In the context of fasteners, the coefficient of friction (CoF) refers to the frictional resistance between contact surfaces in a fastening assembly – primarily the threads of a bolt/nut and the underside of the bolt head or nut face. This seemingly simple property plays an outsized role in how bolts, screws, and other fasteners perform during installation and in service. When a fastener is tightened, friction in the threads and under the head consumes the majority of the tightening effort. In fact, typically only about 10–20% of the applied tightening torque goes into actually stretching (tensioning) the bolt; the other 80–90% is spent overcoming friction in the threads and under the head. Such friction is essential – without it, a tightened nut would quickly spin loose – but it also means the CoF largely governs the relationship between the torque you apply and the clamping force (preload) achieved. This article will delve into why CoF matters for fasteners, how it affects preload and performance (including issues like galling and loosening), differences between thread friction and under-head friction, typical friction values in various conditions (dry vs. lubricated or coated), and how surface treatments or locking features modify friction. We will also discuss the trade-offs between low and high friction and practical considerations for engineers in specifying and controlling friction for fasteners. While the focus is on threaded fasteners (bolts, screws, studs), we will briefly touch on non-threaded fasteners (pins, rivets) where friction is relevant.

Friction and Fastener Installation: Torque-Preload Relationship

During installation of a threaded fastener, the CoF directly influences the torque-tension relationship. In a bolted joint, tightening torque is translated into two main effects: bolt tension (preload) and frictional resistance. The higher the friction, the more of your applied torque gets “wasted” overcoming friction and the less is left to generate preload. Conversely, lower friction means a greater proportion of torque goes into stretching the bolt and clamping the joint. Engineers often express this with an equation or factor (sometimes called the nut factor K):

• High friction (higher CoF): More torque is needed to achieve the same preload, because a lot of the energy is dissipated as heat and surface shear in the threads and under the bolt head. For example, a dirty or rusted bolt with very high friction might require significantly more tightening torque to reach a given tension – and you might think the bolt is tight (based on torque) when in reality the clamping force is low. High friction can thus lead to under-tightening if one isn’t careful, since the bolt may stop turning (torque rises) before sufficient tension is developed.
• Low friction (lower CoF): Less torque is needed to reach a given preload, since there is less resistance to overcome. This makes it easier to achieve high tension in the bolt, but it also means that using a normal torque value on a well-lubricated or coated bolt could over-tighten With very low friction, a small increase in torque can produce a large jump in tension, potentially yielding or breaking the fastener if one doesn’t adjust the torque down.

Because of these effects, controlling and understanding the friction coefficient is critical for reliable preload. The classic rule of thumb is that only about 10% of the tightening torque generates useful bolt tension, while ~90% is lost to friction (roughly split between thread friction and under-head friction). Any change in friction – due to lubrication, coating, surface finish, etc. – will dramatically alter the torque required for the target preload. For example, if you remove the light oil usually present on “as-received” steel bolts, the friction might increase and you’d get much less clamping force for the same torque (perhaps even shearing the bolt before reaching proper tension). Likewise, applying a “super lube” (like a molybdenum disulfide paste or wax) can drop the friction so much that using the original torque could stretch the bolt beyond its yield.

In practice, engineers compensate for CoF by adjusting tightening specifications. A common approach is specifying a torque assuming a typical friction (e.g. “tighten to 50 N·m assuming CoF ~0.15”). If the actual condition differs (say the bolt is greased to CoF 0.08 or bone-dry at 0.25), the achieved preload will be very different. This is why torque tables and standards often state the friction conditions they’re based on. For critical joints, controlling friction through specifications (e.g. requiring lubrication or specific coatings) and using calibrated tightening methods (like torque+angle or direct tension measurement) are important to achieve the desired preload reliably.

Thread Friction vs. Under-Head Friction

In a bolted assembly, friction acts in two primary places: within the threads (between the male and female threads) and under the turning head or nut (between the underside of the bolt head or nut and the surface it contacts, which could be a washer or the clamped part). These are often denoted as thread friction coefficient (μ_thread) and under-head or bearing friction coefficient (μ_bearing). While we often talk about an overall CoF for the fastener, it’s useful to distinguish these because they can differ and each has distinct implications:

• Thread Friction: This is the friction between the bolt threads and nut (or tapped hole) threads. When tightening, the surfaces of the threads slide against each other under high pressure. Thread friction directly resists the turning of the nut/bolt and also puts a torsional stress into the bolt. In other words, as you tighten, the bolt is not only stretched by the tension but also twisted due to thread friction. High thread friction increases this torsional shear stress in the bolt’s shank. This matters because a bolt’s tensile strength is effectively reduced when it’s simultaneously being twisted – a phenomenon that can cause the bolt to yield at a lower tension if thread friction is excessive.
• Under-Head Friction: This is the friction under the bolt head or nut face, where it presses against the joint surface or washer. As the bolt/nut turns, this interface slides (at least until the bolt snugs up fully) and friction here resists that rotation. Under-head friction does not directly twist the bolt’s shank (if you’re turning the nut, the bolt shank sees tension but the nut face friction is acting on the nut). However, under-head friction has a large effect on the torque required because the radius at which it acts is larger (the bolt head or nut radius) compared to the thread’s radius. A rough or galling under-head interface can consume a huge portion of the torque. Differences in surface condition here (like using a smooth hardened washer vs. a rough flange or a painted surface) will change μ_bearing and thus change the tightening behavior.

Typically, about 40–50% of the total frictional resistance comes from the under-head area and about 40% from the threads, leaving only ~10% for tension. But μ_thread and μ_bearing aren’t necessarily equal – for instance, a zinc-plated nut on a plain steel surface might have a different friction under the nut than the friction in the steel-steel threads. It’s common in design to assume they’re similar, but in reality one may be higher than the other. A washer can also alter under-head friction: a hardened, smooth washer can lower and stabilize the friction (since it provides a consistent smooth bearing surface), whereas a lock washer or serrated washer intentionally increases under-head friction (by biting into the material) to resist loosening. Engineers must consider both types of friction: for example, if a lubricant is only applied to threads but not under the head, one friction component drops but the other remains high – this could lead to uneven torque distribution or galling under the head. Ideally, both thread and under-head friction should be controlled within reasonable ranges for consistent tightening.

