Electronic devices require fasteners for securing circuit boards, mounting daughterboards, closing panels, and other uses. There are many types of screws, nuts, and rivets made with a variety of materials. The selection of fasteners must account for the unique needs of electronic applications, including electrical isolation, corrosion and vibration resistance, light weight, non-magnetic characteristics, and blind installation.
This article discusses different nylon fasteners, their selection, and their application. Examples from Essentra Components, including screws, nuts, snap rivets, barbed fasteners, and pop rivets, are used for illustrative purposes.
Why nylon for fasteners?
Nylon is a family of high-performance engineering thermoplastic linear polyamides with high mechanical strength, good fatigue, impact and corrosion resistance, and excellent electrical insulating properties. It is also light weight compared to metals such as aluminum and offers higher wear resistance. These properties make it ideal for electronic applications. As electronic devices shrink, weight reduction is a major design goal. From a reliability and a user experience standpoint, a loose metal fastener in an active electronic device can be a disaster; a loose, non-conductive nylon fastener is just an annoyance.
The most common types of nylon used in fasteners for engineering applications are nylon 6 and nylon 6/6. Nylon 6 is a polymer derived from multiple monomers, each with six carbon atoms. The chemical formula of nylon is (C6H11NO)n. Nylon 6/6 is made from two monomers, each with six carbon atoms: hence the name nylon 6/6.
Nylon 6/6 is the more commonly used material for fasteners due to its lower cost and slightly higher temperature range.
Fastener types
A fastener is a hardware device that mechanically joins two or more objects together permanently or non-permanently. Popular mechanical fasteners are screws, nuts, and rivets.
A screw is a fastener that uses a helical groove (thread) cut into the outside of a cylindrical body. It mates with a reverse helical groove cut into the inside of a mating hole. Turning the screw into the threaded hole advances it into the mating surface. A screw is primarily specified by its major diameter, thread pitch, length, head type, and drive type (Figure 1).
The thread pitch is the distance the screw advances in one turn or the distance between peaks of adjacent threads. In the United States (US), it is commonly specified by the thread count per unit length. The Unified Thread Standard (UTS) uses the inch (in.) as the unit length in North America. UTS screws are specified by their nominal diameter and thread count; for example, ¼-20 is a screw with a diameter of ¼ in. with a thread count of 20 threads per in.
Screws with diameters less than ¼ in. are specified by an integer number, 0 to 16, called its gauge. The larger the gauge number, the greater the diameter.
The major diameter of a screw can be determined by multiplying the gauge number by 0.013 and adding the product to 0.060 in. Thus, a #6 screw has a diameter of (6 * 0.013) + 0.060 or 0.138 in. (3.5 mm).
There is also a metric standard where screws are designated by the letter M followed by the screw’s major diameter in mm and then the thread pitch in mm.
An example of a UTS nylon screw, the Essentra Components 011024H100, is a 1 in. (25.4 mm) long nylon cap #10-24 screw with a hex head. The Essentra Components 0401024HN is a mating #10-24 nylon hex nut. #10 fasteners have a major diameter of 0.19 in. (4.826 mm).
The selection of head types depends on the application. Flat and oval heads with countersunk holes offer edge-free surfaces that do not snag clothing or objects. Socket and fillister head screws generally have higher head height and are often used in counterbored holes. Flat-bottomed screws such as binder, pan, and button heads are used to hold down wires in terminal strips. Hex and socket heads are intended for applications requiring high-torque tightening.
Slotted drives are the most basic drive type and are best driven manually. Phillips, hex, and socket drives are often used with electric drivers to keep the driver’s head from slipping off the screw.
Rivets
A rivet is a non-threaded mechanical fastener that holds components together. There are many types of nylon rivets, including snap-in, barbed or fir tree, push-in, mounting button, and blind or pop. Their installation characteristics include permanent, removable, and blind. Regardless of the type, all rivets require a pre-drilled hole through the joining components. Different rivet styles use a variety of anchoring mechanisms. For example, the Essentra Components SR-3075W snap rivet uses an expanding grommet (Figure 2).
