HLN FAQ - Common Questions & Answers

Find answers to commonly asked questions about HARDLOCK Nut.

Self-locking Performance
Product Selection
Attachment procedure
Maintenance after installation

Self-locking Performance

What kinds of tests have been performed to validate the self-locking effect of nuts?

Hardlock uses an in-house impact vibration test based on NAS (National Aerospace Standard) 3350/3354 and Junker vibration tests to evaluate the loosening of screw-threaded fasteners.
How strong is the Hardlock Nut self-locking effect compared to general hex nuts?

Based on finite element analysis (FEA), the Hardlock Nut has a self-locking effect 1000 times more powerful than a general hex nut. This level of self-locking is equivalent to the connection strength of welding, adhesives and rivets, far beyond a comparison to general hex nuts.

The Hardlock Nut has also been compared to other self-locking components in scientific research. Two such presentations were made at the Pressure Vessels & Piping Conferences held in 2005 and 2006 by the American Society of Mechanical Engineers. This research shows through performance tests and FEA that the Hardlock Nut has a superior self-locking effect against transverse impact vibrations. Click here for a list of research papers.

Can the Hardlock Nut be reused?

Yes, the Hardlock Nut can be reused. In-house Junker tests of up to 50 repetitions demonstrate no loss in the self-locking function.

Even if the self-locking function is operating normally, it should be noted that environmental conditions can cause material fatigue and deterioration.

Before reuse, check for material integrity. If the are no problems, tighten the concave nut onto the convex nut manually until it no longer turns and there is a gap of about one thread between the two nuts. If there is no gap, reuse should be avoided as there is the possibility of concave nut plastic deformation.

Check out the Re-useable testing results here.

Does the Hardlock Nut have a self-locking effect even with an extremely low tightening torque (a low bolt axial force)?

Yes, thanks to its unique wedge force, the Hardlock Nut maintains its self-locking effect regardless of whether the bolt axial force is high lower. This is the Hardlock Nut’s most outstanding feature.
Is Hardlock Nut only produced with metric threads?

In addition to metric, the Hardlock Nut can be produced in unified thread standard (UTS) and British Standard Whitworth (BSW) sizes.
Where is the Hardlock Nut often used?

• In places subject to severe impact and vibration
• In replacement of welds and adhesives when removal is needed for maintenance
• In places where maintenance is difficult after fastening
• In locations with life-threatening conditions
• When a high torque cannot be used for tightening
• In extreme environments


Product Selection FAQ

Is a special bolt needed for the Hardlock Nut?

No, the Hardlock Nut is produced with screw threads of tolerance class 6H, so any bolt or shaft with a screw thread tolerance class of 6g should be compatible.
Does the Hardlock Nut come in different shapes?

In addition to the rim and basic types, we stock the Hardlock Nut in shapes tailored to customer demand, including flat and heavy types. Contact us for more information.
Is the Hardlock Nut Rim type available for inch threads?

No, the HARDLOCK Nut Rim type is not available for inch threads (unified thread standards). Rim type is only available for Metric threads from sizes M5 ~ M30 course pitch with the exception of sizes M14 & M18. Details can be found here.

Are the Inch thread sizes available in the HLN-R Rim Type or just the Basic?

Can the Hardlock Nut be used in any environment?

The Hardlock Nut can be made from various materials, including iron, heat-treated steel, stainless steel, alloy steel, titanium, brass and even polypropylene, to name just a few. And since the Hardlock Nut comes in a wide variety of surface treatments, it can accommodate almost any environment.
Can the Hardlock Nut sustain its self-locking effect even in environments with severe temperature variations?

Yes, the Hardlock Nut sustains its powerful self-locking effect even in severe temperature changes.

When the temperature rises, the material expands, reducing the tightening force, and conversely, when the temperature drops, the material contracts, increasing the tightening force.

Ordinary nuts become loose through such repeated expansion and contraction, but due to its unique wedge design, the Hardlock Nut will never come loose even with no tightening force.

Is seizure or galling a problem?

A special lubricant applied to Hardlock Nuts eliminates seizing and galling.
What is an eccentric double nut?

An eccentric double locknut another name for HARDLOCK Nut is terminology often used in technical reports, research where the product name can not be used.

Attachment Procedure

Are any special tools required?

No, there is no need for special tools to attach a Hardlock Nut. A general wrench, torque wrench or impact wrench is sufficient.
Is it safe even if there is a gap between the convex and concave nuts after tightening?

Yes, the Hardlock Nut self-locking effect works with or without a gap as long as the concave nut is tightened with the specified torque.
What are the specified torques for the convex and concave nuts?

The convex nut is the tightening nut and should be tightened as appropriate for the target application. As the lock nut, the concave nut should be tightened with the specified torque set by Hardlock. Please consult with us if you desire to tighten both nuts with the same torque.

Maintenance after Installation

Does the Hardlock Nut need to be retorqued after attachment?

Generally, retorquing after attachment to wear down the initial asperities is an effective way to increase the performance of a threaded fastener. However, if retorquing is not possible, the tightening torque value can be increased by 10–30% of the initial bolt clamp load when installing.
What should you do if the Hardlock Nut cannot be removed (especially if stainless steel), due to corrosion or foreign matter in the screw threads?

First of all, do not attempt to forcibly remove the nuts. Apply or spray a penetrating lubricant into the gap between the screw threads of the bolt and nut for smooth removal.
To what extent can the Hardlock Nut be reused?

In-house tests showed no reduction in performance after detaching and reattaching 50 times. Whether the Hardlock Nut can be reused depends on environmental conditions, materials and the target application. Before reusing the Hardlock Nut, make sure that there is a gap of one thread pitch after manually installing the concave nut onto the convex nut. If there is not, refrain from reusing. Contact us if you have any questions regarding reuse.
Are self locking nuts one time use?

Yes, most self-locking nuts are one time use. Friction locking nuts such as Nyloc nut and Stover lock nut have distorted threads which loose there distortion after installation. Also Serrated flange type locknuts rely on a clean bearing surface to lock in but the surface is damaged after one use. Learn about the available bolt locking devices and methods here.
Are there any special precautions for removing a Hardlock Nut?

First, apply lubricant to the bolt and nut. Then remove the concave nut while holding the convex nut in place, and then remove the convex nut. Be aware that there is a risk of seizing if you attempt to remove the concave and convex nuts at the same time.
Is there anything that would cause the Hardlock Nut to loosen?

Loosening can be classified into two categories, due to the nut rotation (rotational loosening) and due to a reduction in the tightening (bolt axial) force.

Rotational loosening can be caused by the following:

  1. The concave nut is not tightened to our specified torque
  2. Excessive over-torqueing the concave nut during reuse can cause plastic deformation, reducing the self-locking effect
  3. Improper installation of the convex and concave nuts

Loss of preload can be due to the following factors:

  1. Initial asperities
  2. A depression in the bolt/shaft
  3. Micro-vibration friction
  4. Excessive load or stress
  5. Thermal causes

Loosening does not occur in the Hardlock Nut due to such a loss in the tightening force.