In summary, thread friction affects bolt stress and how much torque is lost in the threads, while under-head friction affects how much torque is lost under the bolt head or nut. Both contribute to the overall CoF of the fastener. A balanced, known combination of μ_thread and μ_bearing allows for predictable tightening. If one is significantly higher (say a very rough flange face but smooth threads), you might need extra torque and could risk scoring the surface or not achieving proper preload. If one is much lower (say a very slick washer under the nut but relatively sticky threads), more of the torque will go into the thread friction and bolt twist, which also isn’t ideal. Thus, specifications for critical fasteners sometimes give separate limits for thread friction and under-head friction, or they require a certain washer or lubricant to manage both.

Impact of CoF on Fastener Performance

Beyond just the tightening process, the coefficient of friction influences several aspects of a fastener’s performance in service. Here are key areas where CoF matters:

Preload Consistency and Joint Clamp Force

Because friction affects how much tension is achieved for a given torque, variation in CoF leads to variation in preload. If some bolts in a joint have a higher CoF (perhaps due to a drier thread or a slightly rougher surface) and others have a lower CoF (maybe better lubricated), then tightening all to the same torque will not give the same bolt tension. The low-friction bolts will end up with higher tension, and the high-friction ones with lower tension. This scatter can be quite large – even with carefully controlled conditions, a ±20-30% variation in achieved preload is common when using torque control. Uncontrolled friction (like differing surface finishes, dirty threads, or inconsistent lubrication) can widen that scatter further. This means some fasteners might be underloaded (risking joint slip or fatigue) and others might be overloaded (risking bolt yield or fracture). Consistent friction is therefore vital for ensuring that all bolts in a critical joint carry their intended share of load. In design, standards often assume a certain “friction class” for fasteners and require that surface treatments or lubrication be specified so that the actual friction falls in that band. In manufacturing and maintenance, it’s important not to mix fasteners with different coatings or to inadvertently change friction (for example, by lubricating some bolts and not others) unless the torque is adjusted accordingly.

Galling and Seizing

Galling is a form of adhesive wear that occurs when two metal surfaces slide against each other under high pressure, causing material transfer or even seizure. Fastener threads are particularly prone to galling when friction is high and no proper lubrication is present. Stainless steel bolts and nuts, for example, are notorious for galling – a stainless nut can seize on a stainless bolt during tightening, effectively “freezing” or even welding itself due to galling. In the context of friction, high CoF (especially between similar alloys) means a lot of rubbing and heat at the thread interface, which can tear away the protective oxides and lead to direct metal-metal adhesion. Reducing friction is the primary way to prevent galling. Applying a lubricant (even a simple grease or a specialized anti-seize compound) dramatically cuts down friction and allows the surfaces to slide without welding together. Many fasteners in galling-prone materials come with a coating or lubricant for this reason – for instance, stainless steel fasteners often are pre-coated with a wax or polymer, and hot-dip galvanized nuts are usually supplied lubricated to avoid galling on the zinc-coated bolts. Thus, a lower CoF in threads helps prevent the surface damage and sticking that galling causes. On the flip side, once galling occurs, friction skyrockets (the surfaces roughen and lock together), often resulting in a bolt that cannot be tightened further or removed without snapping it off. In summary, keeping friction in a moderate range (not too high) and using lubricants where appropriate maintains smooth threading and prevents galling, ensuring the fastener’s durability and reusability.

Vibration Loosening and Self-Loosening Resistance

Fasteners in service often face vibrations and shifting loads that can lead to self-loosening. One mechanism of self-loosening is that if the clamped joint interfaces slip (even microscopically) under vibration, the preload can relax and the nut or bolt head may rotate in the loosening direction. Friction in the threads and under the head provides resistance to this rotation. A higher CoF means more resistance to turning under those small perturbations – in essence, it helps lock the threads in place. This is why many locking features intentionally add friction: for example, a nylon insert lock nut increases prevailing torque (the torque needed to turn the nut even without clamp load) via friction between the nylon and the bolt threads; deformed-thread lock nuts (like oval locknuts) introduce extra metal-to-metal friction; and serrated flange bolts/nuts create high friction under the head that resists back-off. These design elements are exploiting friction to make the fastener resistant to vibrational loosening.

However, friction cuts both ways. If the overall friction is extremely low (say a bolt is very slick), there is a theoretical concern that the fastener could loosen more easily under certain conditions. In practice, as long as a bolt is properly preloaded, pure “spontaneous” rotation is rare because the tension creates a normal force that, even with low μ, still provides considerable resistance. But extremely low thread friction (approaching the borderline of “self-locking” condition for threads) can reduce the margin. In fact, for a thread to be self-locking (not back-drive under static load), the friction coefficient must be above a certain threshold related to the thread geometry. If μ is too low, the thread could behave like a ball bearing and the nut might run loose when jostled or if there’s any imbalance. Generally, fastener manufacturers avoid CoF below roughly 0.08 for this reason – it’s the “too low” regime. So there is a sweet spot: we want friction low enough to tighten efficiently but high enough to maintain a robust self-locking behavior.

