Appliance Design - February 2008 - (Page 45) JOINING throughout the project. Although the riveted joints held during an unbalanced load test, the engineers had to increase the steel thickness from 16 gauge to 11 gauge mild steel to overcome frame buckling. Pan head rivets 5 mm diameter were selected for the design. Lap shear tests showed 2,824 lbs. of static pull strength and 480 in.-lbs. of energy to failure per self-pierce riveted joint. Tests of unbalanced loads at high speeds over 1,000 hours showed no distortion with performance equivalent to welded joints. Another group of appliance engineers was looking for a new way to fasten the corner seams on 300 series stainless-steel refrigerator doors. Originally, a combination of automated and manual plasma welding was used, but with buffing and dressing operations needed to improve appearance, this method could not meet demands for a significant increase in production. In addition, the decision to use prepainted and laminated doors on some models completely eliminated welding as a viable assembly option. These engineers also selected a self-pierce fastening system. SPR causes no damage to the pre-painted and laminated surfaces on the application. This eliminates the need for clean up, which in turn helps shorten production time. The SPR system can also rivet right through a protective sheet on the stainless steel without joint deterioration. This allows the protective sheet to stay in place during assembly and through delivery, minimizing possible surface damage during transit. Backside of SPr joint with trifuricating rivet (left) and standard rivet (right). Instead of choosing between mechanical fasteners and an adhesive, one obtains the benefits of both by “rivbonding,” where SPR is used with intermediate layers of adhesives. Self-pierce rivets are usually made from high-quality grades of steel wire that has been forged, hardened and tempered to provide optimal joint properties. However, the rivets can also be made from austenitic or martensitic stainless steel; copper for special applications; or aluminum when joining soft grades of aluminum material. A variety of head styles can be chosen to meet functional and/or aesthetic requirements. Even threaded and special studs are available, which are ideal for stand-offs and electrical grounding applications. applications Standard rivet designs meet the needs of most applications, although rivet geometry and installation settings must be configured to ensure success. Sometimes more advanced configurations are required for successful SPR joints. For example, to reduce materials to the bare minimum, one group of appliance engineers selected 2.2 mm plastic and 1.2 mm galvanized steel for a new dishwasher design. They considered self-pierce fastening from the start, but variations in the thickness of plastic material caused inconsistent joint strength when using standard selfpierce processes. Also, the steel thickness was at the minimum required to prevent breakthrough of the fastener. The challenge was solved by the development of an automated, trifurcating selfpierce riveting process, using 300 series stainless steel rivets. The process pierces the material, then splits the rivet into three www.applianceDESIGN.com legs on the tail side. Modifications to the die minimizes material drawdown during riveting and prevents sharp edges on the tail side of the joint. In-house testing showed that this installation process consistently produces highstrength joints. The 300 series stainless-steel rivets and net hole (no tolerances) condition create corrosion-proof and leak-proof joints. Trifurcating dies have been used successfully on other applications, as well. They have proved useful with thin sheet applications or those with a thick piece of plastic (top sheet) attached to a thin metal part (bottom sheet). One current use is automotive airbag applications, joining 3.0 mm-3.5 mm ABS to 0.7 mm-1.0 mm steel. Self-pierce riveting is still a great choice for heavier or more traditional materials. During the design of a new commercial laundry washer frame in mild steel, the project engineers determined that they wanted mechanical fasteners to maintain high quality, durable joints possessing high fatigue strength that could withstand heavy cyclic loads. They selected self-pierce riveting, and Henrob Corp. locations worldwide provided engineering support and joint development Design considerations With standard SPR, the head side of the joint can be either flush or raised, while the tail side will have a 1 mm to 3 mm high protrusion, depending on the rivet size. Testing of these SPr joints showed 2,800 lbs. of static pull strength and 480 in.-lbs. of energy to failure. applianceDESIGN February 2008 45 http://www.appliancedesign.com
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