The screw only engages 3 threads — is that really enough?

In the world of mechanical assembly, there’s a standard “rule of thumb”: once the screw is tightened, as long as the nut is exposed or three threads are visible, it’s secure. Especially in furniture assembly, simple equipment repair, and even some non-standard automated production lines, in pursuit of efficiency, craftsmen often stop tightening the screw just as it’s “engaged.”

“Three threads”—this seemingly unimpressive number—can it truly bear the weight of a tight fastener? Is it a clever simplification of engineering mechanics, or an excuse for cutting corners? When we talk about “three threads,” we’re actually discussing a life-or-death struggle involving shear force, tensile force, materials science, and safety redundancy.

Today, we’ll thoroughly dissect this issue and examine the engineering truth hidden behind these “three threads.”

I. The Cruelty of Theory: The Stress on a Thread is Never “Equally Distributed”

First, we must shatter a beautiful illusion: the load-bearing capacity of a threaded connection is not simply the product of the number of engaged threads multiplied by the load-bearing capacity of a single thread.

Ideally, when you screw a screw into a nut, each turn of the thread should evenly distribute the tension. However, in the real physical world, force transmission is highly eccentric. When the screw is subjected to axial tension, the first thread (the one closest to the stressed surface) bears about 60% to 70% of the load, the second thread bears 20% to 30%, and the threads after the third are practically unaffected.

This is the famous phenomenon of “uneven thread load distribution.”

What does this mean? If only three threads are engaged, only the first 1.5 threads are actually under significant stress. Once the external force exceeds the material’s yield strength, the first 1.5 threads will shear and fracture (slip), triggering a domino effect and causing the entire connection to fail.

According to standard formulas in mechanical design manuals, the shear strength of a thread is directly proportional to the engagement length. For a standard metric coarse thread (such as M10), the pitch is approximately 1.5 mm. The engagement length of the three threads is only about 4.5 mm. For high-strength steel (such as grade 8.8 and 10.9), such a length might be barely acceptable under static loads, but under impact loads, it’s practically a paper-thin defense.

II. The Survival Ground for “3-Threaded Screws”: Only Suitable for Light Loads and Soft Materials

Since 3 threads are so dangerous, why does this notion still exist in the market? Because it can indeed survive in a specific “comfort zone.”

1. “Quantity for Quality” in Soft Materials

When you screw a screw into aluminum alloy, plastic, wood, or even cast iron, the strength of the substrate is far lower than that of the screw itself. At this point, the failure mode is no longer screw breakage, but rather the substrate being “pulled out” or “crushed.” To prevent this failure, we need more threads to distribute the pressure. In this case, 3 threads are absolutely insufficient; usually, 6-8 threads or even more are required. However, if self-tapping screws are used on thin plates, due to plate thickness limitations, they may only physically accommodate 2-3 threads. This is a structural limitation, not a lack of sufficient strength.

2. Temporary Fixation Under Extremely Low Loads

If you’re only fixing a non-stressed decorative cover or an electronic component weighing only a few grams, the friction provided by three threads does indeed exceed the force of gravity. However, this is “positioning,” not “fastening.” Engineering-defined fastening connections must consider a safety factor. Typically, critical connections require a safety factor between 1.5 and 3.0. A three-thread connection often has a safety factor below 1.2, essentially operating without safety.

3. The “Salvation” of Fine-Pitch Threads

Fine-pitch threads have a smaller pitch and more threads within the same engagement length. For example, in an M10x1.0 fine-pitch thread, the length of three threads is 3mm, but because the thread profile is finer, the load-bearing capacity distribution is slightly better than with coarser threads. Even so, three threads are still on the edge of their limits.

III. The Nightmare of Failure: Stripped Threads and Fatigue Fracture

Let’s imagine a scenario: an M8 grade 8.8 screw, used to secure the base of a vibrating motor, is only tightened to three threads.

Stage 1: Loss of Preload

In the initial stages of vibration, fretting wear occurs at the contact surfaces of the three threads, reducing preload. Because the contact area is too small, the pressure is exceptionally high, and the peaks and valleys of surface roughness are quickly worn away, causing the screw to loosen.