It’s worth noting that the primary cause of loosening is usually joint interface slip (loss of friction between the clamped parts) rather than thread friction alone. But if such slip happens (for example, an overload or cyclic vibration causing minute movements), a low-friction nut will require less energy to rotate off. A higher friction nut will resist rotation longer, potentially maintaining some preload. In summary, a moderate to slightly higher CoF can improve loosening resistance, which is why lock nuts and threadlocking patches deliberately raise the friction. Engineers must balance this: if using a very low-friction coating for easy tightening, additional measures (like a secondary locking device or threadlocker adhesive) might be needed for vibration resistance. Conversely, relying solely on very high friction to lock a fastener isn’t wise either – if friction is from a rough, galling-prone condition, it could lead to inconsistent preload or other issues. Purpose-designed locking features (nylon inserts, patches, deformed threads, etc.) provide controlled high friction mainly in the loosening direction while still allowing proper tightening.

Bolt Strength, Torsion, and Durability

The coefficient of friction also ties into the mechanical durability of the fastener itself. As mentioned earlier, high thread friction adds torsional stress during tightening. A bolt under high torsion plus high tension is closer to its yield/failure condition than one just under tension. So a bolt tightened in high-friction conditions might fail at a lower tensile load (or require a lower torque to start yielding) than the same bolt in a low-friction scenario. In practical terms, if you have a very high friction (say a dirty, unlubricated heavy bolt), you might twist it off or permanently stretch it before ever reaching the intended preload. From a durability standpoint, over-stressing a bolt can reduce its fatigue life – a bolt that’s been accidentally yielded or nearly yielded during installation may not endure service loads as well. Thus, controlling friction helps ensure we are not unknowingly over-straining the fastener during installation.

Another durability aspect is wear and corrosion. High friction during tightening means more abrasion on the contact surfaces. Threads can experience wear or galling damage if friction is excessive, which might make them less fit for reuse. Also, if a coating is providing lubrication (like a zinc flake coating with an integrated lubricant), extremely high friction might indicate that coating has been damaged or lost, possibly reducing corrosion protection. On the other hand, a properly lubricated assembly (moderate/low friction) tends to tighten smoothly and is easier on the threads and bearing surfaces, preserving them for future re-use or adjustment.

Finally, consider that friction can change over the life of the fastener. A joint assembled with grease (low friction) could see that grease dry out or wash away over years, effectively raising the friction if re-torqued or disassembled later. Corrosion can also increase friction over time (ever tried to loosen a rusty nut?). If a bolt’s CoF goes up in service (due to corrosion products or loss of lube), it will be harder to turn – that can be “durable” in the sense of resisting loosening, but it also makes maintenance harder and raises the risk of the fastener seizing or breaking on removal. Thus, depending on the application, engineers may prefer certain coatings that maintain a stable friction over the service life and do not deteriorate (for example, a coating that both prevents rust and provides lubricity ensures the friction won’t spike years later).

In summary, the coefficient of friction affects more than just the tightening process – it has a cascading influence on whether the bolt achieves proper clamp load, if it will suffer surface damage or not, if it will stay tight under vibration, and whether the bolt is being overly stressed or remains within safe limits. Managing CoF is therefore a key part of ensuring a durable, reliable bolted joint.

Typical Friction Coefficient Ranges for Fasteners

The actual numeric value of the coefficient of friction in a fastener assembly depends on the materials and surface condition (cleanliness, roughness, coatings, etc.). Here we outline typical ranges for CoF in various common conditions for steel fasteners:

• Dry (As-Received) Steel, Uncoated or Plain Finish: Approximately μ ≈ 0.15–0.25. Many plain carbon steel fasteners come with a thin oily film from manufacturing which gives a bit of lubrication; this often puts them in the ~0.15 range initially. If that oil is removed (truly clean and dry steel-on-steel), friction can be on the higher end (~0.2 or more). Dirty, corroded, or rough surfaces can easily push the friction above 0.25 – values as high as 0.3–0.4 have been observed for rusty threads or very rough conditions. (In such cases, torque required to achieve preload is much higher, and there is risk of thread seizure or bolt damage.) For a black oxide (“blackened”) finish without added oil, expect high friction (~0.2+), but black oxide is almost always accompanied by a light oil for corrosion resistance, which yields a moderate μ around 0.12–0.20 in practice.
• Lightly Lubricated Steel: Approximately μ ≈ 0.10–0.15. Applying standard lubricating oils or light greases to threads and underheads can reduce friction considerably. For example, a few drops of motor oil or a squirt of WD-40 on a bolt might bring a previously dry 0.18 friction down to ~0.12. This is why many specifications for torque assume a slightly oiled fastener – it’s more consistent and lower than completely dry. Be aware that different lubricants have different effectiveness: a heavy grease or anti-seize compound might get you to the lower end of this range or below, whereas a thin oil is moderate. Phosphate & oil coated bolts (common in automotive uses) also fall roughly in this range; the phosphate provides a matte, porous surface that holds oil, resulting in a fairly consistent μ around 0.10–0.16.
• Well-Lubricated or Low-Friction Coated: μ ≈ 0.05–0.10 (in some cases up to 0.12). This category includes fasteners that have been deliberately treated for low friction: for example, waxed bolts, fasteners with PTFE (Teflon) coatings, or those coated in a dry film lubricant like MoS₂ (molybdenum disulfide) or graphite. Cadmium plating (now less common due to environmental concerns) historically provided a very consistent low friction, often around 0.12 or even down toward 0.08 when combined with a clear chromate film – one reason cadmium-plated bolts were favored in aerospace. Modern substitutes like zinc plating with a wax topcoat can achieve similar ranges (a zinc-plated, waxed fastener might have μ ~0.10 or a bit under). PTFE or proprietary anti-friction coatings on bolts can push friction to the very low end (~0.05–0.08), which makes tightening extremely smooth. For instance, PTFE-coated structural bolts used in corrosive environments can be tightened to high preloads with much less torque, thanks to those low friction values. It should be noted that when friction goes this low, engineers often specify a reduced tightening torque or use tension control methods to avoid the risk of overshoot.
• Intentionally High-Friction (Locking) Features: μ (effective) ≈ 0.2–0.5. Some fasteners intentionally introduce added friction for locking purposes. For example, a nylon patch applied to threads (a common lock feature in set screws or aerospace fasteners) can create an effective friction such that a significant torque (often 0.3 times the tightening torque or more) is required just to overcome the patch’s drag. Numerically, the patch itself might behave like a local friction coefficient of 0.3–0.4 in that region of the thread. Similarly, nylon insert lock nuts have a section of much higher friction once the bolt reaches the nylon, and all-metal lock nuts that are deformed (oval or slotted) create higher friction metal-to-metal contact in certain areas. Under-head locking features, like a serrated flange nut, increase friction under the head (metal serrations digging in can act like a very high μ, well above 0.3, plus some minor plastic deformation). These features are not usually quantified just by a simple “μ” because they often behave a bit differently (e.g., a nylon insert also has elastic deformation). Instead, standards specify their performance in terms of a required prevailing removal torque. But it’s clear that they fall on the high end of friction. A regular fastener would never be deliberately manufactured with a CoF of 0.4 in normal tightening – it would be too unpredictable – but with lock patches or inserts, we accept high friction in exchange for locking ability (and we tighten those fasteners knowing that a portion of our torque is going into overcoming the locking feature).
• Plated or Coated (normal friction range): Many standard coatings aim to keep friction in a moderate band, often μ ≈ 0.10–0.20. For example, plain electro-zinc plating (without special lubricant) might have μ around 0.15–0.20 as supplied (somewhat lower than completely bare steel because the plating process often leaves a slight film and the zinc is softer than steel, but it can be inconsistent). Hot-dip galvanized fasteners are a special case: galvanized coatings are thick and a bit rough, so by themselves they can create high friction if nuts are not lubricated. Typically, galvanizing standards require that galvanized nuts be overtapped and lubricated (often dyed to indicate presence of lube) to target a reasonable friction (usually around 0.14–0.18). Zinc flake coatings (like DACROMET, GEOMET, etc.) usually include an integrated topcoat that can be formulated to achieve a desired friction coefficient, commonly specified in a range such as 0.09–0.15. The automotive industry often calls for coated fasteners to meet a friction “window” (for example, a spec might say μ = 0.12 ± 0.04) to ensure consistency in assembly.

