as heat, light, or moisture.

Another disadvantage of adhesives is that visual inspection may be difficult because the bond line is generally hidden between the adherends. Voids, contamination, or insufficient coverage may remain sealed and inaccessible.

Adhesive joints are generally difficult to disassemble without damaging the adhesive or substrate. Removing a bonded component is generally difficult and may require heat, chemicals, cutting, prying, or mechanical abrasion. Therefore, adhesives are less suitable when disassembly, adjustment, or replacement are routinely required.

Furthermore, adhesives may be sensitive to their operating environment. Environmental factors such as water, humidity, UV radiation, fuels, acids, solvents, alkaline substances, and other chemicals can weaken particular adhesives. Temperature may also be a limitation, as high temperatures can soften an adhesive, accelerating degradation or reducing its load-carrying capacity. Cold temperatures may cause the adhesive to stiffen or become brittle.

Peel or cleavage stress also generally performs worse than shear or compression stress because they cause the force to concentrate at particular weak points.

Finally, there may be storage and temperature requirements for handling the adhesive, as well as additional instructions to protect it from heat, freezing, moisture, or light.

Advantages of Mechanical Fasteners

Mechanical fasteners are good for use almost immediately after installation. A properly installed screw, bolt, or rivet does not have a curing time before the assembly can be used. Mechanical fasteners are straightforward to visually inspect because the presence, position, and condition of the fastener can be verified. Torque marking, thread engagement, head position, corrosion, deformation, and visible damage can indicate installation quality.

Fasteners are also generally easy to configure because standardized components, tools, and procedures can be used. For instance, fasteners are available in defined materials, dimensions, grades, and strength classes. Installation involves selecting hole diameters, edge distance, tightening torque, spacing, and thread engagement which can be measured and provided for. Mechanical fastening has established design standards and engineering practices. Designers and select a fastener design and estimate the expected performance under tension, bearing or other loads.

Disassembly is likely the biggest advantage of mechanical fasteners which is generally difficult with adhesives.

Additionally, mechanical fasteners are suitable for thick structural components because the thickness provides sufficient space for the fastener to penetrate.

Mechanical fasteners are generally less susceptible to dirt, oxidation, surface energy and chemical compatibility requiring less surface preparation than adhesives.

Disadvantages of Mechanical Fasteners

Mechanical fasteners concentrate stress at the joint where the fastener is installed. Over time, this may cause the component to deform, crack, or tear, rendering it unable to carry the load.

Most mechanical fastenings require holes to be drilled, punched, molded, or threaded into the component, which may damage fibers, coatings, or surface treatments. This may cause minor cracks or residual stress, which can affect the component’s load-bearing capacity. This is especially concerning for thin components that do not have sufficient space to accommodate fasteners.

An additional disadvantage of fasteners is that they add weight to the assembly, as the combined weight of bolts, nuts, washers, nails, or screws can increase the component’s overall weight.

Threaded fasteners can loosen under vibration, repeated loading, thermal cycling, or reduction of clamping force. This can be counteracted by locking nuts, thread locking compounds, safety wire, or periodic tightening. Rivets generally do not loosen the way screws do, but they can still become damaged.

Fastener holes may create leakage routes for water, air, gas, dust, or chemicals unless properly sealed.

Additionally, fastener heads may compromise the aesthetic profile of the joined components. A seamless appearance may not be possible with fasteners, although caps can be designed to cover them.

When Should Adhesives be Used?

Adhesives are suitable when the component assembly needs to be lightweight. This makes adhesive bonding useful in transportation, aerospace, electronics, and sporting goods.

Adhesive bonding is also suitable when the finished surface must remain seamless and smooth. Because the bond line is concealed, adhesives can remove the visible heads and protrusions common with fasteners. Smooth surfaces are important for appearance and hygiene.

