In bolted joints, the use of washers has become something of a default practice. At first glance, a washer seems like a simple component, just a flat disc placed between the nut or bolt head and the joint surface, but underneath this simplicity lies a complex interplay of mechanics and materials that can make or break the performance of your assembly over time.

Let's then take a look at why washers are used, how they impact bolt relaxation, and why the phenomenon of embedding, frequently underestimated, deserves special attention in joint design. 
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 Why Use Washers in Bolted Assemblies?

Washers serve multiple functions in a bolted joint. They’re typically added to:

• Distribute load across a wider surface area
• Protect softer materials (e.g., aluminum, plastic, paint) from damage
• Reduce surface wear during tightening and under dynamic loads
• Bridge oversized holes or irregularities on the joint surface

In theory, all these functions should enhance the durability and reliability of the joint. However, in practice, their impact is not always a net positive, especially when embedding comes into play.
 

Some types of common washers: helical spring washers, flat and countersunk tooth lockwashers, smooth and serrated Belleville washers.

The Hidden Role of Embedding in Joint Relaxation

Embedding refers to the microscopic flattening and deformation that occurs when rough or irregular contact surfaces in a bolted joint compress under the high clamping force of tightening. Though these changes may be imperceptible to the eye, their effect on bolt tension can be significant.

Here’s what happens:

1. Initial Contact – When a bolt is first tightened, the clamping force is transmitted through the washer (if present) and the bearing surfaces.
2. Microscopic Yielding – Over time—or immediately under high load—the micro-asperities (tiny peaks and valleys) on the contacting surfaces deform and collapse. This happens at the bolt head, washer, nut, and sometimes between the plates being clamped.
3. Loss of Tension – As the surfaces "settle" into each other, the effective length of the clamped stack shrinks, and the tension in the bolt drops accordingly. This phenomenon can cause up to 50% or more loss in preload, depending on materials and joint geometry.

 Embedding is particularly pronounced in the following scenarios:

• Elastic joints using soft materials like aluminum or plastic parts
• Surface coatings or paints, which compress or flake under pressure
• Multiple interfaces in the load path—such as when washers are added
• Short grip lengths, where the joint compresses easily and any loss in thickness translates directly into tension loss

In short, embedding isn't just a nuisance; it’s a major contributor to bolt relaxation and, if not accounted for properly, to potential failure of the assembled product.

The Double-Edged Sword of Washers

While washers can reduce surface damage and spread out the load, they introduce an additional interface into the clamped stack. This extra interface is yet another point where embedding can occur, and if the washer is made of a softer material or poorly matched to the bolt hardness, it can become the weak link in the system.

Even worse, if the washer rotates along with the bolt head or nut during tightening, it changes the friction profile, altering the torque-tension relationship and making it harder to predict or control the final preload.

In joints where high precision is required or where relaxation must be minimized (such as in aerospace or precision automation), flanged fasteners are often a better solution than using a separate washer. These integrate the washer into the head or nut, removing an interface while still offering load distribution.

Flanged fasteners.

How Smart Torque Tools Help Mitigate Embedding and Relaxation

While joint designers often try to account for embedding and relaxation in their specifications, the reality on the production floor is rarely ideal. Surface imperfections, operator variation, and real material behaviors often cause preload loss that’s hard to predict and control, unless you're closely measuring what actually happens during tightening.

That’s where transducerized electric screwdrivers like Kolver’s K-DUCER come in.

Real-Time Torque and Angle Monitoring

The K-DUCER continuously monitors both torque and rotation angle during tightening, giving you real-time insight into what's happening inside the joint.

• Abnormal angle gain with low torque can indicate material settling or embedding
• You can analyze torque-angle curves to detect soft joints or excessive relaxation zones
• You can set up running torque monitoring to dynamically detect each joint's conditions and respond appropriately, to guarantee a consistent preload applied to each fastener;

Advanced Tightening Strategies to Manage Embedding

In the most demanding industries (like aerospace and automotive) embedding is treated as a real and significant challenge. For example, NASA’s official procedure for threaded fastener installation mandates a three-step torque application:

< • Snug (approximately 0% of final torque)
• Between 50% and 75% of final torque
• 100% of final torque>>

This phased approach allows the materials in the joint to progressively settle and align under increasing load, helping to minimize the effects of embedding and uneven surface contact. Therefore, it’s not just about reaching the final closing torque value, it’s also about how you get there.

Building on this principle, one of the most effective field-proven strategies, especially in joints involving compressible materials like gaskets, is a two-step tightening process using our K-DUCER line of smart electric screwdrivers.

Here’s how it works:

1. First Step – Joint Conditioning
   The tool applies an initial torque and angle to bring the joint up to the desired preload; it can be the final torque value or, as NASA recommends, around 75% of final torque. This causes early embedding and any initial surface deformation to occur in a controlled way.
2. Pause (or downshift) and tighten
   After the initial tightening, the tool pauses and, after a short pause, resumes tightening to the final torque value. Because the surfaces have already "settled," the joint now behaves more like a solid stack, and less relaxation will happen.

In the case of gasketed joints, this method is particularly beneficial, especially if the final torque value is applied in both steps (effectively making this a double-tightening strategy). After the first tightening, the gasket material compresses and conforms to the joint surfaces. By retightening shortly afterwards, you’re now applying force to a hardened gasket, one that has already undergone most of its deformation. The result is a more stable joint with:

• Higher retained clamp load
• Less long-term gasket creep
• Greater resistance to preload loss

Empirical results show that this double-hit approach can reduce preload loss from ~25% to as little as ~10%, simply by retightening on a more stable surface. It’s a remarkably simple strategy with a major impact and with a smart tool like the K-DUCER, as seen previously, it can be set up in no time, ensuring consistency across every cycle and every operator.

Repeatability and Traceability

Every tightening cycle is logged and traceable, so variations in joint behavior, such as unexpected settling or early signs of embedding, can be flagged, reviewed, and addressed. It’s a data-driven way to eliminate guesswork in your production line.

And when used alongside tools like the K-TESTER for torque verification and quality checks, you create a closed loop of measurement, verification, and improvement. 

Takeaway: the (torque) journey is just as important as the (torque) destination

Washers can serve a purpose, but they also introduce complexity and potential for preload loss through embedding. While joint designers do their best to account for these effects, actual performance depends on how a joint behaves during tightening and over time. Therefore, success lies not just in how much torque you apply, but in how deeply you understand what happens during and after the tightening. 

While determining the precise amount of preload loss can be extremely complicated, most practical issues can be mitigated with the right fastening approach and the K-DUCER can help you achieve just that, by spotting embedding before it causes problems, adapting tightening strategies to match real joint behavior and ensuring stable, repeatable, traceable assembly, cycle after cycle.