In summary, untreated or dry fasteners tend to have higher and more variable friction, around 0.2 or above, whereas lubricated or specially coated fasteners can bring that down to ~0.1 or even less. Most general-purpose bolting falls somewhere in the middle; a widely cited “typical” total coefficient of friction for a clean, production bolt is ~0.12–0.18. Many manufacturers actually strive for around 0.12–0.15 as an optimal value because it’s low enough for efficient tightening but not so low as to raise loosening concerns. Extremely low values (<0.08) are generally avoided unless intentionally needed, and extremely high values (>0.25) are avoided as they indicate problems (unless it’s a deliberate locking element).

How Surface Treatments, Coatings, and Patches Affect CoF

A myriad of surface treatments and thread modifications are used in fasteners, and a key reason is to modify the coefficient of friction (along with corrosion protection and other properties). Let’s look at some common ones and how they influence friction:

• Lubrication (Oils and Greases): The simplest way to alter friction is to apply a lubricant to the fastener. Even a thin oil will fill in surface asperities and ease sliding, reducing μ. Heavy grease or dedicated thread lubricants (like copper-based anti-seize or molybdenum disulfide paste) can reduce friction dramatically. For example, a stainless steel bolt that might gall and seize when dry can be tightened smoothly with an anti-seize compound because the compound provides a low shear strength film between the metals. Lubricants tend to give more consistent results as well, since they are less sensitive to small surface finish differences. The trade-off is that lubrication must be consistent – too much or too little or differences in type can change friction. (This is why specifications often either prescribe a lubrication or insist on assembling dry – to avoid uncertainty.) In critical applications, special extreme-condition lubricants are used: e.g. graphite-based for very high temperatures where oils would burn off, or silicone-based for certain plastics. Overall, added lubricants generally lower CoF significantly, which is beneficial for avoiding galling and achieving higher preload, but requires careful torque adjustments to not overload the fastener.
• Phosphate Coatings with Oil: Phosphate conversion coating (such as zinc phosphate or manganese phosphate) is a common treatment for steel fasteners, usually combined with a light oil. The phosphate by itself is a crystalline, mildly rough coating, but it can absorb and hold onto oil or wax. The combination provides both corrosion resistance and a controlled friction. Phosphate & oiled bolts are known for relatively consistent tightening behavior, often yielding friction around the 0.1–0.15 level. The oil in the pores of the phosphate acts as a built-in lubricant, while the phosphate helps prevent metal-to-metal contact and galling. This treatment is seen in automotive and machinery fasteners as well as high-strength structural bolts that are torque-tightened. One reason it’s popular is that it’s cheaper and safer than plating like cadmium, yet still gives a predictable friction and avoids the stick-slip issues of plain dry steel.
• Electroplated Coatings (Zinc, Cadmium) and Their Topcoats: Traditional electroplating of fasteners (zinc-plating being the most common today, cadmium in older or specialized usage) provides a thin metal layer over the steel. Cadmium plating (with chromate finish) was excellent for providing a low and steady friction – cadmium is a slick metal, and plated surfaces would often have μ around 0.12 or even a bit less, with good consistency (plus cadmium’s lubricity helped prevent galling). Cadmium’s toxicity has led to it being phased out for most uses, except critical aerospace or defense where nothing else quite matches its combo of corrosion resistance and low friction. Zinc plating is more common but pure zinc-on-zinc thread contact can be somewhat higher friction and prone to stick-slip unless treated. Thus, manufacturers frequently apply a sealant or wax over zinc plating (clear or blueish transparent coatings) that act as lubricants. These topcoats can be tuned: a “high lubricity” topcoat may bring zinc’s friction down to ~0.12, whereas an untreated zinc might be ~0.20 and also more erratic. There are also specialty platings like zinc-nickel alloy, which can have different frictional properties (often also used with a topcoat). In summary, electroplating usually reduces friction relative to bare steel but to ensure consistency, a supplementary lubricant is often added. One thing to watch: plating significantly reduces the risk of random very high friction occurrences (like there’s less risk of galling compared to bare steel), but it can introduce the opposite problem of friction being too low if over-lubricated, so standards have evolved to include tests for torque-tension to make sure plated fasteners fall in an acceptable friction range.
• Zinc Flake Coatings (Geomet, Dacromet, etc.): Zinc flake coatings are non-electrolytic (applied by dip-spin or spray and baked), consisting of zinc/aluminum flakes in a binder, usually with an added topcoat. They are widely used in automotive and high-strength bolts because they offer good corrosion protection without the risk of hydrogen embrittlement. One advantage is the ability to tailor the friction by formulating the topcoat. Manufacturers of these coatings often have different topcoat formulas that yield “friction coefficient 0.09–0.14” or maybe “0.12–0.18” depending on requirements. The presence of lubricating substances (like PTFE, moly, or other proprietary lubricants and waxes) in the topcoat gives a controlled, moderate CoF. For example, a zinc flake coated M10 bolt for automotive use might be guaranteed to have a total CoF of 0.10–0.15 against a steel or aluminum part, both to ensure assembly torque accuracy and to avoid stick-slip during automated tightening. Thus, zinc flake coatings essentially allow engineers to dial in the “just right” friction level while also providing corrosion resistance. Without the topcoats, a pure zinc flake basecoat might be somewhat higher friction and subject to variability, so in practice these systems are always paired with friction-modifying top layers.
