- SPECIAL REPORTS
At first glance, the sexy little sportscar from Italy and the big brute truck from America appear to have nothing in common. The all-aluminum Ferrari 360 Modena is a sleek two-seater capable of accelerating from 0 to 60 in 4.3 seconds, while the Freightliner Century Class is a 330-hp tractor unit that can haul 80,000-pound loads from coast to coast. Although the two vehicles are worlds apart, they’re both held together with self-piercing rivets.
The fasteners are becoming an increasingly popular alternative to traditional spot welding. They pierce and fasten in one operation, simplifying assembly and reducing costs while providing a strong, reliable joint. Unlike conventional riveting, self-piercing technology does not require a predrilled hole. The rivet makes its own hole as it is being inserted.
Self-piercing rivets also emulate the results and quality of spot welding without many of the risks, such as toxic fumes, sparks and noise. And they boast static strengths similar to, or better than, spot welding along with superior fatigue performance.
"Pierce riveting is a clean, simple, fast and consistent cold-forming operation," notes Dean Monday, global product manager for Textron Fastening Systems (Rockford, IL). "There is no need for punching, drilling, cleaning or special treatment of material substrates, reducing component and installation costs."
In a single operation, self-piercing rivets join multiple layers of similar or dissimilar materials with varying thicknesses. "These materials include rolled, cast and extruded metals, molded and sheet polymers, nylon, and other durable types of webbing and fabric," Monday points out. "The result is a compact, highly stable, corrosion-resistant bond impermeable to gas, water and other external fluids."
How They WorkDuring the assembly process, a self-piercing rivet is driven into a material stack at a controlled force, piercing the top layer or layers. The rivet radially expands into the bottom layer or sheet, forming a strong mechanical interlock. The entire process takes less than 3 seconds, depending on cylinder stroke and feed tube length.
There are four basic processes in setting a self-piercing rivet. First, the rivet is delivered to the placing head. Once detected, the materials to be joined are clamped together by the riveting tool. The rivet is then driven down, piercing the first material and any intermediate materials. The rivet forces the sheet material into a die and radially flares, forming a mechanical interlock. The rivet does not break through the last material.
Assembly equipment can be stationary, robotic or integrated into an assembly cell, depending on production rates and complexity of parts joined. Typically, equipment is comprised of a support structure engineered to endure setting forces up to 50 kilonewtons, ensuring rivet alignment with the lower anvil or dye. The force used to install the rivet is generated by a hydraulic cylinder that drives a plunger against the rivet head. The anvil forces the rivet to expand radially into the lower sheet. "Because the system is simple, with few moving parts, the equipment has a long tool life—usually more than 250,000 cycles," claims Monday.
High setting forces are essential for rivet installation and critical to alignment of rivets to the lower anvil. Monday says this requires the installation equipment structure to be extremely rigid to prevent excessive bending or deflection during the joining process, which would create a joint of substandard quality. "The anvil is a solid cylinder with a cavity whose volume is similar to the material being displaced by the rivet," he explains. "The cavity crosssectional profile is dependent upon the formability of the bottom sheet and desired degree of rivet flare."
Many different types of tools and equipment are available to insert self-piercing rivets. Manual assembly is done with electric, pneumatic and battery-powered hand tools. Gravity-fed rivet delivery systems are commonly used with a stationary machine. Approximately 70 percent of self-piercing applications use robot-mounted equipment.
"The most common rivet delivery system when the customer is interested in a higher level of automation is either collating the rivet in plastic tape or blow feeding," says Monday. "Blow feeding eliminates the cost of the tape and its disposal."
Typically, the rivet setter and die are mounted in a C-frame, which must be large enough to allow access into the areas to be riveted. The placing head and die can operate independently as long as there is alignment during the placement of the fastener.
Self-piercing rivets are formed from steel and stainless steel wire into semitubular shapes. They are then heat-treated and plated. According to Al Koett, sales manager at AKH Inc. (Indianapolis), the optimal rivet has sufficient strength to pierce the top layers of material without collapse, but is sufficiently ductile to flare when contact is made with the anvil. Popular self-piercing rivet diameters include 3.2, 3.9, 4.8 and 5.3 millimeters, with body lengths of 3 to 9.5 millimeters. Standard head styles include countersunk, oval and tinman.
