Nuts (Hardware): Definition, and Types

What are Nuts(Hardware)?

A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads’ friction (with slight elastic deformation), a slight stretching of the bolt, and compression of the parts to be held together.

In applications where vibration or rotation may work a nut loose, various locking mechanisms may be employed: lock washers, jam nuts, specialist adhesive thread-locking fluid such as Loctite, safety pins (split pins) or lock wire in conjunction with castellated nuts, nylon inserts (nyloc nut), or slightly oval-shaped threads.

Square nuts, as well as bolt heads, were the first shape made and used to be the most common largely because they were much easier to manufacture, especially by hand.

While rare today[when?] due to the reasons stated below for the preference of hexagonal nuts, they are occasionally used in some situations when a maximum amount of torque and grip is needed for a given size: the greater length of each side allows a spanner to be applied with a larger surface area and more leverage at the nut.

The most common shape today is hexagonal, for similar reasons as the bolt head: six sides give a good granularity of angles for a tool to approach from (good in tight spots), but more (and smaller) corners would be vulnerable to being rounded off. It takes only one-sixth of a rotation to obtain the next side of the hexagon and grip is optimal.

However, polygons with more than six sides do not give the requisite grip and polygons with fewer than six sides take more time to be given a complete rotation. Other specialized shapes exist for certain needs, such as wingnuts for finger adjustment and captive nuts (e.g. cage nuts) for inaccessible areas.

A wide variety of nuts exists, from household hardware versions to specialized industry-specific designs that are engineered to meet various technical standards. Fasteners used in automotive, engineering, and industrial applications usually need to be tightened to a specific torque setting, using a torque wrench.

Nuts are graded with strength ratings compatible with their respective bolts; for example, an ISO property class 10 nut will be able to support the bolt proof strength load of an ISO property class 10.9 bolt without stripping. Likewise, an SAE class 5 nut can support the proof load of an SAE class 5 bolt, and so on.

Types Of Nuts

Types of Nuts

1. Cap Nuts

The cap nut, also known as the acorn nut, gets its name from its shape. The nut has a domed top to prevent contact with the external thread.

2. Castle Nuts

Used with cotter pins to prevent loosening, a castellated nut, also called a castle or slotted nut, is not with slots cut into the top. Used in low-torque applications such as holding a wheel bearing in place.

3. Coupling Nuts

A coupling nut is a threaded fastener used for joining two male threads, most commonly threaded rod. The outside of the fastener is a hex so it can be driven with a wrench.

4. Flange Serrated Nuts

A flange nut is a nut that has a wide flange at one end which acts as an integrated washer that does not move or spin. The serrated flange distributes the pressure of the nut over the part being secured and creates a locking action to prevent 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 hex-shaped with internal threads and driven with a wrench.

6. Hex Jam Nuts

A jam nut is often used when a nut needs to be locked in place without clamping to another object. Hex jam nuts are hex-shaped with internal threads, but they are thinner than hex finish nuts.

7. Heavy Hex Nuts

Larger, heavier, and thicker than a standard hex nut. Heavy hex nuts are hex-shaped, internally threaded, and driven with a wrench. 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 known as a keps nut, a k-nut or a washer nut, a keps-k lock nut has an attached free-spinning lock washer. Keps nuts are designed to make assembly more convenient.            

11. Knurled Thumb Nuts

A knurled head thumb nut or thumb nut has a knurled outside surface rather than a hex, which facilitates tightening by hand. Often used in decorative finishes or applications.

12. Nylon Hex Jam Nuts

A low-profile lock nut is hex-shaped, internally threaded with a nylon insert. The nylon material prevents loosening from vibration and cross threads to stop the nut from backing off of the fastener.

13. Nylon Insert Lock Nuts

A nylon insert lock nut is hex-shaped, internally threaded with a nylon insert. The nylon material prevents loosening from vibration and cross threads to stop the nut from backing off of the fastener.

14. Prevailing Torque Lock Nuts (Stover)

Commonly known as stover nuts, prevailing torque lock nuts have chamfered corners and a conical top. The distortion in the top thread resists loosening from vibration. Also called one-way nuts, they can only be installed one way and 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 portions cut out designed to be used with a cotter ping to create a locking mechanism. These nuts are similar to a castle nut but have a lower profile which sometimes makes them a better option.

16. Square Nuts

A four-sided nut that may be flat or beveled on top. Square nuts provide a greater surface contact area which provides more resistance to loosening. Typically mated with square head bolts.

17. Structural Heavy Hex Nuts

Structural hex nuts are comparable to finish nuts but are made to be thicker and much stronger. They are typically used in steel-to-steel structural connections.

18. T-Nuts

A t-nut or tee nut is used to fasten wood, particle, or composite board leaving a flush surface. A long thin body with a flange at one end resembles a T in profile. T-nuts often have 3 or 4 prongs that sink into the surface providing better retention.

Types of nuts

19. Break Away Or Shear Nuts

Shear nuts are cone nuts with a hexagonal gripping point. They are designed with an intentional flaw to snap the hexagonal head off once the maximum torque is reached. Leaving behind a protective cone nut that cannot be easily removed.

20. Tri-Groove Nuts

Tri-groove security nuts have a tapered diameter making them difficult to grip with grabbing devices such as adjustable wrenches or pliers. These nuts require a special unconventional gripping device to install them making them more secure than a typical nut.        

21. Wing Nuts

Wingnuts are threaded nuts with wings on each side of the body allowing for manual turning and installation. Easy hand assembly and used when the nut needs to be removed often.