The most reliable way to prevent a screw from loosening is to ensure it has enough clamping force (called preload) and then add a secondary locking method, whether that’s a threadlocker, a lock nut, or a wedge-locking washer. Most screws come loose for one of two reasons: vibration gradually rotates them out, or temperature changes cause the joint materials to expand and contract at different rates, relaxing the grip over time. The fix depends on your situation, but you have several proven options.
Why Screws Come Loose
Every threaded fastener works by stretching slightly when tightened. That stretch creates a clamping force between the parts being joined. Anything that reduces that clamping force lets the screw back out. Vibration is the most common culprit: repeated sideways movement can overcome the friction between the threads, allowing the screw to rotate a tiny amount with each cycle until it eventually falls out.
Temperature swings cause a subtler problem. If the screw and the material it’s threaded into expand at different rates when heated, the joint can loosen or even damage the surrounding material. NASA research on fasteners joining metal bolts to carbon composites found that a standard stainless-steel fastener installed snugly in graphite cracked the graphite during a single heating cycle because the metal expanded so much more than the surrounding material. You won’t face conditions that extreme at home, but the principle applies to any outdoor or engine-adjacent fastener that sees wide temperature changes.
Tighten to the Right Torque First
No locking device works well if the screw isn’t properly tightened to begin with. Under-tightening leaves too little clamping force, and over-tightening can strip threads or stretch the fastener past its limit, both of which lead to loosening. If your fastener has a specified torque value, use a torque wrench.
One detail people miss: lubrication changes everything. A lubricated bolt converts more of your turning effort into actual clamping force because less energy is lost to friction. That sounds like a good thing, but it means a lubricated bolt tightened to a “dry” torque spec can end up with far more clamping force than intended, sometimes enough to strip the threads or break the fastener. Lubrication can reduce friction by up to 50 percent, so if you’re using any kind of anti-seize or oil on the threads, you need to reduce your torque target accordingly. Manufacturers often specify exactly which lubricant to use for this reason.
Threadlocker (Chemical Locking)
Threadlocker is a liquid adhesive you apply to the threads before assembly. It fills the microscopic gaps between the male and female threads and cures into a solid that resists rotation. It’s one of the most effective anti-vibration methods available and works on screws of nearly any size.
Threadlockers are color-coded by strength:
- Purple: Low strength, designed for small screws under 1/4 inch. Easy to remove with hand tools. Good for electronics, eyeglasses, and instrument screws. Rated from -65°F to 300°F.
- Blue: Medium strength, intended for 1/4- to 3/4-inch bolts. This is the most common choice for general-purpose work like automotive and household fasteners. It can be removed with standard hand tools. Rated up to 360°F.
- Red: High strength to permanent. Used on larger bolts (3/4 inch and up) or anywhere disassembly is rare. Removing a red threadlocker typically requires heating the joint with a torch first. Rated from -65°F to 300°F or 360°F depending on the product.
- Green: A thin, wicking formula designed for screws that are already assembled. You apply it around the exposed threads and it wicks into the joint by capillary action. Low strength, best for small fasteners under 1/4 inch.
If you’re working on something you’ll need to take apart later, like a motorcycle handlebar clamp or a furniture bolt, blue is almost always the right pick. Reserve red for permanent or semi-permanent assemblies.
Lock Nuts
A lock nut adds resistance to rotation on the nut side of a bolted joint. The two main types work differently and suit different environments.
Nylon insert lock nuts (often called Nyloc nuts) have a ring of nylon embedded in the top of the nut. When the bolt threads through the nylon, it deforms the insert, creating friction that resists back-rotation. They’re inexpensive, widely available, and work well for most home and shop applications. The limitation is heat: nylon degrades at elevated temperatures, so these aren’t suitable near engines, exhaust systems, or industrial equipment that runs hot.
All-metal lock nuts use distorted or deformed threads instead of nylon. The threads are slightly out of round, creating metal-to-metal interference that produces a prevailing torque, meaning the nut resists turning even when no clamping load is applied. These handle high-temperature environments where nylon would fail and are standard in automotive and aerospace applications.
Washers: Which Ones Actually Work
Not all locking washers perform equally. The classic split lock washer (the one with a single cut and a slight twist) is the most common, but its reputation exceeds its performance in high-vibration settings. In controlled vibration testing of 66 fasteners with split washers, 4 came completely loose and another 22 dropped below the target tension, though they hadn’t yet started spinning freely. They work by digging into the mating surface and building up a small burr that resists further rotation, which helps in moderate vibration but isn’t foolproof.
Wedge-locking washers (the most well-known brand is Nord-Lock) are a significant step up. These come as a pair of washers with angled cams on one side and radial teeth on the other. The cams lock together so that any back-rotation forces the joint apart, actually increasing tension rather than allowing loosening. In the same vibration testing, none of the 20 fasteners with wedge-locking washers came completely loose. The only practical downside is that the washer pair adds height under the screw head, which can be a problem in counterbored holes where the head needs to sit flush.
If you’re choosing between the two, wedge-locking washers are worth the extra cost for anything exposed to significant vibration, like machinery mounts, trailer hitches, or outdoor structures.
Serrated Flange Fasteners
Some bolts and nuts come with a built-in flange (a wide, flat rim) that has small serrations on the bearing surface. When tightened, these teeth bite into the mating material, increasing friction and resisting rotation. They eliminate the need for a separate washer entirely.
There’s one important caveat: the surface the flange bears against must be soft enough for the serrations to dig in. On hardened steel or very hard materials, the teeth can’t create indentations and the locking effect is minimal. They work best against mild steel, aluminum, or painted surfaces.
Safety Wire
Safety wire is a manual method used primarily in aerospace, motorsport, and industrial settings where failure is not an option. A length of stainless steel wire is threaded through holes drilled in bolt heads and twisted so that any loosening motion pulls the wire taut. NASA’s process specification for flight hardware requires that the wire be routed so it’s under tension whenever the fastener tends to loosen, ensuring zero slack in the system.
For most home and shop work, safety wire is overkill. But if you’re building a race car, maintaining an aircraft, or working on equipment that vibrates severely and can’t be easily inspected, it’s the gold standard for positive retention.
Matching the Method to the Job
For a quick summary of which approach fits common situations:
- Small electronics or eyeglass screws: Purple threadlocker or a tiny drop of blue.
- Furniture, fixtures, general household: Blue threadlocker or a nylon lock nut.
- Automotive (engine, suspension, exhaust): Blue or red threadlocker, all-metal lock nuts, or wedge-locking washers. Avoid nylon lock nuts near heat sources.
- Outdoor structures exposed to weather and temperature swings: Wedge-locking washers or blue threadlocker. Consider matching fastener material to the base material to reduce thermal expansion mismatch.
- High-vibration machinery: Wedge-locking washers, threadlocker, or both together.
- Already-assembled screws you can’t remove: Green (wicking) threadlocker applied to the exposed threads.
In many cases, combining two methods provides the most security. A bolt with blue threadlocker and a lock nut, for example, has both chemical and mechanical resistance to loosening. For critical applications, that redundancy is worth the small added cost and effort.