• PTFE and Other Non-Stick Coatings: Polymer-based coatings such as PTFE (Teflon), Xylan, Fluoropolymer, or similar “non-stick” coatings are used on some fasteners to achieve very low friction and ease of installation. PTFE is one of the slipperiest solids (with static μ as low as ~0.04 against smooth surfaces). A bolt coated in PTFE will feel notably easy to turn – it often has a slick, glossy appearance (sometimes colored, e.g. blue PTFE-coated bolts in certain pipeline applications). These coatings drastically reduce both thread and under-head friction. They are popular in applications where galling must be absolutely avoided (e.g., stainless steel fasteners in seawater service often get PTFE coating) or where you want to achieve a precise preload with minimal torque scatter. Another aspect is that PTFE coatings tend to maintain low friction over time – they don’t rely on an applied oil that might dry out; the polymer itself provides long-lasting lubricity even after multiple tighten-loosen cycles. The obvious caution is, as we’ve discussed, if you substitute a PTFE-coated bolt into an assembly that was designed for a plain bolt, you must lower the tightening torque significantly or you’ll over-tighten. Typically, specifications will give separate torque values for coated vs. uncoated. Aside from PTFE, other proprietary coatings (often fluoropolymer-based or containing lubricating pigments) fall in this category of making a “slippery bolt.” For example, some high-strength structural bolts use a baked-on lubricant coating (not exactly PTFE, but similar idea) to achieve a reliable low K-factor for tensioning with a calibrated wrench.
• Dry Film Lubricants (Molybdenum Disulfide, Graphite, etc.): Dry film lubricants are solid lubricative compounds that can be applied as a thin film on fasteners. Molybdenum disulfide (MoS₂) is common – it’s often included in anti-seize pastes and sometimes in a coating itself (like a moly coating that leaves a greyish-black film). Graphite or boron nitride are other examples. These films adhere to the metal and provide low shear strength layers that facilitate sliding. They are especially useful in extreme conditions: e.g. vacuum or space applications (where oils would outgas), very high temperatures (where greases would burn), or where long-term lubrication is needed without re-application. A MoS₂-coated bolt tightened today will still have that solid lubricant present years later. Coefficient of friction with such coatings can be quite low, in the 0.05–0.1 range, and importantly consistent. The aerospace industry often uses a combination like cadmium plating plus MoS₂ topcoat on critical fasteners – this yields a known low friction and prevents any chance of galling, enabling accurate preload with torque control.
• Threadlocking Adhesives: Although not exactly a “coating” in the same sense, chemical threadlockers (like Loctite) deserve mention. Liquid threadlockers initially act as a lubricant when wet – which can slightly lower friction during tightening (making it easier to achieve higher tension for a given torque). However, once they cure, they bond the threads and effectively prevent rotation (not through friction but through adhesive locking). Engineers sometimes overlook that using a liquid threadlocker can change the tightening friction (it’s usually a small effect, but in very precision assemblies it might matter). It’s a good practice to calibrate or test if a torque spec assumed dry threads but you plan to add threadlocker (in many cases the effect is minor, but it’s there). After curing, the removal torque is higher not due to classical friction but because you’re breaking an adhesive bond. So, threadlockers are more of a chemical locking method than a friction modifier in service, but their initial lubrication effect is worth noting.
• Nylon Patches and Inserts: These are prevailing torque additives – a nylon patch (often a strip or pellet fused onto thread flanks) or a nylon ring insert in a nut. They increase the friction dramatically in the portion of engagement where they are present. When you tighten a screw with a nylon patch, initially the run-down is normal until the threads hit the patch, then you experience a rise in torque as the nylon deforms and presses against the mating threads. This added drag is what keeps the screw from loosening easily. The patch doesn’t usually affect the maximum achievable preload much, because once the joint is clamped the dominant resistance is still from tension and metal-metal friction; the nylon just adds a constant drag. However, it does mean you must expend extra torque to overcome it, and it can cause some heating in that area. Nylon insert locknuts similarly make turning harder. Engineers must account for this by specifying higher installation torque (to both overcome the insert and achieve preload). The coefficient of friction in those zones can be considered very high (equivalent to >0.3), but since it’s localized, we usually just measure its effect as an added prevailing torque requirement (for example, “requires 5 N·m to turn the nut even with no load”). Nylon patches are useful because they don’t gall or damage the threads and are reusable a few times, but they do wear out eventually (losing friction after multiple on-offs).
• Surface Hardening and Roughness: Processes like shot peening, carbonitriding, or Kolsterising (for stainless steel) don’t directly aim to change friction, but by making the surface harder and sometimes smoother, they can indirectly reduce friction and galling. A harder surface is less prone to adhesion (galling) and can have a lower friction coefficient when paired with like materials. Conversely, a very rough surface finish on threads (due to poor machining or damage) will increase friction. So, quality of thread manufacture (smooth rolled threads vs. rough cut threads) affects friction too. This is why high-grade fasteners often have rolled threads and fine finishes – not only for strength but for controlled friction.

