Description

Captive screws are fasteners which remain attached to a panel or parent material, even after they are disengaged from the mating panel or part. They meld with the parent via cold forming, and then fuse with the panel or part into a single entity.

Often, they attach to the panel, part or parent materials with split washers that clip-on after the captive screw is inserted into the panel. Other methods of attachment include threaded panel holes and flared or swaged ferrules.

In terms of applications, captive screws can help speed assembly and disassembly operations, and prevent screws from getting loss or falling into moving parts or electrical circuits.

Captive screws are available in a variety of materials and sizes, but may not be interchangeable with each other.

Types

Captive screws come in a variety of head types.

• A binding head has a rounded top with slightly tapered sides and a flat bearing.
• A button head has a rounded top with cylindrical sides and a flat bearing surface.
• A fillister head or cheese head has a slightly convex top with cylindrical sides and a flat bearing surface.
• A five-sided head is shaped like a pentagon. Typically, captive screws with this type of head are tightened and loosened with special wrenches and are designed for specialized applications, such as the valves of fire hydrants.
• A flat head has a flat top surface and a conical bearing surface.
• A hexagonal head has six equal sides, a flat top, and a flat bearing surface. Hexagonal captive screws are designed to be tightened and loosened with a wrench.
• An oval head has a rounded top surface and a conical bearing surface, usually with an included angle of 82°.
• A pan head has a flat top, cylindrical sides, and a flat bearing surface.
• A recessed pan head has a rounded top, cylindrical sides and a flat bearing surface.
• A round head has a rounded, elliptical top surface and a flat bearing surface.
• Socket head cap screws have a cylindrical head, a hexagonal socket drive, and a flat bearing surface.
• A square head has four equal sides, a flat top, and a flat bearing surface. Square headed captive screws are designed to be tightened and loosened with a wrench.
• A truss head has a low head height, a rounded top surface, and a flat bearing surface.
• Washer head screws have a washer-like bearing surface that distributes the load over a larger area. 

Standards

MS 90402 - Screw, captive

NA0127 - Screw, captive, 100 degrees flush head, offset cruciform recess, alloy steel 

References

Image Credits:

Keystone Electronics Corp.|BRIM Electronics, Inc.