Special threads are designed for unique engineering applications where standard thread profiles are not sufficient. These thread types serve specific functions such as high load-bearing capacity in one direction, self-tapping capability, electrical connectivity, or enhanced durability.
Each of these threads is optimized for particular industries, including heavy machinery, electrical applications, and sheet metal fastening. Below, we explore four key special thread types and their characteristics.
🔹 Related Special Thread Standards
🔹 Buttress Threads (ISO 2904, DIN 513) – Asymmetrical Load-Bearing Threads
Buttress threads feature a unique asymmetrical design, with a steep load-bearing flank (typically 7° to 10°) and a larger relief flank angle (30° to 45°). This geometry allows them to handle extremely high axial loads in one direction, making them ideal for heavy-duty applications such as hydraulic presses and jacks.
✔ High axial load capacity in a single direction
✔ Low backlash, improving efficiency in force transfer
✔ Common in construction, injection molding machines, and power transmission
🔹 Knuckle Threads (DIN 405) – Rounded Thread Profile
Knuckle threads are designed with a rounded thread profile, providing high durability and smooth engagement. They are widely used in applications requiring resistance to wear and impact, such as railway couplings, conveyor systems, and mechanical linkages.
✔ Smooth engagement and disengagement
✔ Highly durable in rough conditions
✔ Works effectively with soft materials and plastics
🔹 Edison Threads (E27, E14, etc.) – Light Bulb Screw Threads
Edison threads, also known as E-series threads, are standardized screw threads used in light bulb sockets. Invented by Thomas Edison, these threads are designed for easy and secure electrical contact, making them the industry standard for lighting applications worldwide.
✔ Quick screw-in mechanism for easy bulb replacement
✔ Secure electrical connection for safety and reliability
✔ Used in household, automotive, and industrial lighting
🔹 ST Threads (DIN 7970) – Self-Tapping Sheet Metal Screws
ST threads are optimized for self-tapping screws, designed to cut into thin sheet metal, plastics, and other soft materials without pre-drilled holes. Their sharp profile ensures a tight and secure fit, making them essential in automotive, construction, and electronic applications.
✔ Self-tapping ability – No pre-drilling required
✔ High thread engagement – Stronger hold in soft materials
✔ Reduces loosening due to vibration