Lock Nuts

Many different types of lock nuts are available for use in a wide range of applications.  While their underlying mechanisms might vary, they all have one thing in common: they produce additional resistance to vibrational loosening forces without using lock washers which improves efficiencies in many manufacturing processes.


Lock nuts typically fall into one of two groups:

  1. PREVAILING TORQUE – an intrinsic feature of the lock nut produces friction between threads of mated components thereby increasing the resistance to loosening forces. The two main categories of prevailing torque type nuts include: all metal and nylon insert lock nuts.
  2. SURFACE-BEARING – a free spinning nut that requires tightening against a bearing surface in order for the locking mechanism to function. Two common surface bearing type lock nuts include: serrated flange and KEPS (K-Lock)

1-Prevailing Torque Type Lock Nuts:

All metal prevailing torque type lock nuts achieve their prevailing torque by altering the shape of the nut in some way – most commonly by distorting the threads of the nut which then bites into the mating part when tightened. These include: 2-way reversible lock nuts, Flex-Type Lock Nuts, Stover Lock Nuts.

A one piece all metal prevailing torque type hex nut with two or three equally spaced round or rectangular impressions compressed into the flat sides of the nut. These compressions create slight distortion in the internal threading that results in an increased resistance when tightened onto mating threads.  Two way reversible lock nuts are considered “two way” because they can be installed in either orientation (top up, top down) These nuts are also referred to as  center lock nuts, and are ideal for production and automated assembly. As a result the locking of the mating part occurs in the center of the nut. This saves on screw length, because threads need not protrude from the nut to lock securely. Since they are all-metal, they are more resistant to higher temperatures and chemical exposure than non-metallic lock nuts (such as nylon insert lock nuts).

Example of a flex-type lock nut

 A one piece all metal prevailing torque type hex lock nut with six equally spaced opposing slots aligned with the corners of the nut that are cut into a cylindrical section of slightly smaller diameter at the top of the nut. It is this slotted section that locks the nut in place by increasing resistance to loosening forces when tightened onto a mated thread.  These are considered as one-way because they can only be installed one way – top up.Since they are all-metal, they are more resistant to higher temperatures and chemical exposure than non-metallic lock nuts (such as nylon insert lock nuts).

Example of a grade C stover equivalent lock nut

A one piece all metal prevailing torque type hex lock nut with a conical top with chamfered corners. The resistance to loosening forces is created by distortion in the top threads.  These are considered as one-way because they can only be installed one way – top up. Since they are all-metal, they are more resistant to higher temperatures and chemical exposure than non-metallic lock nuts (such as nylon insert lock nuts).

Example of a nylon insert lock nut

Nylon insert torque prevailing type lock nuts contain a captive (permanent) undersized nylon (or other polymer) insert that grips the mating threads when tightened and generates the prevailing torque. The nylon insert also dampens vibration between the bolt and nut further securing the mated components and provides a gas and moisture seal. Because the threads of the mating bolt deform but do not cut into the nylon so nylon insert lock nuts may be re-used a limited number of times. These nuts are considered one-way lock nuts because they can only be installed one-way—top up.  Unlike all-metal nuts, the nylon insert limits the use at elevated temperatures or when exposed to certain chemicals.

2-Surface-Bearing Lock Nuts:

Example of a serrated flange lock nut

A one piece all-metal lock nut with a flanged bearing surface that increases the surface area substantially and has circular serrations radiating outwards.  Unlike prevailing torque tyes, this bearing lock nut does not lock onto mating threads. It is a free spinning nut until the serrations contact and displace material on the mating surface upon tightening. Depending on surface finish, localized surface corrosion may occur.  The serrations are designed not to interfere with the tightening of the nut, but once tightened against a bearing surface, the teeth produce a ratchet-type locking mechanism resistant to vibrational and other loosening forces.  The serrated flange eliminates any need for a washer or lock washer. The increased flange size relative to the rest of the nut distributes the load over a greater area. Furthermore it is ideal for irregular or over-sized holes.  Since they are all-metal, they are more resistant to higher temperatures and chemical exposure than non-metallic lock nuts (such as nylon insert lock nuts).

Example of a K-Lock nut

An all-metal hex nut pre-assembled with a free spinning lock washer often external tooth. Unlike prevailing torque types , this bearing lock nut does not lock onto mating threads.  Rather it is a free spinning nut that must be firmly tightened against a bearing surface so the teeth of the lock washer can bite into the surface onto which it is being tightened.  It is the washer that resists vibrational loosening forces. Depending on surface finish, localized surface corrosion may occur as the lock washer bites into and removes the surface treatment.  Since they are all-metal, they are more resistant to higher temperatures and chemical exposure.