After a snap rivet is inserted into a hole, the head is pushed in, and the grommet expands to anchor it. A rivet is specified by its head diameter, length, rivet diameter, and grip range. Grip range is the thickness of the fastened components over which the rivet will work correctly. The Essentra Components SR-3075W has a head diameter of 0.253 in. (6.4 mm), a length of 0.295 in. (7.5 mm), a rivet diameter of 0.118 in. (3 mm), and a grip range of 0.197 to 0.236 in. (5 to 5.99 mm). This type of nylon rivet can be removed by pulling up the head.
A barbed or fir tree rivet, such as the Essentra Components 27XT1250250, uses a ribbed shank to secure the components (Figure 3).
The ribs of the barbed rivet are angled to compress on insertion but lock into the sides of the hole to prevent withdrawal. These rivets work with metal, foam, rubber, plastic, and even wood components.
The Essentra Components 27XT1250250 is intended for a 0.125 in. (3.18 mm) hole. It has a length of 0.321 in. (8.15 mm) and a grip range of 0.062 to 0.250 in. (1.58 to 6.35 mm).
Another type of barbed rivet is a two-piece ratchet rivet, such as the Essentra Components BR1-226-01. This type of fastener works well to join thin components. It operates by joining the two ratchet rivet halves to form a single fastener (Figure 4).
The Essentra Components BR1-226-01 requires a hole diameter of 0.281 in. (7.14 mm). It is 0.226 in. (5.74 mm) long and has a grip range of 0.235 to 0.297 in. (5.97 to 7.54 mm).
Push rivets, such as the Essentra Components 61PR400600 (Figure 5), provide fast, reliable, and permanent attachment. These blind-side rivets do not require access to the reverse side of the joined components.
Push rivets are installed by inserting the body of the rivet into the panel hole and pressing the attached plunger into the assembly using an installation tool to expand the rivet. The Essentra Components 61PR400600 is 0.445 in. (11.3 mm) long and mounts in a 0.187 in. (4.75 mm) hole. It has a grip range of 0.078 to 0.375 in. (1.98 to 9.53 mm).
The Essentra Components 27AC0011 mounting button or canoe clip is an easily installed fastener for securing sheet metal parts or light components without tools (Figure 6).
The installation is as simple as pushing the rivet body into a pre-drilled round hole and pressing on the rivet head until it locks in place. This rivet fits into a 0.192 in. (4.9 mm) hole and is 0.515 in. (13.08 mm) long. It has a grip range of 0.165 to 0.185 in. (4.2 to 4.7 mm).
A pop or clinch rivet is another blind-insertion rivet that doesn’t require access to the reverse side of the panel. The Essentra Components CR32-2-1 precision molded nylon unit is an example (Figure 7).
The CR32-2-1 has a mandril that passes through the rivet body. The rivet is inserted into the hole, and a pop rivet tool is used to pull the mandril through the body of the rivet. This causes the rivet body to collapse, expand against the hole, and lock the components together. This rivet has a diameter of 0.126 in. (3.2 mm) and a length of 0.378 in. (9.6 mm). It requires a hole diameter of 0.130 in. (3.3 mm) and has a grip range of 0.157 to 0.276 in. (4.0 to 7.0 mm).
Each type of rivet is offered in a range of diameters and lengths to match an application’s requirements.
Specifying rivets
The Essentra Components catalog makes it easy to select the rivet size needed. The user begins by picking a type of rivet. Then, based on the thickness of the joining components, the catalog provides a range of suitable hole diameters and rivet lengths for that grip range (Figure 8).
A sample of snap rivet selections for a hole diameter of 0.083 to 0.087 in. (2.1 to 2.2 mm) shows seven suitable models. For a grip range of 0.205 to 0.225 in. (5.2 to 5.7 mm), the rivet measuring 0.295 in. (7.5 mm) long (line 7) is required to achieve the necessary holding thickness.
Conclusion
Nylon fasteners offer many advantages for electronics, appliances, and automotive applications. They are lighter than their metal counterparts, are non-conductive and corrosion resistant, and are available in many types and sizes.