Recently, changing business trends have become a key element in engineering departments across the globe. Designs are driven by market demand, profitability, legal regulations, and a myriad of other issues. For the engineer this presents a host of new challenges, even including fastener selection.
"One of the most significant trends we see is global environmental regulations," says Mike Hollis, Sr. Materials and Fastening Engineer at Delphi Automotive. "In Europe, for example, you may have to dismantle and reclaim materials you sell, which means you would have to select fasteners that can be removed."
This would increase usage of threaded fasteners and snap-fit designs, while reducing the reliance on fastening technologies that do not permit disassembly.
In addition, environmental regulations will impact the selection of plating and lubricants, which can directly affect torque/tension specifications, change fastener performance, and possibly require new installation equipment. Adhesives are also impacted as companies move from solvent-based to water-based formulas that are more environmentally friendly. Unfortunately, the water-based formulas may not have all the performance benefits of solvent-based formulas. Plus, there is a learning curve when new technologies are introduced.
Other issues affecting fastener selection include increasing assembly productivity, a move towards lighter weight materials, shorter product design cycles, vendor consolidation, standardization and globalization. Also, the fastening challenges will vary by industry. In the automotive industry, their challenges include environmental regulations, cost, global manufacturing, and fuel conservation. Some aerospace engineers face the task of developing low-observable (stealth) military aircraft. In electronics, it's increasingly shortened product design cycles, smaller products, and high levels of product cleanliness.
How does an engineer develop a robust product in the face of these ever-increasing challenges? The most significant step is to consider fastener selection early in the design stage. This allows you to choose from a broader array of fasteners and make adjustments in product design when it is the least expensive. This is particularly important when using new materials, such as engineered plastics and magnesium.
As an example, an automotive company asked us to solve a fastening problem it was having with an instrument panel after the design was fixed and mold tooling was made. The plastic material they were using would not accommodate any existing fastener designs. To solve the problem, we had to develop a custom-designed fastener, which increased the price of the application.
Perhaps the fastest-growing material is magnesium. Originally used in automotive applications, magnesium's light weight and heat sink properties make it an ideal material for a variety of products, such as electronics, power tools, and many other applications. However, its brittle nature makes it extremely difficult to fasten. Most companies are pre-tapping holes in the housing and using machine screws to fasten magnesium components. This adds cost, as tapping operations can cost up to 5˘ a hole, increase the risk of cross threading, and provide a less than robust joint. To alleviate the issues of fastening magnesium, Camcar Textron developed the Mag-Form® thread-forming fastener, which provides strong mating threads without hole tapping operations.
It is also critical that the engineer identify and prioritize their expectations of the fastener and the joint. Often, engineers expect the fastener to do more than it is designed to do. This has become more common as fasteners are downsized and loaded to a greater percentage of their capacity, which means there is often less safety margin in the joint. I've seen instances where the engineer tried to use an aluminum fastener to take advantage of its weight and corrosion-resistance properties, but found it wouldn't meet the strength requirements of the joint. Unfortunately, the "universal screw" for all applications does not exist.
However, with the advent of new fastener designs, the engineer can leverage a variety of fastener features into the same envelope. For example, computer hard drive manufacturers require fasteners that overcome galling, eliminate cross threading and are ideal for high-speed, automated assembly conditions. A common fastener design is a Camtronic® anti-galling fastener, with a special anti-cross-threading point and the TORX PLUS® Autosert® Drive System. By combining multiple features on the same fastener, the engineer is able to solve a multitude of potential manufacturing and quality problems.
While a fastener with multiple features may have a higher per-piece price, it is often significantly less expensive in terms of overall fastening cost. We completed a product teardown on a refrigerator and saved the manufacturer $1 million annually by proposing a higher-cost, yet higher-performing, Crimptite® self-tapping fastener. This allowed them to reduce the thickness of the sheet metal on the back of the appliance, while virtually eliminating strip-out.
It is possible to reduce costs and enhance joint design by fastener commonization. This requires maintaining a fastener design database and using fasteners from that list for new product designs. By using fasteners that have worked successfully in the past, you can reduce the amount of trial and error, leverage purchasing volumes, and reduce inventory levels. This is particularly valuable for companies with global design and manufacturing locations.
When implementing common fastener designs, there is a tendency to utilize standard fasteners or inefficient fastening methods.
"A manufacturer of a global positioning system uses 40 machine screws with brass inserts on one of their units because that's what they've always done," says Duane Baumler, Camcar Textron Electronics Product Specialist. "A less costly design would have utilized thread-forming screws for plastics and eliminated the brass inserts."
Design for assembly and manufacturing (DFA/DFM) also impacts fastener selection, particularly when design and manufacturing are in different countries. World class companies are integrating manufacturing and design engineering, because manufacturing efficiency is so critical to company performance.
"What we design here has to consider the markets, laws, and regulations of other countries," said Delphi's Mike Hollis. "This means we have to design products that are modular, flexible and interchangeable."
Fasteners are becoming an increasingly critical element of a product's design. Through proper design and fastener utilization, an engineer can enhance product quality, lower in-place costs and improve assembly productivity. The real key is early supplier involvement.
Checklist for fastener selection
Involve suppliers early. Leverage the engineering expertise of your fastener suppliers at the beginning of the design stage.
Leverage existing parts list. When possible, use fasteners that are currently in your system to reduce inventory, per-piece costs and design time.
Consider component manufacturability. Can the fastener or component you've designed be cost-effectively manufactured?
Identify and prioritize expectations of the fastened joint. This will help to ensure critical performance characteristics are achieved. It will also drive fastener selection.
Test. Test. Test. To verify performance, it is essential that you test critical joints prior to manufacturing.
Look at fasteners and assembly early in the design stage. By the time the CAD drawing for the end product is complete, up to 80% of the product's cost is fixed, making changes extremely expensive.
Use DFA/DFM principles. Design for assembly and manufacturing techniques can ensure manufacturability of completed designs.
Identify service issues during design. Consider what aspects of the end product will require service and design to allow access to and removal of fasteners
Consider all fastening technologies. It's essential you choose the most appropriate fastening technology, including threaded fasteners, rivets, adhesive and snap-fits.