Mike LaPensee, vice president of applications and marketing at Henrob Corp. (Farmington Hills, MI), says 5-millimeter rivets tend to be the most popular size in North America. In Europe and Asia, 3-millimeter rivets are more popular.
Numerous BenefitsSelf-piercing rivets offer numerous benefits to assemblers. For instance, they have the same static tensile and peel strength as spot welds, but twice the fatigue life. Process monitoring can be integrated with self-piercing technology to verify that every joint created is of high quality. "Longer fatigue life and higher joint quality repeatability can reduce the number of fastening points required over spot welds in certain applications," says Monday.
Unlike spot welding, self-piercing rivets do not generate heat. This is important when using materials that degrade in thermal processing. Spot welding also can destroy coatings on steel, leaving it vulnerable to corrosion. Koett says self-piercing rivets don’t alter metallurgical properties like welding can.
Self-piercing rivets also eliminate heat problems associated with spot welding pure aluminum and aluminum alloys, which melt below 800 C. And, the fasteners are compatible with galvanized and coated surfaces, minimizing damage and eliminating recoating or replating, unlike spot welding.
"Self-piercing systems don’t require extensive monitoring equipment or smoke exhaust equipment associated with welding," notes Jerry Valka, president of Alternative Technologies Corp. (Warren, MI). He also points out that assemblers can eliminate costly electrode maintenance by switching to riveting.
"Traditional fastening methods are not as advantageous or are impossible in today’s new material joining requirements," adds Textron’s Monday. "Self-piercing technology requires no drilled or punched holes in the materials being joined, which reduces component cost. High joint strength allows the end user to reduce the number of fastening points, further reducing manufacturing cost while maintaining a consistent quality product."
Typically, a self-piercing rivet is more expensive than a similar size squeeze or buck rivet. "However, because the rivet insertion is an automated process vs. a manual process for bucking and squeezing, the in-place cost is generally less," claims Henrob’s LaPensee. "Compared with threaded fasteners and pin-and-collar fasteners, self-piercing rivets are less expensive. Coupled with the automated insertion, they result in a lower cost assembly method.
"Cycle time for a self-pierce riveting system is generally the same as spot welding steel and the same, or less, than spot welding aluminum," adds LaPensee. "It takes 1.3 seconds for a complete rivet and feed cycle at a 30 millimeter tool opening."
Self-piercing rivets also are environmentally friendly. "The joints are cleaner and quieter compared with other ways of joining metals, such as welding," notes Steve Bleakley, director of marketing for assembly systems at Emhart Fastening Teknologies (New Haven, CT). "It does not produce any harmful fumes or smells."
Material MattersSelf-piercing rivets can be used to fasten both dissimilar metal joints, such as steel to aluminum, as well as dissimilar materials, as long as the bottom material layer is a ductile material. An example is automotive sunroofs where the frame front and rear pieces are molded thermoplastic that are fastened to aluminum extrusion side rails.
"Sheet aluminum or steel with a total joint stack thickness of up to 6 millimeters for mild steels, such as 1008 and 1010, and 12 millimeters for mild aluminum, such as 5000 and 6000 series, are ideal of self-piercing rivets," claims LaPensee. "While this is ideal, there are many exceptions in production today, such as high-strength steels, aluminum castings, and multiple layer joints with metallic and nonmetallic layers."
According to Textron’s Monday, total material stack up depends on the materials, but generally ranges from 0.6 millimeter to 10 millimeters. "New application requirements are constantly expanding fastening capability ranges," explains Monday.
According to LaPensee, the minimum thickness for steel and aluminum is 1.6 millimeters. "Self-piercing technology desires to rivet from thin to thick material layers," says LaPensee. "If this is not possible, we recommend that the bottom layer thickness be no less than one-third the joint stack thickness."
The self-piercing process requires a somewhat ductile material as the bottom layer or tail side of the riveted joint. "Because of this, we can successfully rivet from thermoplastic into aluminum or steel but not plastic to plastic," notes LaPensee. "One possible solution for a nonmetallic to nonmetallic joint is to add a metal piece to the bottom of the joint."
