Screw thread

Dubendorf floodgate

Screw thread handedness

Right-hand grip rule

Different Thread Types Update

Lead and pitch

Helical screw single double triple quadruple start

Screw thread refers to a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter a tapered thread. Screw threads have several applications, including fastening objects together, in screws, bolts, and nuts, and as a critical component in machines such as vises, lathes, and engines.

Types of Screw Threads

Screw threads can be categorized into various types based on their design and purpose:

• Metric threads: These are standardized threads defined by the International Organization for Standardization (ISO), characterized by their pitch (distance between threads) and diameter. Metric threads are widely used globally for all types of applications.
• Unified Thread Standard (UTS): Predominantly used in the United States and Canada, this standard includes Unified Coarse (UNC), Unified Fine (UNF), and Unified Extra Fine (UNEF) threads.
• British Standard Whitworth (BSW): The first standard to define screw threads, introduced in Britain by Sir Joseph Whitworth in 1841. It has a unique profile compared to metric and UTS threads.
• Acme thread: A trapezoidal thread form used for translation motion in machines like lathes, vises, and lead screws. It is designed to withstand high loads.
• Square thread: A thread form with a square cross-section, used for high load while offering low friction.
• Buttress thread: Designed to handle high axial force in one direction, these threads are used in applications such as artillery shells and presses.

Design and Nomenclature

The design of a screw thread involves several parameters:

• Major diameter: The largest diameter of the thread.
• Minor diameter: The smallest diameter of the thread.
• Pitch: The distance from a point on one thread to a corresponding point on the next thread, measured parallel to the axis.
• Lead: The distance a screw thread advances axially in one turn.
• Thread angle: The angle between the flanks of the thread, which varies depending on the thread type.

Manufacturing

Screw threads can be produced by several methods, including:

• Cutting: Using tools such as taps and dies to remove material and form threads.
• Rolling: Forming threads by displacing material using thread rolling dies, which is more efficient and produces stronger threads.
• Grinding: Used for precision thread production, especially for hard materials.

Applications

Screw threads are essential in many mechanical devices and systems. Their applications range from simple fastening devices, like screws, bolts, and nuts, to complex machinery such as automotive engines, airplanes, and precision instruments.

Standards

Several organizations, including the International Organization for Standardization (ISO), the American National Standards Institute (ANSI), and the British Standards Institution (BSI), have developed standards to ensure compatibility and interchangeability of threaded products.