In summary, surface treatments and coatings are chosen not just for corrosion protection or appearance, but often specifically to engineer the frictional behavior of fasteners. The goal might be to lower friction (for easier, more consistent tightening and galling avoidance) or to deliberately add friction (for self-locking capability). Modern fastener coatings, like those used in automotive production, are usually optimized to a target friction range that gives reliable clamp without risk of self-loosening or bolt damage. It’s important when designing and servicing joints to know what coating/condition your fasteners have and how that affects friction – and to not arbitrarily swap one for another without accounting for the change in CoF.

Low vs. High CoF: Finding the “Just Right” Balance

As we’ve discussed, very low or very high friction in fasteners each comes with pros and cons. Engineers often talk about a “Goldilocks zone” for fastener friction: not too low, not too high – just right. Let’s compare the extremes and the middle ground:

• Very Low CoF (Pros and Cons): Low friction (say μ in the 0.05–0.1 range) makes tightening a bolt more efficient. Pros: You can achieve the desired preload with less torque, which can be advantageous if you’re limited by the torque capacity of tools or if you want to minimize torsional stress on the bolt. Low friction also usually means less variance in friction (if achieved through good lubrication or coating), improving consistency of preload. Galling is virtually eliminated when friction is very low and surfaces are well-lubricated – fasteners turn smoothly and come apart easily as well, even after time. Cons: The downside is that low friction can make it easier to overshoot into the bolt’s yield regime; the margin for error in torque is smaller. It also raises a theoretical concern for self-loosening: if a shock or vibration does try to turn the fastener, a low-friction interface offers less resistance, so a preloaded bolt may lose more preload per slip event compared to a stickier bolt. In critical applications, if friction is extremely low, designers may incorporate locking mechanisms or specify careful torque-angle tightening to avoid any chance of the fastener relaxing. Additionally, extremely low friction values (like approaching 0.05 or below) are usually achieved with special lubricants that must remain in place – if those conditions change (lube dries up, etc.), the friction could jump unexpectedly. Thus, sustaining a low CoF can require maintaining the lubricant condition throughout the fastener’s life.
• Very High CoF (Pros and Cons): High friction (μ > 0.25) in a fastener provides a lot of resistance to turning. Pros: One immediate effect is that a high-friction fastener is less likely to come loose on its own – the high resistance in the threads and under the head act like a brake. This is why, historically, mechanics sometimes just relied on “friction” by leaving bolts unlubricated, figuring they’d stay put (and many cheap hardware-store nuts are a bit on the rough side, providing more friction so they don’t shake loose easily under light duty). High friction also means that during tightening, the bolt will tend to stop rotating as soon as the parts make contact, which might give a tactile sense of “snug” even if preload is low – though this can be false security. Cons: The negatives generally outweigh the positives for high friction: you need to apply much more torque to reach the same preload, which can strain tools and the fastener. The bolt is under more torsional stress for a given tension, meaning you’re closer to material limits; effectively, you lower the achievable preload before yielding the bolt. High friction interfaces are prone to galling, as mentioned – two rough dry surfaces can seize up mid-tightening, leading to stuck or broken fasteners. And with high friction, the preload attained is sensitive to small differences: one bolt might have slightly less friction and thus end up with a higher tension than its neighbor that had more friction – so preload scatter can be worse, not better, if surfaces aren’t uniformly high-friction. In many cases, what we perceive as “high friction helps prevent loosening” is better achieved by a designed locking feature, rather than just relying on random high friction, because the latter also jeopardizes achieving correct preload.
• Moderate CoF (the Goldilocks Zone): For most engineering purposes, a medium friction level (roughly μ ~0.10–0.18) is preferred. At this level, the fastener is lubricated enough to tighten predictably and reach proper preload without extreme torque, but not so slippery that it’s on the verge of unwinding easily or causing huge jumps in tension with a minor torque change. This balance allows using standard torque tools and getting consistent results. Many standards (like the German VDI 2230 guideline for bolted joints) define classes of friction and recommend staying in a middle band. For instance, a common target is around 0.12–0.14 as a mean friction for critical fasteners, with acceptable range perhaps 0.10 to 0.18. In this zone, if a one-time overload causes a slight slip in the joint, the nut/bolt likely will not lose much preload because the friction is enough to resist freely spinning – it might relax a little but not run loose. Meanwhile, tightening torques are reasonable and within tool capability, and the bolts are not excessively stressed by torsion. This is why most fastener suppliers for automotive or industrial use aim to supply parts with a known friction in this moderate range. They often achieve it by coatings or lubricants that aren’t extreme low friction but ensure it’s not high either. As an example, if testing shows a batch of bolts has an average CoF of 0.08 (too low), the supplier might apply a different finish to raise it, or caution the user to adjust tightening procedures. If a batch had 0.25 (too high), they’d likely reject or reprocess it (perhaps add wax) because that could cause installation problems and inconsistent clamping.

