Blind fasteners are a type of fastener that is used in moddern assembly. Blind fasteners are often called the unsung heroes of modern construction because you will not be able to see the blind fastener once the job is complete, but blind fasteners holds many things together. Blind fasteners hold everything from aircraft skin to kitchen cabinets together, and do so without leaving a visible trace on the good side of the material.
If you have a situation in which you must join two sheets of material, but you only have access to one side of the material, blind fasteners are you best option. If the backside of the material must remain smooth and not include any fasteners, blind fasteners are still an option. Blind fasteners work by expanding, threading, and/or locking from the accessible face of the material to be fastened.
Blind fasteners have the same strength as nuts and bolts that is used in conventional joinings. Thus, if you understand the varieties of blind fasteners, you will save yourself time, money, and frustration. The choice of which blind fastener to use will depend upon the thickness of the materials to be joined, the loads that will be placed upon the joined materials, whether the joint will need to come apart at a later time, and how much money you are willing to spend on each blind fastener.
Each of these factors will lead to several different major types of blind fasteners to be considered for the various application. Rivets have been used for a long time, and many people do not realize how often rivets are used. Blind rivets, as their name suggest, are used when only one side of the material to be fastened is accessible.
Common Types of Blind Fasteners to Use
1. Blind Rivets
A blind rivet consists of a hollow rivet body that is placed into an predrilled hole in the material, and a mandrel that a rivet gun pulls through the hollow rivet body. The pulling of the mandrel through the hollow rivet body causes the end of the rivet body to mushroom outward, and the mandrel snaps at a weak point along the rivet. The benefit of blind rivets is that they can be used with single-side access to the material to be fastened.
Thus, blind rivets do not require the use of a bucking bar or a second person to hold a dolly against the blind side of the material with which the blind rivet is to be secured. Because blind rivets work in this way, aircraft manufacturers began to use blind rivets many decade ago. The joint formed by blind rivets is resistant to vibration, so the blind rivet joint will not loosen when the material is exposed to cyclic loads.
Thus, blind rivets are often used on HVAC systems and on aluminum boats. However, the downside of blind rivets is that the joint formed with the blind rivet is permanent. Once the blind rivet is set, it cant be undo.
2. Pop Rivets
Pop rivets are the name that most people use to refer to blind rivets. More specifically, pop rivets are the small, aluminum varieties of blind rivets that are sold in most hardware store. The name pop rivets refers to the sound that the mandrel makes when it snaps at the weak point along the blind rivet.
Pop rivets are used in applications that require relatively little strength to fasten the two sheets of material together. For instance, pop rivets can be used to attach signs, weather stripping, and metal bracket. Because many people like to use them, pop rivets can be installed in just a few seconds with an inexpensive hand tool.
However, the soft aluminum varieties of pop rivets cannot handle heavy shear load or extreme temperatures. The steel or stainless steel versions of pop rivets will allow for more strength to be present within the fastener. However, the installation of those versions of pop rivets require more installation pull force.
The thickness of the materials to be fastened together must be within the grip range of the pop rivet. If the thickness of the materials to fastened together is outside of the grip range of the pop rivet, the pop rivet will either not fully expand to allow for the fastener to lock into place, or the rivet may waste some of the material to be fastened and create an ugly looking button on one of the blind sides of the material. Another type of blind fastener is threaded insert, which are also referred to as rivet nuts or nutserts.
3. Threaded Inserts
Threaded inserts provide a variety of benefits when utilized in sheet metal. A threaded insert looks like a cross between a rivet and a nut. To utilize a threaded insert, the sheet metal is first predrilled.
You place the threaded insert into the predrilled sheet metal hole, and a tool of the proper size is used to pull the threaded section of the insert. As the insert is pulled, the body of the insert expands outward against the sheet metal that is to be fastened. When the tool is removed, the sheet metal will have a threaded hole in which another fastener can be placed.
The design of the threaded insert allows those who use the fastener to transform a thin sheet of metal into a site where bolts can be placed that are able to be tightened and loosened repeatedly. For instance, one of the jobs of automotive technicians is to use threaded inserts to mount brackets to the firewalls of the vehicles in which they work. The firewalls have a thin sheet of metal on their back side, which is inaccessible after the cars are assembled.
While this type of fastener has a setting tool that is more expensive than the tools needed to secure blind rivets, the reusability of the fasteners justifies the higher cost of those blind fasteners. Drive rivets utilize a different principle than any of the other types of blind fasteners. Rather than pulling a mandrel through a blind rivet to expand the blind side of that rivet, the drive rivet is simply hammered into the material with a hammer.
4. Drive Rivets
The pin that is included in the drive rivet will spread the slotted shank of the drive rivet against the sheet metal on the blind side of the material to be fastened, forming a bulge in the blind side that lock the fastener in place. Thus, no special gun is required to install drive rivets. Only a hammer and a punch (or even just the pin alone) are required.
Mechanics prefer drive rivets for tasks like attaching interior panels to trucks and heavy construction equipment. Drive rivets are much faster than other types of blind fasteners. After predrilling the panel, the mechanic merely has to insert the drive rivet into the hole, tap the rivet with a hammer, and the job is complete.
