Bolt and Nut

Nuts are fasteners with threaded holes that secure to bolts, screws, or studs, holding parts together. Once securely fastened, a nut prevents axial movement in the connected materials. Types of nuts include:

Cap Nuts
Also called acorn nuts, these have a domed top and cap the exposed ends of shafts, axles, and rods.

Hex Nuts
Six-sided nuts used for multiple threaded applications. Variants include finished hex, semi-finished hex, hex flange, and slotted hex.

Jam Nuts
Half the length of standard hex nuts, these lock in place without clamping to another object.

Lock Nuts
Also known as nylon lock nuts or prevailing torque nuts, these are used with other nuts to prevent them from loosening.

Push Nuts
These can be capped or uncapped and are installed with a special nut driver. Removal will destroy the fastener.

Coupling Nuts
Hollow threaded fasteners, also called extension nuts, used to join two male nut threads and driven with a wrench.

Speed Nuts
Also known as sheet metal nuts, these consist of two metal pieces that function as one.

Square Nuts
Threaded nuts with four sides and various internal platings.

T-Nuts
These have a flange at the end of a long threaded body, often with 3 or 4 prongs to sink into material for better retention.

Wing Nuts
Threaded nuts with wings on both sides, allowing for easy manual application and removal.

If you cannot determine that a part meets any of the four primary criteria, there are nine supplementary criteria you can examine. A fastener that satisfies five of the nine supplementary criteria can be classified as a screw.
In general terms, screws are more tightly toleranced than bolts. If you have access to the blueprint or specifications for the part in question, you can compare the tolerances to those of industry-standard tolerances for bolts and screws to determine which is closer to the part in question.

Point: The configuration at the end of the shank has a chamfer or other specially prepared point to facilitate entry into the hole and engagement with the internal thread.

Bearing Surface: The under-head bearing surface should be smooth and flat to reduce friction or scoring when tightening the head.

Under-Head Fillet Radius: The angle of the junction of the head with the body should be controlled to ensure proper seating of the head.

Head Angularity: The squareness of the under-head bearing surface with the shank should be controlled to ensure proper seating.

Thread Concentricity: Threads should be concentric with the body axis.

Thread Length: Measured as the axial length of the full-form threads, in addition to up to two incomplete threads at the start of the thread, the thread length should be sufficient to fully engage with the tapped hole.

Body: The body, or unthreaded part of the shank, should be controlled in size and roundness to fit into tightly toleranced holes.

Shank Straightness: The shank should not have a bow or camber that would prevent it from fitting into tightly toleranced holes.

Length: The overall length should be controlled to prevent the screw from bottoming out in the hole.

The selection of a fastener is determined by the intended use and function.

Bolt
Bolts are widely used in detachable connections in mechanical manufacturing and are generally used in conjunction with nuts (usually with one or two washers).

Nut
A nut is a small metal object with a hole in the middle that has internal threads. Nuts serve as fastening devices. It is important to note that, although nuts are used as fasteners, they cannot function without bolts. To understand this, consider the shape of walnuts: their outer shells can be shaped in various ways, but they are mostly hexagonal or circular.

Screw
A screw is usually used alone (sometimes with a washer) for tightening and should be screwed into the internal thread of a component.

Stud
Studs are primarily used to connect parts with significant thickness and are necessary in situations where a compact structure or frequent disassembly makes bolt connections unsuitable. Studs are generally threaded at both ends (single-end studs are threaded at only one end). Typically, one thread is securely screwed into the component, while the other end is matched with a nut for connection and fastening. They also help maintain a fixed distance.

Wood Screws
Wood screws are specifically designed to be driven into wood to connect or fasten materials.

Self-Tapping Screws
Self-tapping screws do not require pre-tapped holes; instead, they form the internal thread as they are screwed in.

Washer
A washer is placed between the supporting surface of bolts, screws, nuts, etc., and the workpiece's surface to prevent loosening and reduce stress on the supporting surface.

Retaining Ring
A retaining ring is primarily used for positioning, locking, or stopping parts on a shaft or in a hole.

Pin
Pins are typically used for positioning but can also connect or lock parts. They can function as overload shearing elements in safety devices.

Rivets
A rivet has a head at one end and no threads on the shank. During use, the rod is inserted into the hole of the connected piece, and then the end of the rod is riveted to secure the connection.

A typical bolt consists of two main components: a shaft and a head shaped to fit a tool like a wrench. Bolts have an externally threaded shaft, which may be entirely threaded or divided into threaded and unthreaded portions. The unthreaded portion is often called the shank or grip length. The most common bolt heads are hex-shaped, although other variations exist for specialty applications.
A nut is a fastener with an internal thread and an external surface shaped to fit a tool. Like bolt heads, modern nuts are usually hex-shaped. Nuts are screwed onto bolts to form a bolted joint.
The purpose of a bolted joint is to prevent parts from moving relative to each other, with each component playing a specific role:
● The bolt's shaft prevents lateral (sideways) movement of the parts.
● The bolt's head and the nut prevent the parts from moving away from each other.
● The clamping force generated by a correctly torqued nut and bolt joint prevents radial (rotating) movement of the parts.
When installing a bolt with a nut, there are two main stages. In the first stage, the nut easily screws onto the bolt until the bolt head and the nut are in contact with the surface of the joined parts.
In the second stage, tightening the nut becomes significantly harder. This is because further tightening—or torquing—elongates the bolt. Similar to an elastic band, this elongation generates tension in the bolt, known as the preload force. The preload force creates a clamping force that holds the joined parts together.
Achieving the correct preload force is essential when installing a nut and bolt. In other words, you need to apply the proper amount of torque. Inadequate preload can cause parts to move relative to each other or allow the nut to unscrew from the bolt. Conversely, excessive preload can strip the bolt's threads, leading to joint failure.