In practical terms, a low-friction fastener might allow 20–30% more preload for the same torque compared to a high-friction fastener. However, the low-friction one might need locking features to secure it, whereas the high-friction one might not loosen but also might not deliver needed preload at all. Thus, the trade-off is clear: low friction gives strength (preload) but needs caution for security; high friction gives security (locking) but needs caution for strength. Engineers often mitigate the downsides: for low friction cases, use locknuts or secondary retention; for high friction cases, use higher torque or better still, reduce the friction by lubrication but then use a lockwasher or locknut to keep it in place – effectively separate the functions of achieving preload and locking, rather than letting random friction handle both.

Design, Specification, and Quality Control Considerations

When dealing with fasteners in engineering design and assembly, controlling and specifying the coefficient of friction is as important as specifying the bolt strength. Here are some practical considerations for engineers and quality control regarding CoF:

• Specifying Surface Condition in Design: A designer should indicate in drawings or specifications the required condition of fasteners – for example, “lubricate threads with oil before assembly” or “use zinc-plated, waxed nuts” or “phosphate-oil coated bolts”. By specifying this, the designer communicates the assumed friction condition under which the torque or preload requirements were determined. If a torque value is given without context, assembly techs might not know that it assumed a light oil on the threads. Clear instructions prevent scenarios like someone assembling a joint dry when the torque spec was meant for lubed (leading to under-tensioning), or vice versa. In critical bolted joints (like engine head bolts, structural connections, pressure vessels), the assembly procedure often explicitly calls out lubrication (e.g., “lightly oil the bolt threads and underhead”) because without it, the friction could be too high and the intended preload won’t be reached before the bolt yields.
• Friction Coefficient Requirements and Standards: Some projects will directly specify an allowable range for the friction coefficient of the fasteners. For instance, the automotive sector might refer to a standard such as ISO 16047 or DIN 946 for determining friction coefficients of bolt/nut/washer assemblies. They may require that the fastener supplier provide parts that, when tested, show a total CoF between say 0.10 and 0.18 under defined conditions. Structural steel bolting standards (like those for high-strength structural bolts in bridges/buildings) implicitly control friction by requiring specific coatings and a pre-installation verification test: you put the bolt, nut, washer through a tightening in a calibrated device (often a Skidmore-Wilhelm tension calibrator) to ensure that at the specified tightening method, the proper tension is achieved. This kind of test effectively checks that the friction of that bolt-nut-washer combination is within the expected range. If it were too high, the required tension wouldn’t be met; if too low, the bolt might elongate too much or break in the calibration test. Thus, quality control tests often involve torque-tension measurements on sample fasteners to validate CoF. Manufacturers will do this to certify that their coating process is consistent – adjusting things like a lubricant additive if results drift out of spec.
• Avoiding Uncontrolled Friction Variations: In production or maintenance, it’s vital to avoid inadvertently changing the friction condition. This means, for example, do not mix fasteners of different finishes in the same joint (one might be waxed, another not, causing uneven preload). It also means training technicians to apply torque to the correct condition – e.g., if a spec is for dry, don’t decide to grease it “to be safe” (you’ll over-tighten), and if a spec is for lubricated, don’t skip the lubricant (or you’ll under-tighten). In situations where fasteners are reused, consider that the act of tightening can change the surface: a previously lubricated bolt may lose some lubricant after one use, or a patch locknut might have less friction on second use. Procedures might call for reapplying lubricant on reused fasteners or replacing locking nuts after a certain number of uses to ensure the friction (and thus performance) stays consistent.
• Torque Calibration and Calculation: Engineers often use a nominal friction (or nut factor K) to calculate required torque: for example, using K ≈ 0.20 for an “unlubricated” bolt vs K ≈ 0.15 for a “lubricated” one. These values embed assumptions of CoF. A smart approach in design is to calculate expected preload for both the high and low ends of the possible friction range to see the extremes. If the scatter is too great (e.g., if friction uncertainty could lead to preload that is either too low to hold, or so high it yields the bolt), then you know you must tighten up the specification – perhaps by requiring a specific coating or by using a more reliable tightening method than simple torque. In assembly, if extremely accurate preload is needed, methods like torque + angle tightening or direct tension indicating devices are used to mitigate friction variability. Torque-angle tightening works by using torque just to snug and then turning a controlled angle into yield; this way friction affects the snug point but the final stretch is controlled by angle, which is more consistent even if friction varies. Direct tension methods (hydraulic tensioners, twist-off bolt sets, or load-indicating washers) bypass friction issues by measuring or generating tension directly.
• Documentation and Testing: It’s good practice to document the frictional condition of critical joints. For instance, in a maintenance manual: “All bolts to be installed with antiseize compound XYZ; torque values are given for that condition.” In testing, performing a few torque-preload measurements during development can validate that the assumed CoF is correct. If you find that in reality a “dry” bolt was giving much less preload, that indicates perhaps the bolts were rougher than expected or there’s a need to lubricate. Conversely, if during a trial assembly the torque seems too low or the bolt starts to yield early, maybe friction was lower than expected and the torque needs to be reduced or spec changed.
• Environmental and Service Conditions: Consider how the service environment will affect friction over time. If a bolt is expected to be adjusted or retightened in the field after years, think about whether corrosion might have increased friction – maybe stainless or a coated fastener is preferable to keep things moving freely. In high-temperature applications, a regular grease will burn off and leave you effectively with a dry, high-friction situation; thus one would specify a high-temp lubricant or a coating like moly that can withstand the heat. In vacuum (spacecraft), you can’t use oil (it will vaporize), so you must use dry lubricants to keep CoF low. The bottom line is that the specified CoF should be valid not just at initial installation but through the life cycle as needed.
• Non-Threaded Fasteners: Although our main focus is on threaded fasteners, design considerations extend to them as well. For example, a dowel pin pressed into a hole relies on interference and friction to stay in place – if the surfaces are too smooth and lubricated, the pin might slide out, whereas if they are lightly rough or coated, the friction (and maybe slight micro-welding) keeps it fixed. In riveted joints, friction doesn’t come into play in the same calculable way as with a torqued bolt (since rivets are installed by deforming them, not by turning against friction), but friction is still relevant in the sense that a rivet holds plates together and the friction between those clamped plates allows the joint to resist shear loads (this is sometimes called a “friction grip” joint in structural terms). So even though we don’t specify a CoF for a rivet in installation, we do rely on the fact that the rivet’s clamping creates interfacial friction that prevents movement. If those surfaces get oily or painted with a low-friction coating, the joint might slip more easily until the rivet bears in shear. Therefore, sometimes surface preparation (like clean, maybe slightly rough faying surfaces) is specified for riveted or tensioned bolted joints to maximize useful friction between parts. Another example: clevis pins or hinge pins often have a coating or lubrication to ensure they can rotate freely if needed (low friction for motion) or sometimes to ensure they don’t gall if they are a tight fit. While we don’t usually quantify friction for a pin the way we do for a bolt, the engineering intuition about friction still matters (e.g., if a stainless steel clevis pin is galling in a stainless clevis, adding a PTFE washer or grease can solve it by reducing friction).

