What is Nuts (Hardware)?- Definition, and Types

What are Nuts(Hardware)?

Fasteners defined generally as “nuts” are fasteners featuring a hole threaded throughout the thickness of the part. Most often, nuts are used with a mating bolt to hold two or more parts together.

The fastened couple is maintained in an axial position by the friction between their threads (plus a bit of elastic deformation), the elongation of the bolt, and the compression of the parts being held together.

When parts must be held together without the potential of vibration or rotation to work a nut loose, many different ways of locking the nut are utilized: lock washers, jam nuts, locking adhesives (e.g., thread locking fluid sold under the name “Loctite”), different types of safety pins (split pins) or locking wire with castellated nuts, nylon inserts (e.g., nylock nut), and various forms of slightly oval threads.

Historically, the first shape that appeared was square, both as bolts and nuts. Square is also the most common shape as it was much easier to make, especially by hand.

Square nuts while not common, are utilized occasionally for unique reasons when one is looking for the maximum amount of torque and thus grip for a given size: with square nuts, one can apply a tool to the larger surface area and also utilize greater leverage at the nut length as square nuts have a larger and equal length for each side.

Hexagonal is the most common shape used today, for similar reasons as the bolt head: six distinct sides give a good granularity of angles to approach from (good for tight spots), and if more (and smaller) corners were created, they are at risk for rounding off. With a hex 1/6 of a rotation allows to obtain the next side and maintain optimal grip.

Shapes with more than six letters do not provide enough grip, while shapes with fewer than six letters require even more time to complete one rotation.

Other shapes have been designed generally for certain applications, such as wingnuts for finger adjusted tightening, and captive nuts (e.g., as in cage nuts) for difficult-to-access locations.

There is an assortment of nuts, from those used at home hardware stores, to those designed for specific industries and technical specifications.

In most instances of automotive, engineering or industrial fastened assemblies, tightening must be done to a specific torque specification (which means the use of a torque wrench).

Nuts are rated by grades with strength ratings that are compatible with a bolt grade; for example, an ISO property class 10 nut will be able to resist an ISO property class 10.9 bolts proof strength load without the nut stripping. Likewise, a nut from SAE class 5 can resist an SAE class 5 bolts proof load, etc.

Types Of Nuts

1. Cap Nuts

A cap nut or acorn nut is named due to its shape, with the dome on top keeping the external threads from engaging.

2. Castle Nuts

A castellated nut, castle nut, or slotted nut is like a regular nut, but has slots cut into the top that prevent the nut from loosening with cotter pins. Castellated nuts are low-torque applications usually with the objective of holding one part in place to another part, think wheel bearing.

3. Coupling Nuts

A coupling nut is just a threaded fastener meant to join together two male threads, most commonly a threaded rod. A coupling nut is a hex fastener from the outside, so you can drive it with a wrench.

4. Flange Serrated Nuts

A flange nut is a nut with a wide flange on one end that acts as a built-in washer that does not turn. The serrated flange gives a nut better pressure distribution over the part being held securely, and provides some locking action to minimize unwanted loosening.

5. Hex Finish Nuts

Hex finish nuts are used for fastening to a hex cap screw, socket cap screw, or bolt. The most common nuts, hex finish nuts are hexagonal with internal threads and turned with a wrench.

6. Hex Jam Nuts

A jam nut is used when you want to lock a nut in place without clamping it against something else. Hex jam nuts are hexagonal with internal threads, but they are thinner than hex finish nuts.

7. Heavy Hex Nuts

Heavy hex nuts are larger, heavier, and thicker than a standard hex nut. Heavy hex nuts are hexagonal, have internal threads, and are turned with a wrench. Heavy hex nuts are often used with hex cap screws and carriage bolts.

8. Hex Machine Nuts

A machine nut is hex-shaped with internal threads. Smaller than a hex jam or hex finish nut, they are used with machine screws under 1/4″ diameter.

9. Hex Machine Nuts Small Pattern

A machine nut is hex-shaped with internal threads. Smaller than a hex jam or hex finish nut, they are used with machine screws under 1/4″ diameter.

10. Keps-K Lock Nuts

Also sometimes referred to as a keps nut, k-nut, or washer nut, a keps-k lock nut has a free-spinning lock washer attached to it. Keps nuts are intended to make assembly easier.    

11. Knurled Thumb Nuts

A knurled head thumb nut or thumb nut has a knurled surface on the outside face, rather than a hex face, making it possible to hand-tighten. This type of fastener is used frequently in decorative finishes or applications.

12. Nylon Hex Jam Nuts

A low-profile lock nut consists of a hex-shaped nut that is internally threaded and has a nylon insert. The function of the nylon material is to prevent the fastener from loosening due to vibration, and the cross threads have the effect of preventing the nut from backing off of the fastener.

13. Nylon Insert Lock Nuts

A nylon insert lock nut is a hex-shaped, internally threaded fastener with an attached nylon insert. The nylon material works in two ways to prevent loosening, by mitigating vibration and cross threads, preventing the nut from backing off of the fastening.

14. Prevailing Torque Lock Nuts (Stover)

Also referred to as stover nuts, prevailing torque lock nuts feature a conical top and chamfered corners that create a distortion in the top thread to resist loosening caused by vibration.

Often referred to as one-way nuts, they can only be installed one way. These are often used in high-temperature applications because they are all metal with no nylon insert.

15. Slotted Hex Nuts

Slotted hex nuts are nuts with sections cut out intended to be used with a cotter ping to achieve a locking mechanism. They are similar to a castle nut but they are a bit shorter, which sometimes makes them the better option.

16. Square Nuts

A square nut, which has the possibility of being flat or chamfered on top, is four sided. Square nuts provide a larger surface contact area which gives resistance to loosening. Normally mated to square head bolts.

17. Structural Heavy Hex Nuts

Structural hex nuts are similar to finish nuts, but designed to be thicker and much stronger. A structural hex nut is typically used as part of steel-to-steel structural connections.

18. T-Nuts

A t-nut or tee nut is used to fasten wood, particle, or composite board while creating a flush surface. The T-nut has a long thin body and a flange at one end which resembles a T in profile. T-nuts typically have 3 or 4 prongs which sink into the surface allowing for better retention.

19. Break Away Or Shear Nuts

A shear nut is a cone nut with a hex-shaped grip point which has a predetermined weakness designed to shear off the hex head once maximum torque is produced on the nut. Shear nuts leave a cone nut that is very difficult to remove.

20. Tri-Groove Nuts

Tri-groove security nuts have a tapered diameter which makes them harder to grip with dealing tools like adjustable wrenches or pliers.

In addition to the tapered diameter, tri-groove nuts require a special unconventional gripping device to be installed which would make them a lot more secure than a typical nut. 

21. Wing Nuts

A wingnut is a type of nut with wings (or “ears”) on each side of the body to hold onto when turning manually to install. Easy to hand assemble and used when frequent removal of the nut is needed.

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