Stage 2: Cracks Caused by Stress Concentration

Due to the short engagement length, the transition zone between the screw shank and the thread root is located at the section with the highest stress. This is the “eye of the storm” of stress concentration. Under alternating stress, tiny cracks will initiate at the root of the third thread.

Stage Three: Instantaneous Fracture

When the crack reaches a critical size or a severe vibration or impact occurs, the first two threads undergo shear stripping due to overload. The remaining load-bearing capacity instantly drops to zero, the screw breaks like a noodle, or the entire threaded hole is pulled out.

This is why connections with only three threads are absolutely prohibited in aerospace, automotive chassis, and lifting equipment. NASA standards even require that, in critical areas, the thread engagement length must not be less than 1.5 times (for steel) or even 2 times (for aluminum) the bolt diameter. For an M10 bolt, this means at least 15mm of engagement length, which translates to approximately 10 threads.

IV. The Source of the “Three Threads” Myth: The Visual Judgment of Exposed 3 Threads

Why is there the claim that “three threads are enough”? This is likely a misinterpretation of the construction specification regarding “exposed 3 threads.”

Many construction specifications do indeed require “2-3 threads of the screw to be exposed above the nut.” Note that “exposed” here refers to the portion extending beyond the nut, not the portion engaged inside the nut!

• Exposed 3 threads: This is for easy inspection of the screw’s tightness (if no threads are exposed, it may be a false tightening), and also to prevent the screw from jamming (jamming leads to uncontrollable preload).

• Engagement length: This is crucial. A standard hex nut’s height is approximately 0.8 times its diameter, and the number of internal threads is typically between 6 and 9. Therefore, when you see 3 threads exposed, at least 6 threads are actually engaged inside the nut – this is the safe truth!

Many non-professionals mistakenly interpret “3 exposed threads” as “only 3 threads are under load,” which is the root cause of this misconception.

V. Where is the real bottom line?

So, if you don’t use a nut and screw directly into a threaded hole (blind hole), what is the minimum number of threads required?

Based on the consensus of the *Mechanical Design Handbook* and engineering practice:

1. Steel to steel (equal strength): Minimum engagement length Lmin ≈ 0.8d～1.0d (d is the nominal diameter). Taking M10 as an example, at least 8 mm-10 mm is required, approximately equal to 5-6 coarse threads.

2. Steel to cast iron: Cast iron is brittle, requiring a longer engagement length to distribute pressure. Typically, Lmin ≈ 1.2d～1.5d, approximately 7-9 threads.

3. Steel to aluminum/plastic: Requires an even longer engagement length, typically Lmin ≈ 1.5d～2.0d, approximately 10 threads or more.

What if your threaded hole depth is limited, and you can’t achieve 6 threads?

Engineers have only three remedial measures:

1. Use a wire thread insert (wire thread insert): This is the most ingenious invention. It can not only repair stripped holes but also distribute the load-bearing capacity of thin-walled holes over a larger area. Even so, it’s recommended to retain at least 3-4 intact threads for effective engagement.

2. Adding adhesive (thread binder): Anaerobic adhesive fills the gaps, increases friction, and distributes the load. However, this does not increase shear strength; it only prevents loosening.

3. Enlarging the hole and tapping to get a larger screw: This is the simplest but most reliable method. If an M6 only has 3 threads, enlarge the hole and tap an M8. Even if an M8 also only has 3 threads, its load-bearing area is much larger than that of an M6.

Conclusion: Don’t gamble with probability.

Back to the initial question: Is it really okay to screw a screw out 3 threads?

The answer is: Under extremely ideal conditions—non-critical, vibration-free, purely static, and with extremely high substrate strength—it “may” not break immediately. But in engineering practice, it is absolutely “unacceptable.”

The essence of mechanical connections is building trust. When you design a product, you’re promising users, “This part won’t come loose for ten years.” A three-threaded connection is like using a thin thread to tether a wild beast. It might be docile today, but a single unexpected vibration, a drastic temperature change, or an unexpected overload will snap the thread.

So, next time you pick up a wrench and see only three lonely threads struggling to hold on in a threaded hole, stop. Tighten it a couple more turns, or replace it with a longer screw. Because in the world of mechanics, redundancy isn’t waste; it’s life-saving insurance.

Even an extra thread demonstrates respect for the laws of physics.