Adhesives may be more suitable for thin materials because fasteners will often damage or tear the surface area when the material is not thick enough. Examples may include thin metal sheets, plastic films, flexible laminates, and lightweight panels.

Adhesives may also be more suitable if dissimilar materials need to be joined. Adhesives can be particularly effective at bonding dissimilar materials such as metal to plastic, which may be difficult to do with fasteners alone.

Furthermore, adhesive bonding is preferable when a seal is required to limit the passage of liquids, gases, dust, and other contaminants. This reduces the need for gaskets or sealants in fastening applications.

Moreover, adhesives are preferable when the load needs to be distributed across a large surface area rather than concentrated at individual points. Lap joints, bonded flanges, laminated panels, patches, and skin-to-core joints can transfer force neatly across large areas. Shear and stress types respond better to adhesives than peel or cleavage.

Finally, adhesives should be used when drilling holes or punching staples into the material is unacceptable. Holes can damage fibers and protective coatings, creating leakage paths.

When Should Mechanical Fasteners be Used?

Mechanical fasteners are suitable when time is limited and the joint needs to carry a load immediately after installation. Once a screw, bolt, or rivet is installed correctly, it immediately provides load-carrying capacity.

Likewise, if disassembly is a future requirement, mechanical fasteners may be the preferred choice. Disassembly allows for inspection, maintenance, and repair or component replacement. It is typically quick to replace a damaged bolt or screw using ordinary tools, while adhesive repairs may require time for surface preparation or curing.

Additionally, if visual inspection is a requirement, mechanical fasteners may be the preferred choice. The correct installation of mechanical fasteners can be checked without advanced equipment, but it is still possible to measure things such as torque, thread engagement, or rivet dimensions.

Mechanical fastening is also suitable for applications where high temperatures must be resisted to prevent softening or degradation. Adhesives may soften or degrade at high temperatures, whereas mechanical fasteners are better able to withstand heat.

Mechanical fastening is better suited for heavy structural load applications where the components are thick and can accommodate large bolts, pins, or screws. While adhesives can also carry weight, they require appropriate joint geometry. If surface preparation is not convenient, fasteners may be preferable.

How to Choose Between Adhesives and Mechanical Fasteners

Deciding between an adhesive and a mechanical fastener requires a multi-step process. Alternatively, you may choose a hybrid bonded-fastened joint. This type of joint provides the advantage of continuous load distribution while the fasteners provide support at critical points. You may often find adhesive-bolted, adhesive-riveted, adhesive-screwed, or weld-bonded joints.

To make a selection, you should first identify the types of loads the joint must carry by paying careful attention to their magnitude, direction, duration, and frequency. Determine the nature of the stress that the joint will be subject to in the sense that it is peel, shear, tensile, compressive, or torsional.

The second step is to identify the types of materials being joined. The surface chemistry, porosity, strength, stiffness, thickness, thermal expansion, temperature limits, and corrosion behavior all affect the choice you make. Dissimilar materials are better suited for adhesive bonding, while thick components are better suited for mechanical fasteners.

If the joint requires disassembly in its life cycle, then fasteners may be a more practical choice. If the joint is intended to remain permanently affixed, then adhesive bonding may be more suitable.

Operating environment is also a key consideration because environmental conditions like temperature, humidity, UV radiation, water, chemicals, vibration, impact, and thermal cycling affect the integrity of the joint,

Overall, you should pay attention to the total production and lifecycle of the joint to ensure you account for any constraints or considerations that could affect the integrity of the bonding mechanism.

Conclusion

This article provides information on adhesives and mechanical fasteners, how they differ, their advantages and disadvantages, and whether to select one, the other, or both for your joining method. Selecting the suitable method requires taking into account the considerations outlined in this article and analyzing the joining method from inception through preparation, installation, and post-configuration. Evaluating the merits of each method throughout the entire life cycle will enable you to make a well-informed and prudent choice about whether to use adhesives, mechanical fasteners, or a hybrid approach.