An example of this occurred at Lotus Cars (Hethel, England) on a car body with a joint of fiberglass to fiberglass. "We modified the lower upset die tooling to accept a steel disk similar to a quarter dollar coin," explains LaPensee. "Once the rivet was inserted into the fiberglass joint, the result was a three-layer thick joint consisting of fiberglass on fiberglass on steel. The tail of the rivet was flared and anchored into the ductile steel."
Randy Leedy, project manager at Emhart Fastening Teknologies says a self-piercing rivet joint has virtually the same static strength as a spot-welding joint. "However, in dynamic situations, such as vibrating conditions that occur in a car, the self-pierce rivet joint has higher strength and stability under load," explains Leedy. "Under vibrating load conditions, the service life is twice as long as that with a conventional spot-welded joint."
Because self-pierce riveting is a mechanical fastening process, LaPensee says the joints need to be of a lap-type configuration. "In terms of part size or configuration, the only stipulation is that the riveter’s actuation cylinder, C-frame and nosepiece can access the joint," he points out. "Tooling access to both sides of the application is required. It’s also important to allow for die clearance, specify adequate flange dimensions and avoid closed box sections."
Assembly ApplicationsThe technology behind self-piercing rivets was originally designed for the construction industry more than 30 years ago. The first manufacturing application was the assembly of garage doors in Germany.
The fasteners have been slowly growing in popularity during the past decade, primarily due to the introduction of new materials, such as aluminum, high-strength steel, plastic and laminate steel.
"There has been tremendous growth in applications in a wide range of industries since 1990," says Monday. "This is directly attributable to the increase in joining dissimilar materials in production processes, and the need for fatigue resistant joints in high-cyclical loaded structures."
According to Monday, the typical automobile has 3,000 to 5,000 spot welds. But, increasing use of high-strength, thin-sheet steels, aluminum and composites has rendered this traditional procedure obsolete in applications where fatigue strength and joint reliability are required. "Pierce riveting fulfills the need for an assembly process that meets the modern challenges of light construction required by the auto industry and other industries," claims Monday.
Growing use of mixed materials has made the auto industry think beyond traditional spot welding in structural parts. Automotive applications planned or in production include hoods, doors, trunk lids, tailgates, seat rails, side-impact bars, space frames, window frames and sunroof frames.
While the largest growth potential is in the area of vehicle assembly, Emhart’s Bleakley points out that a number of other products contain a mix of aluminum and steel alloys, coated metals and plastics. Manufacturers of step ladders, air cargo containers, garage door frames, air conditioners, refrigerators, office furniture, bus bars and electrical enclosures have embraced self-pierced rivets or have expressed great interest in the joining technology.
"None of these advances in light construction would be possible without pierce riveting technology, which enhances structural integrity without the debilitating and hardening effects of spot weld heating," claims Monday. "Traditional fastening methods are not as advantageous or are impossible to use with today’s new material joining requirements. With pierced rivets, the ability to join together a wide range of materials offers product designers the latitude to create lighter weight, stronger, custom-tailored product with improved quality and increased manufacturing options."
European manufacturers have been more willing to embrace the technology. For instance, automakers such as Audi AG (Ingolstadt, Germany) and Jaguar Cars (Coventry, England) use self-piercing rivets to assemble space frames in aluminum-intense vehicles.
However, manufacturing engineers in the United States have been more reluctant to switch from spot welding to riveting. "Most engineers are more familiar with spot welding," says Bleakley. "They are comfortable with welding and know its parameters.
"Self-piercing technology is still new," adds Bleakley. "Unfortunately, it’s always compared to spot welding. Many people have a perception that welding is free. On the other hand, a fastener has a part number and a cost associated with it. As engineers start to recognize all the costs related to manufacturing, they realize that pierce riveting makes sense. We see a huge market as more people become familiar with the technology."
Henrob’s LaPensee says rivet manufacturers need to do a better job educating the marketplace. "Because self-piercing rivet technology is made up of both an assembly process and a fastener, it typically requires more disciplines to be involved to implement compared to other mechanical fasteners or welding," he explains.
LaPensee claims that design engineers, fastener engineers, manufacturing engineers and capital equipment buyers all need to be involved. "When you get them all in one room and present the engineering and manufacturing benefits, along with the payback via true in-place cost analysis, it becomes a sure thing," concludes LaPensee.