Parts can be fastened by a rivet if flat parallel surfaces exist for both the rivet clinch and head, and there is adequate space for the rivet driver during clinching.
When a blind rivet is set, a self-contained mechanical feature expands the rivet's shank, securing the parts being joined. These rivets are most often installed in joints which are accessible from only one side. Blind rivets are also used to simplify assembly, improve appearance, or decrease cost where both sides of the joint are accessible. Blind riveting offers portability -- the riveting can be brought to the work. This is valuable for large assemblies.
Blind rivets should be used where:
- Fastener removal is not necessary for maintenance.
- High vibration exists.
- A temporary fastener is needed.
- Uniform clamping is desirable.
- Repair fasteners for field use by untrained personnel are needed.
Blind rivets are classified as pull mandrel, threaded, or drive pin, according to the methods by which they are set.
Physical characteristics: Blind rivets have three different core styles, retaining different amounts of the pin or mandrel after they are set. A filled rivet contains enough of the mandrel or pin that the break point of the mandrel or the end of the pin is approximately flush with the top of the rivet head. This style provides high shear strength. A semifilled rivet contains a short length of the mandrel in its core. A hollow rivet has a completely empty core and is advantageous when light weight is important. It is also useful in electronic applications because it eliminates the possibility of loose mandrels disrupting circuits and leaves a grommeted hole for ventilation or to pass wires through.
Diameter is based on the measurement of a blind rivet's shank, usually in 0.03125-in. increments. Grip range is the range of material thickness that can be joined properly with a blind rivet of a given length. Manufacturers furnish specific recommendations. Sealing properties vary considerably, depending on the style of the blind end. If sealing is critical, it should always be checked under operating conditions.
Design considerations: Joint design factors that must be known include allowable tolerances of rivet length versus assembly thickness, type and magnitude of loading, hole clearance, and joint configuration.
Power tools install most rivets efficiently. However, drive-pin rivets are most quickly installed with a hammer. Manual installation tools can be used efficiently with little or no training.
Blind-rivet joints are usually loaded in shear, which the rivets can support better than tensile loading. Rivets subject to vibration perform more efficiently if manufacturer-specified minimum hole clearance is maintained.
Materials can be as thin as 0.020 in. with some rivets. If one component is of compressible material, rivets with extra-large head diameters should be used on that side of the application to distribute the load over a larger area.
Installation costs: Blind rivets cost slightly more than conventional tubular ones, but they allow faster installation and reduced assembly costs that can make up the initial price difference. Originally for fastening applications where only one side of the workpiece was accessible, these rivets are often used in other applications to reduce assembly time.
Break-stem blind rivets consist of a body and mandrel. To set the rivet, the placement tool engages the mandrel and provides an axial pull. This causes the mandrel head to upset the tail of the rivet, forming a blind head. When the mandrel reaches its designed tensile load, it breaks at a predetermined point on its shank. The portion in the tool is then discarded.
Cost savings provided by break-stem blind riveting are somewhat reduced because rivets usually are handled individually, and the broken mandrels must be removed from the work area. Broken mandrels sometimes are left in a workpiece, resulting in further production delays and associated costs. These disadvantages can be overcome by cartridge-fed blind-riveting systems. Such systems install packaged sets of rivets with a tool having a built-in permanent mandrel. Production rates for this system may be many times faster than those of break-stem systems, with a proportionate reduction in assembly costs.
Vacuum collection is another way to overcome the problem of broken mandrels. For example, one system uses pneumatic power to install rivets as well as to generate a vacuum that holds rivets in place before installation and vacuums away spent mandrels as the rivet is installed.
Drive-pin rivets also eliminate the broken-mandrel problem. The pin remains in this type of blind rivet, increasing its shear strength as well.
Rivet advantages: Some advantages of rivets include:
- Initial rivet cost, labor costs, and machine time to set the rivets in parts are lower than those of threaded fasteners. This is true because rivets are made in large volumes on high-speed heading machines with little scrap loss.
- Dissimilar materials of various thicknesses can be joined.
- Rivets can have a variety of finishes such as plating, parkerizing, or paint.
- Rivets can serve as fasteners, pivot shafts, spacers, electric contacts, stops, or inserts.
- Parts that are painted or have other finishes can be fastened by rivets.
On the negative side, tensile and fatigue strengths of rivets are lower than comparable-diameter bolts and machine screws with nuts, though tensile strength for …-in.-diameter blind rivets can be over 2,300 lb. However, rivets are the least susceptible to vibration loosening. That is why aircraft manufacturers rely heavily on rivets. While riveted joints are weatherproof, they are not normally sealed against air or water under pressure. A sealing compound, rivet coating, or special washer may be used but at added cost.
Rivets produced in volume are not normally made with the same precision as screw-machine parts. Normal tolerances on major dimensions are 0.005 in., although closer shank-diameter tolerances can be held. Rivets should not be used where dimensional variation must be maintained as low as 0.001 in.