Lock Nuts

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Lock Nuts Basics

A lock nut, also known as a self-locking nut, locking nut, or in some cases, a prevailing torque nut, is a nut used to secure bolted joints, that under vibration and torque, will resist loosening. Styles of lock nuts include nylon insert lock nuts, all-metal lock nuts, serrated flange lock nuts, nylon insert jam lock nuts, as well as galvanized Tri Lock nuts and Anco pin lock nuts.

Lock Nuts Grades

Lock nuts are available in several different grades and finishes based on the mechanical and chemical properties of the material, and the intended application of the lock nut.

Nylon Insert Lock Nuts

Nylon insert lock nuts are hex nuts that have a nylon collar insert in one end of the nut. The nylon insert increases friction on the screw thread, which resists loosening. The nylon insert lock nut is an economic locking nut and ideal in applications where vibration or motion could loosen or undo the nut.

Nylon insert jam lock nuts, aka “half lock nuts”, or “low profile lock nuts”, are low height jam nuts that have a layer of nylon on the interior threads, the same as a full size nylon insert lock nut. The nylon insert jam lock nut is typically used in applications where there is not sufficient space for a standard nut.

Lock nuts are available in the following grades and materials:

  • ASTM A563 Grade A / SAE J995 Grade 2
  • ASTM A563 Grade C / SAE J995 Grade 8
  • 18-8 Stainless Steel
  • 316 Stainless Steel
  • Brass

All Metal Lock Nuts

All metal lock nuts are resistant to high temperatures and high chemical exposures, as they are made from one material.

Grade C All Metal Lock Nuts

All-metal crimped lock nuts are not temperature or chemical limited (beyond that of the material the nut is manufactured from) in applications compared to a nylon insert lock nut. They have indentions formed by a special stamping tool that warps the top threads of the nut, which creates very tight threads to provide a locking mechanism. Grade C lock nuts shall be marked with six equally spaced (60°) identical symbols on the top side of the nut.

Serrated Flange Lock Nuts

Serrated flange lock nuts eliminate the need for a lock washer or washer, as the flange serves to distribute the bearing load over a greater area. The bearing surface has circular serrations that fan out so that once the serrated flange makes contact with the mating surface, the serrations cut into and displace the mating material upon tightening. Serrated flange lock nuts do not require grade markings.

Tri Lock Nuts

Tri Lock style lock nuts are all metal prevailing torque lock nuts with a conical top and a flat bottom bearing surface with chamfered corners. The thread distortion on the top thread of the nut makes it frictionally resistant to rotation. Grade A Tri Lock nuts do not require grade markings.

ASTM A563 Grade DH Heavy Anco Lock Nuts

Anco pin lock nuts have an added stainless steel ratchet pin inserted into the side opposite the nut’s bearing surface. The ratchet pin slides along the threads as the nut is threaded onto the bolt, which prevents the nut from backing off. The Anco style lock nut has an added feature in that the ratchet pin can be bent so that the nut can be removed, if needed. Anco lock nuts are manufactured from heavy hex ASTM A563 Grade DH nuts that have an added stainless steel ratchet pin. Grade markings consists of the grade symbol "DH".

Common Finishes of Lock Nuts

Lock nuts are available in a variety of finishes and coatings, which play a role in the suitability of the fasteners in different applications, such as enhancing resistance to corrosion from environmental elements or chemicals. Coatings also greatly effect consistency of torque values for installation, and ease of disassembly at the end of service.

Galvanized Lock Nuts

Lock nuts are coated with a sacrificial zinc coating that acts as an anode to prevent the fastener underneath from corroding.

  • Hot Dipped Galvanized: Fasteners are dipped in a bath of molten zinc.

Plated Lock Nuts

Lock nuts are coated with a thin layer of zinc, typically via electroplating.

  • Plated lock nuts will not corrode as quickly when covered with this protective coating, even when a scratch or cut exposes the steel to air or moisture. The zinc or cadmium plating will always tarnish and corrode first.