Jerry G. Perkins
Rocky Hill, Conn.
Threaded fasteners can be found on practically all mechanical components, in one form or another. They have become an even bigger part of a design today, as environmental concerns boost the importance of disassembly and recycling. However, even after centuries of improvement, threaded fasteners still fail.
One failure mode is tension relaxation. Temperature changes, for example, cause bolts and substrate materials to expand and contract, reducing bolt tension and lowering clamping force in an assembly. Another source of bolt failure is self-loosening caused by sliding between contact surfaces.
Common approaches to eliminate these weaknesses include installing bolts with high shaft length/diameter, or l/d ratios, and using hardened and tempered washers to reduce surface pressure and settling on bearing surfaces.
When hardware modifications can’t do the trick, engineers turn to alternative assembly methods that prevent failure by prestressing joints with conical spring washers or cup springs, or relaxing assemblies and securing them with loss prevention or prevailing torque-type fasteners.
Threadlocking adhesives prevent bolts from loosening by completely filling microscopic gaps between interfacing threads. Single-component liquids are easy to apply, and the anaerobic adhesives cure to a tough solid on contact with metal ions. The reaction creates an interfacial connection and locks fasteners by diffusing into the metal’s rough surface.
With a variety of adhesives to choose from, designers must consider operating temperature, thread size, chemical and environmental factors, adhesive strength, bolt reusability, and whether the adhesive needs primer to cure.
When applying adhesives, workers must wet the total length of the thread. Thread size and geometry determine how easily parts wet, along with adhesive viscosity. Assemblies must also be free of oils or cleaning systems that prevent adhesives from curing.
One advantage of threadlocking adhesives is bolt reusability, particularly in maintenance and repair work where bolts are frequently disassembled and reused. Each adhesive has break-loose torque values ranging from low, which may be disassembled using normal tools, to high, which is difficult to disassemble. Threadlockers with low to medium strength are easily loosened without damaging bolts, and are reusable after removing old adhesive.
In addition to preventing movement, threadlockers can extend joint life by blocking moisture, gasses, and corrosive fluids.
An additional benefit of using threadlockers is assembly costs because engineers use standard bolts in place of complex locking bolts. Liquid adhesives also work regardless of bolt size and diameter, eliminating extensive inventories of mechanical fasteners.
Developments in threadlocking adhesives streamline assembly and save time. Eliminating precleaning steps, the most recently developed threadlocking adhesives are more forgiving of oily surfaces.
Advanced anaerobic threadlockers also cure more easily on inactive surfaces such as brightly plated metals, and endure temperatures up to 500°F.
Another development is preapplied fastener coatings that make it easier for workers to secure joints. An alternative to manual or automatic adhesive dispensing methods, the coatings feature microcapsules containing adhesive which are preapplied to an assembly’s threads as a dry film. When assembling fasteners, the capsules crush and release adhesive threadlocker. Besides boosting assembly speeds and reducing costs, precoated bolts also improve quality because a more consistent quantity of adhesive dispenses from the coating.
Nuts and bolts of design
When designing components that will be assembled with nuts and bolts, it is important to determine the required clamping force and its duration along with how much force it will take to remove the fasteners. When a bolt meets resistance, such as clamping a flange, it continues to rotate until there’s a balance between the torque and the sum of bolt tension and friction. To express this relationship mathematically
T = KDF
where T = torque, D = nominal diameter of bolt, F = induced force or clamp load; and K = an empirical constant that compensates for thread friction. Friction and, therefore K, varies significantly since it results from extremely high pressure between surfaces that may be rough, smooth, oxidized, chemically treated, or lubricated. For example, oily steel has a K value between 0.11 and 0.17 or ±20%. Engineers use test data to determine proper torque values for bolts. Technical data for lubricants and other thread-treating materials will often have K values plotted in torque-tension curves.
Factors which affect K are fastener material, insertion speed, bolt and adhesive quality, thread tolerance, and surface finish. In choosing the right bolt, the expression
K × D
represents the slope of a torque-tension curve. Because D is also a constant, each torque-tension curve has a computable constant K, and K is the same for all diameters. Knowing the friction constant K, designers can compute the torque tension relationships for other bolt sizes.
Besides using graphical methods to determine bolt stress, engineers use the expression
S = F/A
where S = bolt stress, F = force, and A = cross-sectional area of bolt. With this equation, a complete fastener design can be approximated for rigid clamping by using material stress limits.
Another design consideration is fatigue resistance. A joint clamped tighter than applied loads will not experience cyclic loads on the bolts. However, a trade-off exists between overdesigning a joint to accept higher clamp loads, and designing for lower loads which could not fatigue even if below the limit.
Joint overloading also causes fasteners to self-loosen. As few as 50 side-to-side movements of a joint reduce clamp loads by 20%. As clamp load decreases, joints are more susceptible to high cycles and lower loads. Therefore, fastening techniques and bolt size need to considered together. Many securing methods affect the clamp load as well as self-loosening tendencies. For example, when workers secure bolts against self-loosening, a few hundred overload cycles will not affect performance.
Gasketing breaks the rules
However, in a flexible joint with a soft gasket, joint and bolt dynamics are quite different. A much greater proportion of the externally applied tension load is added to the bolt preload described by the expression
P = Pi + CFa
where P = final load on the bolt, Pi = initial preload or clamping load developed through tightening, Fa = externally applied load; and the constant
C = (EbAb/Lb)/((EbAb/Lb) + (EgAg/tg))
where Eb = modulus of elasticity of the bolt, Eg = modulus of elasticity of the gasket, Ab= effective cross-sectional area of bolt, Ag = loaded area of gasket, Lb = effective length of bolt, and tg = gasket thickness.
The value of C falls between 0 and 1 and changes with various gasket parameters. For example, if the gasket is hard, thin, and large, the term EgAg/tg will be greater thanEbAb/Lb and C approaches zero. When no gasket is between members in a rigid joint, C equals 0. For very soft gaskets, C approaches 1. The calculations are valid as long as the gasket stays in contact with joint members. If the bolt stretches to the point where the gasket is no longer in contact, the bolt preload equation is simply P = Fa.
The most important single factor that can eliminate cyclic stress variation due to cyclic loading is proper pretensioning or preloading of the fastener. Test results indicate that rigid members bolted together by relatively elastic bolts offer the best method to prevent fatigue failure.
The fatigue strength of a gasketed joint is determined by fastener and flange fatigue limits. A properly torqued bolt will not fail in fatigue in a rigid joint. Initial bolt tension will remain constant until the external tension load on the joint exceeds the bolt load. Furthermore, without appreciable stress variation, regardless of the number of load cycles on the joint, fatigue failure won’t occur.
In contrast, considerable flexibility creates variable stress in screw or bolt fastenings resulting in greater flexibility of the connected parts. If bolts flex too much, variable stress may be large enough to cause eventual fatigue failure of the fastener regardless of the initial bolt preload.