A bolt that stays tight on a vibrating pallet truck, machine guard or conveyor is rarely just a “better bolt”. The dependable answer is a correctly designed fastener assembly with the right bolt, adequate preload, a suitable locking method and joint faces that do not settle or move in service. For bolt head and access considerations, see our socket head versus hex bolt guide.
Vibration loosening can begin when clamp load falls through embedment, gasket compression, thermal cycling or movement between the clamped parts. Once the joint slips sideways, repeated transverse vibration can rotate conventional threaded fasteners loose.
Why bolts loosen under vibration
A tightened bolt stretches slightly and clamps the joint together. That clamp force creates friction between the joint faces. If preload is too low, the parts can move. Excessive tightening is not the answer because it can damage threads, crush softer materials or overload the bolt.
Use a tightening method based on bolt size, property class, material, finish, lubrication and joint design. For critical applications, follow the equipment manufacturer’s specification or obtain competent engineering advice rather than borrowing a torque figure from another assembly.
Which fasteners resist vibration loosening?
Flange bolts and flange nuts
Flange fasteners spread bearing load over a wider area and can reduce local settlement. They are useful on rigid steel brackets, frames and machinery, but a plain flange is not a positive locking device. Correct preload still matters.
Prevailing-torque locknuts
Nyloc nuts use a polymer insert to resist rotation and suit many general low-to-moderate temperature applications. Check chemical and temperature suitability, and replace the nut if its locking action has weakened.
All-metal prevailing-torque nuts suit higher temperatures and demanding service, but their prevailing torque must be allowed for when establishing the tightening procedure. Otherwise, less useful clamp load may be produced than expected.
Serrated flange fasteners
Serrations bite into the mating surface and resist rotation. They can work well on suitable unpainted steel, but may damage coatings and softer materials such as aluminium, plastics or thin sheet. They are not ideal where regular removal or an unmarked finish is required.
Wedge-lock washers
Correctly installed wedge-lock washer pairs provide mechanical resistance to loosening and are widely used on vibrating machinery and fabricated assemblies. Fit them as a matched pair with the cam faces together, following the manufacturer’s instructions.
They need sound bearing surfaces and enough bolt length for proper thread engagement after the washers are added. Confirm suitability for slotted holes, soft surfaces, coatings, temperature and repeated reuse with the washer manufacturer.
Positive locking methods
Safety wire, tab washers and castellated nuts with split pins physically restrict rotation. These methods can suit defined safety-critical or high-shock applications, but must follow the equipment specification and an appropriate inspection regime.
Bolt grade and material
Choose property class for the load and joint before selecting a locking method. Metric carbon-steel classes such as 8.8, 10.9 and 12.9 offer different strength levels, but higher strength is not automatically better. Thin or soft clamped parts may deform before a high-strength bolt reaches the intended preload.
Stainless steel offers corrosion resistance but has different strength, friction and galling behaviour. Use suitable lubrication or anti-seize only where the application permits it, then account for its effect on preload. Lubrication can significantly increase bolt tension at the same applied torque.
Zinc-plated, galvanised and other coated fasteners also have different friction characteristics. Our fastener coatings guide explains the main finish considerations. Do not transfer tightening values between finishes without checking them.
Fix the joint before adding a locking device
A locknut or washer cannot compensate for a flexible bracket, damaged threads, poor bearing surfaces or an unsuitable joint design. Check bolt length and thread engagement, use appropriate washers where required, and avoid clamping across damaged threads.
If the joint carries repeated sideways load, use suitable locating features such as dowels, shoulders or fitted fasteners where the design requires them. A standard bolt is primarily intended to clamp the joint, not repeatedly absorb movement through its threads.
Practical selection guide
- For rigid joints with moderate vibration, correct preload with suitable flange fasteners may be sufficient.
- For regular machinery vibration, consider an appropriate prevailing-torque locknut or wedge-lock washer system.
- For high-consequence applications, follow the original equipment manufacturer’s specified locking method.
- For heat, chemicals or corrosion, verify the temperature and material limits of every component.
- After initial running, inspect joints where coatings, paint, gaskets or soft materials may settle.
The best solution is not simply the strongest bolt available. It is a complete fastener system matched to the load, materials, vibration, environment and inspection needs, then installed using a controlled tightening method.