In the UK, where they are distributed exclusively by Staytite since 1980, the vibration-resistant nuts have found a purpose in safety-critical applications in rail track infrastructure, among others. The two-piece nuts, said by the manufacturer to reduce loosening from vibration “longer than any other fastener or locking nut on the market” do so thanks to a unique fastening method that was inspired by the Japanese wood building technique, which makes use of wedges to create a rigid joint.
Inventor Katsuhiko Wakabayashi – who incidentally also designed the Staytite locking nut – incorporated a wedge concept in the design of the two-piece, all-metal nut. The top of the lower nut has a cone that is eccentric in shape; on one side it is thin, on the other thick. When the concentric upper nut runs down on the thread on top of the lower nut, it is pushed against the wedge of the lower piece, and into the thread, which locks the fastener.
This transverse gripping force is very different to the ‘prevailing torque’ type of locking nuts, such as the Staytite nut, or those fitted with a nylon insert. Both of those have a special insert that deforms in contact with the threads, and so requires additional torque to overcome, but do not jam all of the threads, so remain vulnerable to coming undone.
By contrast, the Hardlock nut offers higher performance. Vibration testing results are available on Staytite’s dedicated Hardlock website,
www.hard-lock.co.uk.
In fact, Hardlock fasteners are so reliable they have been approved by Network Rail for use on rail points, crossings and fishplates (rail expansion connections), replacing fasteners with a nylon insert and requiring fewer inspections.
Because much of the force applied in a Hardlock nut is in compressing the bolt stud, Hardlock nuts do not require extra torque to install or remove (and they are reusable, too). To install a nut, users first run down the lower nut by hand until it seats, then use a torque wrench to tighten it to the required preload. Next, users run down the upper nut, also by hand, until it stops, and then use a torque wrench to achieve the correct tension. (There is no need to keep turning to eliminate the gap between the two parts). A flange on the base of the upper nut helps prevent the wrench from slipping down to the lower nut.
While installing two conventional nuts can also offer good locking performance, the Hardlock nut is easier to install correctly, as it does not require any special competence or double-wrenching. And, compared to threadlocking adhesives, Hardlock nuts do not require any wait for an adhesive to cure, the makers point out.
Staytite offers Hardlock nuts in grade 8 low-carbon mild steel, stainless steel (A2/304) and titanium, although the functional principle does not depend on the material. Staytite maintains stocks of Hardlock nuts, offering UK customers overnight delivery for popular sizes. It also provides Hardlock-equipped grubscrews for plastic moulding, diecasting and robotics applications, as well as bearing nuts for rotating equipment.
BOX: Get the best out of your Hardlock nuts
Hardlock offers a few tips of how best to install Hardlock nuts to optimise fastening:
- Make sure parts and threads are clean and undamaged
- Apply anti-seize lubricant on bolts, shafts and nut tapers
- Ensure at least two thread turns extend above the top of the assembly
- Never install or remove both upper and lower nuts at the same time
- In cases where relaxation and settlement cause loss of initial preload, install disc spring washers with Hardlock nuts