While drive rivets are not the strongest type of blind fastener, they have good resistance against shear forces, and they are very inexpensive to purchase. However, the downside of drive rivets is if those fasteners have to be removed from the material to which they are attached. In this case, a drill and a pry tool will have to be used to remove the drive rivet.
5. Blind Bolts
Blind bolts are used when the structure to which the fasteners will be attached must be able to withstand serious load. Blind bolts are essentially high-strength blind fasteners that contain a locking mechanism that allows the blind bolt to lock into place. Blind bolts often work by having a core bolt extend into the material to be fastened, and the bolt will pull a sleeve or a collar until the sleeve or collar locks into place.
The locking of the blind bolt into place involves the shearing of the visible collar of the blind bolt. Engineers who are designing structures that can withstand significant tensile or shear loads often use blind bolts. For instance, blind bolts are often used in the construction of bridges, railroad cars, and wind turbines.
Like pop rivets and threaded inserts, blind bolts require the use of a specialized tool to install the fastener. Thus, blind bolts will not be found in most hardware stores. However, if a structure requires both one-sided access to the components and structural integrity to the components of that structure, blind bolts are the only blind fastener that will meet those requirements.
However, the obvious trade-offs for using blind bolts are the high cost of purchasing the blind bolts, and the purchase of the tool that is required to install those blind fasteners. Expansion anchors are used in concrete and masonry materials. Thus, blind fasteners in this category are considered blind because the business end of the blind fastener is hidden within the drilled hole in the masonry material.
6. Expansion Anchors
One type of expansion blind fastener uses a slotted sleeve and a tapered plug. The expansion blind fastener is dropped into the drilled hole in the concrete, and then a bolt or nut is tightened. As the bolt is tightened, the plug is pulled upward within the hole, forcing the sleeve of the blind fastener to expand against the sides of the hole in the masonry material.
Contractors use blind fasteners of this type when they are mounting heavy machinery, handrails, and other fixtures onto concrete whose back side does not exist. The holding power of the blind fastener is strong if the hole is of the correct diameter and depth. However, the downside to blind expansion fasteners is that the installation of the blind fastener is critical to the success of the installation of the blind fastener.
If the installation of the blind fastener was sloppy, then the blind fastener will not be able to provide structural support to the component being mounted to the masonry material. Thus, the blind fastener will not be able to perform its function of providing support, and will merely be an expensive decoration for the structure. Self-drilling blind fasteners are used in construction sites.
7. Self-drilling Blind Fasteners
These blind fasteners are also often referred to as Tek screws. Self-drilling blind fasteners have a drill-point on one end and a hex washer head on the other. To install a self-drilling blind fastener, the fastener can simply be driven into the material with an impact driver or a screw gun.
The drill-point of the fastener will drill into the material, the fastener will cut its own threads in the material, and the washer will seal the joint against water. Roofers and metal building erectors use these types of blind fasteners. For instance, the self-drilling blind fasteners allow those individuals to fasten corrugated metal panels to metal beams without having to drill a hole in the metal panels first.
These blind fasteners are not truly blind in the same way as the others discussed in this article, because the fastener is visible on the outside of the metal panel. However, the self-drilling blind fasteners have a benefit in that they can save time when attaching metal panels. However, the thickness of the metal panel must be within a certain range for the self-drilling blind fastener to still have its advantage.
If the metal panel is too thick with respect to the diameter of the self-drilling blind fastener, then the self-drilling blind fastener should be predrilled into the metal panel. Another class of blind fasteners are speed clips and plastic blind fasteners. These types of fasteners are typically used in interior surfaces of vehicles, or for the attachment of electronic components.
8. Speed Clips and Plastic Blind Fasteners
These one-piece fasteners have a snap-in feature that allows them to snap into a square or round hole in the component to be fastened. The rest of the blind fastener has plastic legs or barbs that expand against the interior of the component. Some of the examples of plastic blind fasteners are the plastic Christmas-tree clips that are used to attach the interior door panels of cars, or the push-in retainers that are used to fasten wire looms.
Plastic blind fasteners are useful because they dont require tools to be installed. A flat-blade screwdriver or even one’s strong fingers can be used to pop the blind fastener out of the component. Thus, plastic blind fasteners are not used to support any structural loads.
However, plastic blind fasteners are fast, cheap, and reliable. For instance, plastic blind fasteners can be used to attach interior trim components, deaden sounds radiating from the vehicle, or to attach lightweight cover components. However, plastic blind fasteners should not be re-used more than a couple of times.
The plastic barbs will fatigue if used repeatedly in the same location. Each of the blind fasteners discussed above solve the same problem of being able to join two components of a product without being able to see the fastener, or without leaving a visible mark on the good side of the materials. However, each of those blind fasteners does so in a different way.
Some fasteners are created to allow for the quick installation of components. Others are designed to provide significant strength to the joint that they form between the two components to which they are attached. Others are created to be re-used over time in the same locations.
Regardless of which type of blind fastener is used, there is a sense of satisfaction in knowing that there is a joint between the components of a product that cannot be seen, but that joint is performing the same function of the joint that would be formed by the visible fastener. Although the blind fasteners may not make the news today, the appropriate choice of blind fastener will determine if a product will hold together for decades, or whether it will come apart at a bad moment. Take the time to choose the correct blind fastener for each application, and you will find that the less-visible half of the product will continue to stand strong beside the more-visible half.