In conclusion, treating friction as a controlled parameter in design and assembly leads to more reliable fastener performance. It’s not enough to specify a bolt’s size and grade; specifying (or at least understanding) its frictional condition is equally important for critical applications. Quality control measures, such as torque-tension testing and verifying coatings/lubricants, ensure that the bolts delivered and used in the field behave as expected. By actively managing CoF – through design choices, assembly instructions, and testing – engineers can greatly improve the consistency and safety of bolted joints.

Friction in Non-Threaded Fasteners (Pins, Rivets)

Non-threaded fasteners don’t involve torque-driven tightening, so the role of friction is different, yet still worth a brief mention. For pins, dowels, and interference-fit fasteners, friction is the main mechanism holding them in place. When a straight pin is pressed into a slightly undersized hole, the pin elastically deforms the surrounding material and is held by frictional force. The CoF between the pin and hole surfaces will determine how easily the pin can slip or be removed. If that CoF is reduced (say the pin and hole are greased), the required press-in force drops but so does the resistance to working its way out. If the CoF is increased (dry, or surface treated for roughness), the pin will have more resistance to slipping out but may be harder to install and could gall the hole. Thus designers balance tolerances and surface finish for an appropriate friction – for example, using a slightly rough phosphate-coated pin which both prevents corrosion and provides a grabby surface to hold in a smooth reamed hole.

In the case of rivets and lockbolts, these are installed by plastic deformation (bucking or pulling) rather than torquing, so we don’t discuss a “friction coefficient” in the tightening process. However, friction still appears in subtle ways. When a rivet is set, the friction between the rivet material and the hole can affect whether the rivet hole experiences any rotation or whether the rivet shank seizes as it expands. Certain rivets have serrations or grooves that bite in and create high friction to prevent the rivet from spinning while being peened over. For high-strength structural rivets or tensioned shear connectors, the clamping force generated by the rivet results in friction between the joined plates, which is what allows the joint to resist shear without slip. That’s analogous to a bolted joint: the friction between plates (due to clamp) carries the load until a slip threshold. While this isn’t “friction of the fastener’s surface” per se, it’s related to how a fastener’s tension results in frictional holding in the joint.

Additionally, consider retaining rings or clips (also non-threaded fasteners): these often rely on friction against a shaft or groove to stay put. For example, an O-ring style clip might use an interference fit with a groove – surface finish and friction matter there too (a polished shaft with oil on it might let a retaining ring slide off more easily than a dry, slightly rough one).

In summary, for non-threaded fasteners, we typically don’t calculate a coefficient of friction in use, but we still manage friction qualitatively: ensuring pins have appropriate surface condition for the hold we need, avoiding lubricants where we want a pin to stay, or adding lubricants where we want movement (like a hinge pin). Galling can also occur with press-fits (e.g., a soft aluminum pin might gall in a steel hole if too much friction), so similar logic applies: lubrication or hard coatings might be used to mitigate that. Thus, even outside of threaded connections, understanding and controlling friction is part of good fastening practice.

Conclusion

The coefficient of friction in fasteners is a fundamental factor that underpins how bolts and other fasteners behave, from the moment you tighten them to the long years of service in the field. In threaded fasteners, CoF governs the all-important conversion of tightening torque into clamping force – a process that is surprisingly inefficient (with most of the energy lost to friction) and yet highly sensitive to small changes in that friction. We’ve seen that too much friction can hinder achieving proper preload and risk damage (galling, twisting off bolts), while too little friction can lead to over-stressing and potential loosening. Thus, achieving a “just right” friction level is critical. Engineers accomplish this through material choices, coatings, and lubricants, as well as through specifying how fasteners are to be installed.

Friction affects not only installation torque but also the performance and reliability of the joint: it influences whether threads will seize up or turn smoothly, whether a nut might shake loose or stay put, and whether the preload in a bolt is consistent and maintained over time. By understanding factors like thread vs under-head friction, typical friction values for different finishes, and how treatments like PTFE, zinc flake, or nylon patches alter the friction, one can make informed decisions in design and maintenance – selecting the right fastener finish for the job and applying the correct assembly methods.

In practical terms, always remember to account for friction in your bolted joint calculations and procedures. If you change a fastener’s coating or apply lubrication, revisit the torque specs. If you encounter joints that are failing or coming loose, consider whether an incorrect or inconsistent CoF might be to blame. And utilize the wide array of technologies available (from anti-seize compounds to locking patches) to tailor the frictional properties to your needs – minimizing it when you want easy, accurate tightening and raising it when you need vibrational locking, all while keeping within safe limits. In summary, the coefficient of friction is a key “hidden” design parameter in fastener applications, one that plays a huge role in installation behavior and joint performance. Mastering its implications leads to safer designs, more efficient maintenance, and fewer surprises in the life of bolted assemblies.