Captive fasteners are designed for permanent retention within their target assembly or housing, often even upon servicing, providing a secure joint and avoiding loss or damage that might be caused by a loose part.
Many captive fasteners use thread locking, press-fitting, or broaching to accomplish their anchor-hold within the larger assembly housing. However, some captive fasteners meld with the material into which they are joined, either through cold forming or welding.
There are several types of captive fasteners available including:
Captive nuts - Captive nuts are internally-threaded, captive fasteners designed for quick-connecting with studs. They are good for blind locations and can usually be attached without damaging panel finishes. They are often used in electronic and industrial enclosure applications.
Pallet nuts - Pallet nuts are also known as "tee nuts" or "knock-in nuts." Pallet nuts are usually made from metal or plastic and have a large flange with teeth that get hammered into place by the user. The teeth dig into pallet material, holding the joint fast.
Weld nuts - Weld nuts are captive fasteners designed for welding to flat surfaces for permanent retention within an assembly. Weld nuts have threaded holes through the base and come in several types.
Captive pins - Captive pins are often used in electronic circuit board applications, where they are pushed like a rivet into their target panel hole. They feature strong, self-clinching heads that deform softer mating materials. These heads resist torque and assembly push-out forces, and may also rest flushly against a panel or board.
Captive screws - Captive screws are fasteners designed for ease of installation, often knurled for finger use and retained within their functional environment when not engaged. Captive screw (or bolt) retention prevents loss or damage within the captive screw's parent assembly, or damage to components in the nearby working environment.
Captive standoffs - Captive standards are internally-threaded fasteners designed to secure, support, or provide spacing for circuit boards and similar assemblies upon use of a mating screw.
Captive studs - Captive studs are externally-threaded fasteners designed for permanent assembly integration. They are sometimes used to provide retention for outer-surface metal housing attachments to body structures.
Captive fastener features include: self-clinching (used for ductile materials) and broaching (used for non-ductile materials).
Self-clinching - Captive fastener has an integrated mechanism (non-broaching) for deforming material and permanently locking into place. Example uses include sheet metal integration and printed circuit board (PCB) fastening.
Broaching - Captive fastener - nut, screw or stud - accomplishes joining through a material removal process at time of installation. An example of a broaching captive fastener is a printed circuit board (PCB) captive nut that has knurled cutting-shanks. The nut is pressed into a drilled hole, and accomplishes fastening via interference fit.
Each type of captive fastener becomes a permanent part of the panel or item to which it is installed. This permanent installation eliminates loose hardware problems. Both a self-clinching fastener and a broaching fastener will withstand high push-out and torque-out forces. Weld nuts self pilot into pre-drilled holes and are round, thus eliminating need for indexing. A weld nut is best suited for applications needing high strength.
Some captive fasteners may be installed by pressing the fastener into a pre-drilled or punched hole. In this case, a squeezing force is next applied to embed the fastener in the panel. The squeezing process causes displaced panel material to flow into the shank of the fastener (through a cold flow process) locking the fastener to the panel. The amount of time that force is applied to captive fasteners is important; enough time must be allowed for the material to flow. Because of this, hammers cannot be used to apply force to the captive fastener as the force does not last long enough for the cold flow process to take place.
Captive fasteners are valuable as they add speed during initial assembly and field servicing, thus keeping costs low. Generally captive fasteners use less space and require fewer assembly operations than caged or anchor nuts. Captive fasteners can be used whenever a component must be easily replaced or where hardware cannot be accessible. It is important to keep in mind that the fastener must be compatible with the panel or sheet material being used. In addition, the hardness of the fastener must be greater than the hardness of the panel material.
Captive fasteners must adhere to certain standards and specification to ensure proper design and functionality. Additional standards can be found at the IHS standards store.
DEF STAN 81-73: Guide on use of captive fasteners.
Image Credit: BRIM Electronics, Inc. | ASM Corp. | Keystone Electronics Corp.