Self-piercing rivets help automakers assemble aluminium, plastics, high-strength steel and other difficult materials.
To increase fuel economy, automakers constantly look for ways to reduce overall vehicle weight.
One of those ways is to use lightweight materials, such as aluminium, plastics, composites and high-strength steels, in car and truck structures. However, these new materials can be difficult to join with spot welding.
Self-piercing rivets are one answer to the problem. Self-piercing riveting is a process for joining two or more materials with an engineered rivet.
Unlike conventional riveting, self-piercing rivets do not require predrilled holes. The rivet is driven into a material stack at a controlled force, piercing the top layer or layers and flaring into the bottom layer to form a strong, secure joint.
The process offers several benefits to assemblers. It can join dissimilar materials, such as plastic and aluminium, and hard-to-weld materials, such as coated sheet metal. It can also join three or more layers of material.
The low-noise, low-energy process is consistent and repeatable. It does not create fumes or sparks, and it is easy to automate. Cycle times are equal to, or better than, those for spot welding.
And, it produces better joints in both aluminium and high-strength, low-alloy steel, compared with spot welding.
A self-piercing riveting system consists of a hydraulic power pack, rivet setter, upsetting die, C-frame, and hoses, control triggers and other accessories.
The power pack provides hydraulic pressure to the rivet setter and controls the sequencing of the tool. The power pack is controlled either by a remote manual switch or a programmable logic controller.
Using force supplied from the power pack, the rivet setter squeezes the rivet into the materials to be joined. The rivet setter can be equipped with various accessories for automatic rivet feeding and for operation in fixed or mobile, manual or automated assembly. It can even be attached to a robotic arm.
The rivet setter and die are mounted in a C-frame, which can withstand riveting loads of 60 kilonewtons. The C-frame must be large enough to allow access into the areas to be riveted.
For many manual applications, the rivet setter can be hung from an over-head tool balancer and moved around the workplace as required.
Alternatively, the riveting equipment can be fixed and parts handled manually or robotically.
Self-piercing riveting equipment was originally designed for manual operation.
The rivets are fed automatically into the tool using a plastic rivet tape. An indexing mechanism for the tape is integrated into the nosepiece.
Although this equipment is reliable for manual and low-volume semiautomated applications, certain features are necessary for high-volume automated assembly--especially for robot-mounted equipment.
These features include:
- Sprocket-driven tape for extended operation
- Home position sensor
- End-of-tape sensor
- Partial retraction of the rivet setter for faster cycle times
- Exit tape cutter
- Powered spool winder
- Tool equaliser
- Tool changer.
Self-piercing rivets were originally designed for the construction and white goods industries, where they continue to be used with great success. However, the technology initially proved unsuitable for the automotive sector for several reasons:
- Breakthrough of the rivet tail, which could allow a leak path into the joint
- Insufficient joint fatigue strength
- Noncontrolled distortion
- Flange "quilting" and seam gaps.
To overcome these shortcomings, Henrob worked with Audi to improve the process. A new rivet design and a high-force, preclamping rivet setter were developed, and the new technology was first used in Audi's all-aluminium A8 vehicle.
Today, many automotive OEMs and their suppliers use self-piercing rivets to join steel and aluminium parts in primary vehicle structures and subassemblies.
These companies include Ford, General Motors, DaimlerChrysler, BMW, Freighliner, Mercedes, Volvo, Peugeot, Mazda, Honda, Toyota, Magna, Tower and Meritor.
During development of the Audi A8, the method for joining the aluminium structural panels emerged as a critical issue. The panels had to meet certain fatigue life and crash test requirements.
Spot welding techniques could not provide the required joint characteristics, so other fastening methods, including clinch joining and self-piercing riveting, were evaluated.
Following extensive crash, fatigue and environmental testing, self-piercing riveting was adopted as the preferred single-point fastening method.
Though approximately 70% of the single-point joints are riveted, not all the joints could use this technique because of access limitations. The body had originally been designed for spot welding.
At Freightliner, the traditional method of assembling truck cabs was to drill holes in the skin and structure and manually fit pin-and-collar fasteners or blind fasteners. This was labour-intensive, expensive and prone to variations in quality.
During the design of the new Century Class cab, Freightliner decided to use self-piercing rivets as the primary fastening technology to reduce production costs.
By adopting self-piercing rivets, Freightliner:
- Eliminated manual skin drilling and rivet setting
- Improved fatigue life of joints
- Automated the assembly process
- Replaced expensive multipiece fasteners with less expensive rivets
- Achieved consistent joint quality.
To reduce vehicle weight, Volvo Truck chose to use thinner gauge, high-strength steel for the structure of its new truck cab.
During development, engineers noted that the fatigue life of the spot-welded joints did not increase in line with the strength of the steel.
Fatigue failure occurred where the engine cover was joined to the fire wall.
Volvo's first response was to add reinforcing plates in the affected areas, but that added weight and cost to the design. Volvo then tested a cab structure assembled with self-piercing rivets without reinforcement plates.
This structure design survived twice the normal test duration without failure. By using self-piercing rivets, Volvo eliminated the reinforcing plates and generated a net savings of $244,000 per year.
A sunroof frame manufactured for a Japanese carmaker is assembled from extruded aluminium side rails and a moulded plastic front header.
This material combination was difficult to join in an efficient, automated manner using conventional fastening techniques.
By eliminating the need to drill holes in the header and side rails, self-piercing rivets allowed the assembly process to be automated. Production time and cost were drastically reduced.
Self-piercing riveting has gained acceptance as a joining technology for automotive vehicle structures and subassemblies. The technology can improve joint properties and reduce overall production costs.
In many cases self-piercing rivets can be incorporated into existing product designs.
However, for increased benefit, the process and tooling requirements of the technology should be taken into account during the product design stage.