Key Considerations When Choosing Specific Styles of Lock Nuts:

  • Select the correct lock nut for the specific task at hand
  • Good design ensures savings in the long run
  • Re-use (need to disassemble/reassemble components)
  • Localized corrosion – (Surface finish onto which the nut is tightened)
  • Length of thread exposure (top-lock vs center-lock)
  • Exposure to chemicals
  • Exposure to extreme temperatures
  • Size and shape of hole


24 Hour Delivery Door-To-Door Service With UPS

Aspen Fasteners stocks full lines of lock nuts in steel. stainless steel and metric  in addition to most other screws, nuts, bolts and washers. Available for same day shipping from distribution centers across the USA. And remember, it’s always free shipping. Expedited next day delivery is also available for those rush requirements.

For more detailed information on Lock Nuts and product specifications click here.

If you don’t see what you are looking for, please contact us and we’ll be happy to help you find what you need.

This entry was posted in Product Insight and tagged , , , , , , , , , , . Bookmark the permalink.

14 Responses to Lock Nuts

  1. Julie Seo says:

    1. Can a two-way reversible lock nut be re-used?
    2. Can an all metal lock nut be re-used?
    3. My use requires me to tack-weld the lock nut on. What lock nut is best for this application?

  2. kyle reich says:

    how many times can a nylon insert lock nut be re-used before it no longer works safely?
    and also the same of an all metal lock nut?

    any information on this would be great thank you.

    • Expert says:

      It is impossible to set number of times that nylon nuts can be reused. The general rule of thumb is 5 times up to 15 times. But it depends on the application, temperature and other factors. Can be more or less. Flexloc style are rated for 15 times. In reality you must inspect the nut for wear and make the decision to reuse or replace. Usually the nut is the cheapest component in any installation and replacement cost in infinitely less than the damage that can happen with an overused or worn lock nut. If any question, replace is the safest decision.

  3. John Robida says:

    To Jim Perry, if you use a lock washer underneath the nylon lock nut, but do not tighten the lock nut down enough to compress the lock washer completely, the lock washer will act as a shock absorber for the lock nut which will not lose it’s elasticity. And yes there are different strengths of nylon in lock nuts for various amounts of vibrations.

  4. John Robida says:

    numerous is less than 5, it depends on how long the nut is fastened, after a year and the nut is never re torqued, the nylon gives up its elasticity and no longer has the grip intended to keep it tight.

  5. Nancy aubé says:

    Did a flange nut serrated grade 8 is the same as a flange lock nut serrated gr. 8 ???

    • Expert says:

      Yes that would be the same

      They are available here–http://www.aspenfasteners.com/Lock-Nuts-Flange-Serrated-Grade-8-Steel-Yellow-Zinc-s/199.htm

  6. Jim Perry says:

    I have used 5/16 x 24 standard hardness nylon insert lock nuts on a race car drive shaft. After several weekends of racing these locknus (new when installed) are not as tight as when installed. Is there one type that resists loosening better than another?

    • Expert says:

      Thanks for the question

      The best option for your application is probably a Flex Type Lock Nut
      An all-metal, one-piece, hex-shaped lock nut with a round collar at its back end. The collar is segmented with opposed slots cut into it above each corner of the nut. When the screw or bolt reaches the collar, the slotted portion expands which creates the prevailing torque locking action. Flex Type nuts maintain their locking strength through 15 removals and re-applications. The temperature service limit for steel nuts is 550°F (450°F if zinc or cadmium plated). They have superior resistance to vibration compared to all other lock nut varieties and do not gall thread.

      You can find them here:

      Fastener Experts

  7. VINCENT RUSSO says:

    at what temperatures will the standard nylon lock nuts fail?

    • Expert says:

      The continuous service temperature for a nylon insert lock nut depends entirely on the type of nylon used in the locking ring inserted in the nut. The standard nylon insert lock nut contains a nylon insert made of nylon 6/6 which maintains structural and functional properties up to 250⁰F (121⁰C). Specialty nylon insert lock nuts are also available for higher temperature requirements. It is best to check with the manufacturer to determine the functional temperature range of the specific nut.

  8. Stephen Pharo says:

    If the nylon lock nut deforms the threads of the mating bolt but not the nylon insert of the nut then do should the mating bolts be replaced if the nut is removed.
    Also does it matter how many threads of the mating bolt are showing through the nylon lock nut when tightened?
    Thank you

    • Expert says:

      Please be aware that it is the bolt that deforms the nylon ring in the nylon insert lock nut and not the other way around. Also, being nylon with a decent degree of memory, the nut is reusable and allows for numerous service cycles.

      A good rule of thumb is minimum 2-3 full bolt threads protruding past the nylon in order to ensure a good lock.

Leave a Reply

Your email address will not be published. Required fields are marked *

Spam protection by WP Captcha-Free