Despite seeming trivial, loose fasteners are a common issue in marine craft of all types.
Typical root causes of threaded fasteners loosening include:
- Vibration
- Mechanical load cycling
- Thermal load cycling
- Incorrect, damaged, or missing thread-locking
- Incorrect preload
OEMs should remember that while frequent maintenance is beneficial, every maintenance activity carries the risk of error — including improper fastener installation. Loose fasteners can result in serious incidents and warranty claims, which are often difficult to investigate. After all, nobody wants to admit to incorrectly installing a fastener.
Common thread-locking methods include:
- Plastic-insert lock nuts (commonly called Nyloc nuts)
- Lock washers (including Nord-Lock washers)
- Thread-locking compounds (such as Loctite)
- Mechanical methods (including wires, pins, and tabs)
Marine maintenance technicians know that onsite thread-locking can involve uncertainties. For example:
- Specified thread-locking elements or compounds unavailable.
- Unknown calibration date of a torque wrench.
- Unspecified thread-locking methods or torque values in the equipment manual.
- Poor working conditions and tight schedules increase the risk of improper installation.
The primary objective of locking a fastener is to preserve its preload.
Preload
In most situations, a fastener will not fail in service if it is tightened so that its preload (clamping force) is greater than any applied load during operation.
While this may sound simple, preload is often misunderstood or overlooked. According to the Naval Ships’ Technical Manual, Chapter 075 – Fasteners, extensive studies have shown that the only sizes the average mechanic can reliably tighten “by feel” are 1/2-inch and 5/8-inch nuts. Smaller nuts are typically overtightened (sometimes yielding), while larger nuts are usually undertightened.
Threaded fasteners are tightened until the required clamping force is achieved. Although torque values are specified in manuals, it is not the torque itself that secures the connection but the clamping force it generates.
To achieve correct torque values:
- Fasteners must be installed using a calibrated torque wrench with a range suited to the specified torque. According to MIL-HDBK-1222F, torque wrenches are most accurate between 25% and 75% of their rated range.
- When multiple fasteners are used, the tightening sequence of the group is usually specified in the equipment manual.
Plastic-Insert Lock Nuts
Plastic-insert lock nuts are the most common type of self-locking nut, identified by a plastic collar at the end. The collar elastically deforms over the thread, increasing friction in the connection.
Two plastics are commonly used: polyamide (nylon) and polyimide (vespel), each with different temperature limits.
Pros
- The locking element is built into the nut, so no extra components are required.
- Locking can be achieved without high preload.
Cons
- Reusability is debated. Some OEMs require replacement after each use, others do not specify. As a rule of thumb, they are usually good for ~10–15 reinstallations. For critical applications, compare the breakaway torque against manufacturer specifications.
- May require a longer bolt due to increased nut height.
Lock Washers
Lock washer types include curved or conical spring, helical spring, toothed, and wedged.
- Helical, curved, or conical spring washers flatten when tightened, adding friction through spring force.
- Toothed lock washers flatten during tightening, with teeth biting into mating surfaces to prevent rotation.
- Wedged lock washers (e.g., Nord-Lock) consist of two interlocking washers with wedges and teeth, preventing loosening under vibration.
Pros
- Spring types can accommodate some relaxation of preload while maintaining clamping force, reducing risk of rotation.
Cons
- Increased risk of crevice corrosion in seawater.
- Correct orientation is critical for conical spring and wedged types.
- Reusability is difficult to assess.
- Adds extra components to the bill of materials.
Mechanical Methods
Mechanical locking methods include wires, pins, and tabs inserted through holes or grooves in fasteners.
Pros
- Excellent at preventing total disassembly.
Cons
- Poor at preserving preload.
- Proper torquing while aligning holes is difficult or impossible.
- Adds components to the bill of materials.
- Corrosion-resistant standard tabs, pins and wires can be difficult to find off-the-shelf.
Thread-Locking Compounds
Thread-locking compounds are anaerobic, one-component adhesives. They cure in the absence of air, bonding fasteners and preventing rotation.
Pros
- Compatible with many fastener materials.
- Available in a wide range for specific applications.
- Can reduce corrosion between threads.
Cons
- Surfaces must be free of oil and contaminants.
- Easy to overlook in BOMs or parts catalogues.
- May reduce sacrificial anode effectiveness by increasing resistance.
- Require curing time: fixture strength in minutes, full cure in up to 24 hours depending on temperature.
- May require a primer or activator when applied to passive materials such as certain stainless steels.