Getting The Right Fit

Understanding the nuts and bolts of an operation is essential for any engineer. Yet the physical nuts and bolts used in a product or component can all too often come as an after thought in the design process. A single aircraft can use several million fasteners while the automotive industry get through billions every year. If designers specify the wrong kind, it can make a dramatic difference in cost and weight, affect reliability and even compromise safety. With numerous fasteners on the market, it is important to have the right one for the job. In non-critical applications, engineers often select fasteners on an arbitrary basis; either because they are in stock or, simply, are what is normally used. “Design engineers have a lot of duties and you can’t expect them to be a specialist in fastener connections,” says Thomas Jakob, a design engineer at Arnold Umformtechnik. The German based company largely supplies to the automotive industry and it says that a close working relationship with its customers is vital for both industries to thrive. “There are a lot of unknowns and new innovations,” adds Jakob. “The screw dimension is not normally very complicated. It’s more the connection of different materials that can cause issues such as corrosion or relaxation. “So it is an advantage to the design engineer when he has a strong partnership with a fastener specialist. Our customers usually have a very specific idea of how to make a product. But we can go in and suggest areas where they can actually save money by doing something a different way or by using a different fastener.” Arnold is keen to make its customers’ production lines as streamlined as possible and, as a result, recently carried out a study that led to the development of a thread-forming screw – the Taptite. The self-tapping threads showed enormous potential to eliminate complete process steps involved in the machining of nut-threaded components. The screws use a trilobular shaft and produce a low tapping torque ideal for use with light metals. It not only cuts out the need to drill and form a thread in each hole, but also all but eliminates the cutting emulsion and measuring instruments that were previously needed. It has also developed an alternative to welded-in bolts known as the Rivtex Clinch System. These consist of clinching studs and self-piercing nuts. “Clinching parts are mostly used where you press in a bolt,” says Jakob. “Very often you see welded bolts on a part but that is really expensive to do. You also have problems with thermal energy and the heating of the connection, and that effects the corrosion protection.” Although both of these innovative screws are more expensive, the lifetime cost saving from eliminating processes is vastly greater, especially given the throughput of the automotive industry. The company is also exploring the possibilities presented by more exotic materials. This is being driven by the demand from the automotive industry to lighten structures. “The weight of cars is actually increasing because of airbags, more safety equipments, more motors and electronics,” says Jakob. “Yet the requirements for CO2 emissions are getting tougher and tougher. So one thing to do is bring down the weight of the car. So we must start to discuss with our customers the possibilities presented by new materials like magnesium, aluminium and plastic.” However, for the aerospace industry weight has always been a big consideration. As a result, more exotic materials are all too common on its fasteners. But supplying to the aerospace industry provides its own set of unique challenges. Fasteners are becoming precision-engineered components with very fine tolerance requirements. And the relatively small production volumes, in comparison to the automotive industry, mean meticulous quality assurance methods can be put in place. “The quality standards are almost on a microscopic level,” says Kevin Peacock, an application engineer for Spiralock – a US based fastening company that supplies to the aerospace, medical, oil and gas, and machinery industries. “The cleanliness, the testing, it’s a very thorough process. “You can’t get away with any little scratches or micro-cracks. The fasteners need every bit of strength they can get out of the material. Any small or even microscopic defect could cause a failure. On a spacecraft launch, for example, that could be catastrophic.” Spiralock use x-ray inspections, eddy current tests to check for microscopic cracks, and visual inspection under a microscope. Even something as small as a burr has to be removed as it could become dislodged and possibly cause all kinds of damage. The company has made a name for its self in the fastener industry as a result of its innovative thread design. Most locking threads use some sort of interference between the male and the female to provide the locking force. However, over time, these can loosen because the tolerance between the male and the female threads creates a gap. This tiny gap between the two opposing ‘V’ shaped threads leaves room for movement under vibration. “Spiralock’s thread profile means the male and the female make spiral contact all the way around the thread,” says Peacock. “The aerospace and aircraft industry needed something that is re-useable and satisfies the locking requirement. There are other locking mechanisms out there but they are basically one-time use or can’t hold up to the heat and the stresses experienced in aerospace.” Where am I going wrong? Advice from a pro… The most common pitfall is that designers don’t understand the load that they are trying to hold together with the fasteners. “Ultimately it is the amount of clamp force that they are going to need,” says Kevin Peacock, an application engineer for Spiralock, a fastener specialist. “And the material in the bolt and nut has to give you well above the safety factor to be able to hold that together.” But as well as strength, engineers must also consider what environmental conditions the fastener has to cope with such as heat, vibration or corrosion. Another common pitfall that Peacock and his team come across is relaxation. He explains that if the surface finish between the bolt and the component has rough peaks and valleys, over time these can work themselves out and allow the fastener to move. Over time, it can gradually work itself loose. “People think they are going to save money by not machining a surface but end up costing themselves more money,” he says. “The other common problem is designers don’t use long enough bolts. “A bolts actually behaves much like a spring. When it is tightened down it actually stretches. The longer the bolt, the more stretch and spring it has. It gives it more resistance to the loads, the shock and the vibration. It’s known as short grip length.” When bolts do fail Peacock says that it is often due to miscalculation and a lack of understanding by an engineer. “They can underestimate the clamp force and tension that is needed on the bolts,” he says. “That’s normally the biggest cause of failure out in the field. “Ninety percent of the time its improper torque being applied. But it is a very simple formula to figure out. Quite often engineers will prefer to take figures off a chart, which can be totally inappropriate for what they are doing.” Pointers * T = KDF. This will give you an excellent approximation for most applications. T = Input Torque, K = Friction Factor, D = Nominal Diameter, F = Clamping Force * It is important to consider the complete connection over the lifetime of a unit. It is therefore vital to get the right material pairing of the bolt and nut, the right screw design, the right thread and the right corrosion protection.