Installation of Fasteners
Proper manufacturing and selection are important first steps to prevent fastener failure. But, installation is where the battle is won or lost—by not overtightening or undertightening fasteners, and maintaining their thread integrity. “Overtightening can lead to breaking the fastener during assembly, which is obvious, or it can lead to thread stripping, which might not be so obvious,” notes Medcalf. “The latter is particularly dangerous because the integrity of the bolted joint may be compromised and it goes undetected by the assembler.”
Menke says a proper understanding of the fastener lubricity (friction and torque coefficients, etc.) helps the end-user obtain the optimum clamp load when using torque. However, unwanted lubricant on the fastener, such as a bit of oil from the assembler’s hands, can cause overtightening. This changes the torque-tension value of the joint by increasing the clamp load.
When a fastener is undertightened to a lower clamp load, cyclical or fluctuating loads in excess of that clamp load can soon lead to fatigue.Medcalf says that undertightened fasteners undergo more external loading than expected. This means undertightening can lead to anything from small annoyances, such as squeaks and rattles, all the way up to catastrophic fatigue failure.
Undertightening can result from embedment relaxation, which occurs, for example, when a bolt embeds in a soft material and can’t completely compress the joint. Improper nut running is another cause of undertightening, says Avellon. When nuts are driven down too fast, there is a rebound effect and the joint relaxes somewhat. Also, tightening the bolt head rather than the nut can lessen clamp load by 10 to 15 percent due to torsional relaxation of the bolt length.
Threads need to be kept as clean as possible, notwithstanding a slight lubricant to ease installation and disassembly. Even minute particulate matter on threads will lower a fastener’s coefficient of friction during installation. This increases joint tension and stress, making the fastener susceptible to failure. A fastener coefficient of friction of 0.15 is average, but varies based on lubricant and fastener plating.
Cross-threading can sometimes be remedied by re-engaging the parts at a different angle. However, if thread geometry is inaccurate, the parts may need to be repaired or replaced. If cross-threading is left unfixed, the fastener may not properly seat and its threads will be unable to sustain the clamp load. Galling occurs when pressure and friction cause bolt threads to seize to the threads of a nut or tapped hole. According to Stephan, a fastener that has seized up from galling is typically impossible to remove without cutting the bolt or splitting the nut.
“Manufacturers used to have engineers who specialized in fastening, but many fastener specialists are reaching the end of their careers,” concludes Medcalf. “Because fastening is a complex engineering discipline, if that knowledge is not replaced there is certainly a risk of more failures.”
Quality fastening requires perfect matching of the fastener to the materials being joined, as well as the fastener (if it’s a bolt) to the nut. Using a Grade 8 bolt with a Grade 5 nut, for example, will produce lower-than-expected clamping force. “Proper fastener selection is more critical than ever because of product miniaturization,” claims Joe Stephan, application engineer at Bossard North America. “Fasteners are being asked to handle the same loads but in a smaller space with lighter and thinner materials.” Re-using bolts and nuts that ‘look good’ also isn’t recommended, says Avellon. He recalls visiting one government-run vehicle-repair facility and finding three grades of nuts in the same bin used by workers.
“Threads on re-used nuts are slightly softer than those of the mating bolt,” explains Avellon. “Plus, these threads are compressed and continually lose clamp load with each additional use due to increased thread friction. You can see the effects of this on wheel studs, which often fail after 120,000 miles or so.” Engineers too often focus only on fastener diameter, length, amount and aesthetics during joint design, instead of how these characteristics can lead to failure.
A product designer should never assume that specifying large-diameter or high-strength fasteners means fewer such fasteners are required for an assembly. Four years ago, a small Savanna VGW propeller aircraft crashed in Helena, MT, due to two undersized bolts that failed during a touch-and-go landing.
The 0.25-inch-diameter bolts are supposed to hold the landing wheel’s nose fork in place, but failed because of overload due to repeated use. After the failure, the landing wheel bent and the plane flipped over and skidded across a large section of turf. The pilot suffered minor injuries, but the plane had extensive damage. “Without real-world testing, it can be easy to make assumptions based on internal design guides and VDI 2230 guidelines [German fastening standards] about the required frictional characteristics [for] an application,” says Steve Adkins, director of engineering at Optimas Solutions in North America.
[Doing so can] cause excess or insufficient clamping loads, which is all important in this regard. From a purely joint design perspective, I’ve found the majority of fastener failures we encounter can be attributed to the incorrect calculation of assembly torques.”
Avellon recommends not specifying different types of bolts or too-long bolts in an assembly. In the former case, the harder fasteners will end up carrying most of the load. Threads on too-long bolts have fewer shock absorbers in the joint, in effect, resulting in metal fatigue. Aesthetics is a legitimate consideration. Manufacturers often want the fastener heads to enhance the appearance of an assembled product. For example, Harley-Davidson uses both chrome-plated and stainless steel nuts and bolts to assemble motorcycle sprockets, consoles and air cleaners.
These fasteners must still be able to resist vibration and environmental moisture. Another way that manufacturers can prevent fastener failure is to only purchase fasteners that fully conform to all design specifications. A few years ago, Boeing began more strictly enforcing its policy of rejecting all nonconforming fasteners supplied to the company. The move was in reaction to the FAA’s plan to fine Boeing $2.75 million for installing nonconforming fasteners on its model 777 airplanes in 2008. At that time, Boeing was experiencing an unexpected shortage of nuts and bolts, but wanted to maintain its production schedule.