What Is a Sprew?
A sprew is a part designed to reinforce female threads by coiling a diamond-shaped stainless steel wire around them. Utilizing the spring back effect of the wire significantly enhances the accuracy of screw fastening. By correcting lead and angle errors of the threads, stress is evenly distributed throughout the length of the threaded hole, thus strengthening the connection between the base metal and the screw.
Uses of Sprews
Primarily, sprews are employed to reinforce female threads, especially in materials like light metals, cast iron, and plastics, where the native threads may be weak. The use of sprews to create a rolling female thread of stainless steel equalizes stress distribution, thereby enhancing the fastening force and ensuring a more secure screw attachment.
Sprews also improve the durability of female threads against wear, corrosion, vibration, and heat, protecting the base material and ensuring reliable thread fastening. Furthermore, they are utilized for repairing defective female threads, easily restoring stronger threads than the original ones.
Additionally, sprews contribute to reducing the diameter of male threads and the length of female threads, which results in lighter and more compact products, lowering manufacturing costs and enhancing product quality.
Principle of Sprews
Sprews are particularly effective in reinforcing products made from low-strength materials like aluminum or plastics, preventing thread breakage and enhancing durability. A special tap is used for setting the threads of a sprew, which is characterized by its free outside diameter – the diameter of the sprew before insertion, typically 15% to 20% larger than the insertion diameter.
The structure of a sprew includes the free turns (number of turns in its free state), the first coil (which guides and twists into the tap groove hole), the tongue (the part twisted in with an insertion tool), and a notch (used for folding out the tongue in through-hole applications).
As industrial products evolve to become smaller and lighter, the importance of materials and structures in design and manufacturing increases. Sprews play a vital role in addressing issues like wear, fracture, and galling in weaker or brittle base materials such as light alloys, nonferrous metals, and various plastics, thus preventing common problems in these applications.