Fasteners are essential components of many engineering applications, from aerospace to construction. They are used to join or secure parts together, and they come in various shapes, sizes and materials. One of the most important aspects of fasteners is their threading, which is the helical ridge that runs along the shaft or body of the fastener.
Threading determines how well a fastener can resist loosening, vibration, corrosion and fatigue. It also affects the torque, preload and clamping force of the fastener. Therefore, understanding the threading theory of fasteners is crucial for engineers who design, select or use them.
In this blog post, we will cover the basics of threading theory, including the types, dimensions, standards and calculations of threads. We will also discuss some common problems and solutions related to threading, such as thread stripping, galling and locking.
Types of Threads
There are two main types of threads: metric and imperial. Metric threads are based on the International System of Units (SI), and they use millimeters as the unit of measurement. Imperial threads are based on the British or American systems, and they use inches as the unit of measurement.
Metric threads are further classified into coarse and fine threads. Coarse threads have larger pitch (the distance between two adjacent threads) and smaller depth (the height of a thread) than fine threads. Coarse threads are easier to manufacture and assemble, but they have lower strength and resistance to vibration than fine threads.
Imperial threads are also divided into coarse and fine threads, but they have different names. Coarse threads are called Unified Coarse (UNC), while fine threads are called Unified Fine (UNF). There are also other variants of imperial threads, such as Unified Extra Fine (UNEF), National Pipe Thread (NPT) and British Standard Whitworth (BSW).
Dimensions of Threads
The dimensions of threads are defined by several parameters, such as:
- Major diameter: The largest diameter of the thread
- Minor diameter: The smallest diameter of the thread
- Pitch diameter: The diameter where the width of the thread and the width of the space between threads are equal
- Pitch: The distance between two adjacent threads
- Lead: The distance that a thread advances along its axis in one turn
- Angle: The angle between two adjacent thread flanks
- Depth: The height of a thread
- Crest: The top surface of a thread
- Root: The bottom surface of a thread
These parameters can be measured using various tools, such as calipers, micrometers, gauges and optical devices. They can also be calculated using formulas or tables based on the type and size of the thread.
Standards of Threads
The standards of threads are established by various organizations, such as the International Organization for Standardization (ISO), the American Society of Mechanical Engineers (ASME), the British Standards Institution (BSI) and the Deutsches Institut für Normung (DIN). These standards specify the dimensions, tolerances, designations and markings of threads for different applications and industries.
Some examples of thread standards are:
- ISO metric screw thread: The most widely used standard for metric threads, which defines the basic profile, dimensions and tolerances of coarse and fine threads from M1 to M68.
- Unified Thread Standard (UTS): The most common standard for imperial threads, which defines the basic profile, dimensions and tolerances of UNC, UNF, UNEF and other variants from #0 to 6 inches.
- ISO 724: The standard that specifies the basic dimensions of ISO metric screw threads in terms of pitch diameter.
- ASME B1.1: The standard that specifies the general purpose inch screw threads in terms of pitch diameter.
- ISO 898-1: The standard that specifies the mechanical properties of bolts, screws and studs made of carbon steel and alloy steel with ISO metric screw threads.
- ASME B18.2.1: The standard that specifies the dimensional characteristics of hex bolts, hex cap screws and hex nuts with inch screw threads.
Calculations of Threads
The calculations of threads involve determining various factors that affect the performance and functionality of fasteners, such as:
- Torque: The rotational force applied to a fastener to tighten or loosen it
- Preload: The axial force applied to a fastener to create clamping force between parts
- Clamping force: The compressive force that holds parts together
- Stress: The internal force per unit area that acts on a material due to external load
- Strain: The deformation or change in shape or size of a material due to external load
- Modulus of elasticity: The ratio of stress to strain in a material
- Coefficient of friction: The ratio of frictional force to normal force between two surfaces
- Thread engagement: The percentage or length of thread contact between a fastener and a hole or nut
- Thread fit: The degree of tightness or looseness between a fastener and a hole or nut
- Thread class: The designation of thread fit based on the tolerance and allowance of the thread
These factors can be calculated using various formulas, tables, charts or software, depending on the type and size of the thread, the material and surface condition of the fastener and the parts, and the loading and environmental conditions of the application.
Problems and Solutions of Threads
Some common problems that may occur with threads are:
- Thread stripping: The failure of the thread due to excessive stress that exceeds the shear strength of the material
- Galling: The damage of the thread due to friction and adhesion between metal surfaces that results in seizure or welding of the fastener
- Locking: The prevention of the thread from loosening due to vibration, shock or thermal expansion
Some possible solutions to these problems are:
- Thread stripping: To prevent thread stripping, one can increase the thread engagement, use stronger materials, reduce the torque or preload, or use thread inserts or coatings.
- Galling: To prevent galling, one can reduce the torque or preload, use lubricants, use anti-seize compounds, use different materials for the fastener and the parts, or use thread coatings or platings.
- Locking: To prevent locking, one can use locking devices, such as lock washers, lock nuts, lock wires, lock pins or lock rings, or use thread locking compounds, such as anaerobic adhesives or sealants.
Conclusion
Threading theory is an important topic for engineers who work with fasteners. It covers the types, dimensions, standards and calculations of threads, as well as some common problems and solutions related to threading. By understanding threading theory, engineers can design, select and use fasteners more effectively and efficiently for their applications.