Strength and Grades of Fasteners Part 2

FASTENER TECH – THE NUTS AND BOLTS OF IT (part 2)
By Bill Ansell (Billavista)
Photography: Bill Ansell
Technical Drawings: Lonny Handwork

Why an Article on a Topic as “Simple” as Nuts and Bolts?
Because an understanding of fastener tech forms a solid foundation for all extreme off-road tech. It’s a great place to start – from the ground up. Much of what we do as builders and maintainers of off-road vehicles is strictly custom, often one-off, stuff. We don’t have the luxury of detailed instructions or the benefit of a major manufacturer’s years of engineering and research. In short, we’re on our own so we need to have a good solid understanding of fastener tech in order to answer questions such as: What size and type of thread should we use to attach our custom suspension links? What type of locking mechanism should we use on our beadlock wheels? Should we use studs or bolts to attach our steering arms to the knuckles?

Table 2 –Torque specs for SAE coarse and fine thread fasteners

* The upper end of these values represents approximately 85-90% of the fastener’s maximum torque

What Grade?
This is a simple question, despite persistent myths to the contrary. The answer is, SAE Grade 8 from a reputable national manufacturer. Un-graded and Grade 2 fasteners have no place whatsoever on a 4x4 as they are weak and unreliable, and while Grade 5 fasteners may exhibit the necessary strength in some applications, in others they do not, and the lower strength and possible misuse of them is simply not offset by the marginal cost benefit. Grade 8 bolts exhibit greater tensile, yield, and shear strength as well as greater fatigue resistance and, just as important, are capable of greater torque specs and therefore much greater pre-load and clamping strength.

There exists an often quoted myth, that Grade 5 bolts are better in shear than Grade 8 since they will bend before breaking. Not true. Shear strength of alloy steel is approximately 60% of its ultimate tensile strength. Reference to Table 1 shows that the yield strength of a Grade 8 bolt is higher than the ultimate strength of a Grade 5 bolt. The Grade 5 bolt will therefore always fail first whether in tension or shear. The only “gotcha” with the Grade 8 bolt is that, being harder, it is more “notch sensitive.” This means it is more sensitive to build up of stress concentrations caused by notches, nicks, and gouges leading to fatigue and failure. This becomes a non issue if good quality new fasteners are always used and periodically inspected.
Different grades of fasteners can be identified by the markings on their heads (Table 1). Of course, matching grades of nut and bolt/stud must be used together.

Note that many manufacturers (e.g. Caterpillar, Bowman) manufacture bolts to specifications that exceed those for SAE Grade 8 bolts. These fasteners (Figure 7) are often marked in a similar fashion to SAE graded hardware by means of dashes embossed on the head of the bolt. Despite this, it is not technically correct to refer to these bolts as “Grade 12” or such, as no such SAE specification exists. Bowman calls their line “Bowman Special Alloy.”

Figure 7 - Bowman Grade 8 bolt & Bowman “Special Alloy” bolt

Which Thread – Coarse or Fine?
While there exist many different classes of threads, the only classes likely to be of interest to us are Class 2A/2B and Class 3A/3B (the ‘A’ denotes external threads; the B denotes internal threads). Class 2A/2B is the recognized standard for normal production of the great bulk of commercial bolts, nuts and screws. Class 3A/3B is used where a close fit between mating parts for high quality work is required. This class is usually only found on certain specialized engine hardware (e.g. connecting rod bolts) or aerospace fasteners. The vast majority of our fasteners will be in Class 2A/2B. The thread class should be matched between nut and bolt. When tapping a hole, be sure the tap cuts the same class of thread as the screw or stud you intend to use. SAE fasteners come in a choice of either Unified National Coarse (UNC) or Unified National Fine (UNF). Sometimes the older designations NC and NF are still used. The differences are as follows:

* UNC fasteners are the most common, easiest to find, quickest to assemble, and most resistant to cross threading and thread fouling. They are easier to disassemble when corroded and are also less susceptible to thread stripping - making coarse threads a good choice for threading into cast pieces.
* UNF fasteners have a larger minor diameter than UNC, giving them a corresponding slightly larger tensile stress area and therefore tensile and therefore tensile and shear load carrying capability. They are not appreciably more resistant to vibration loosening than UNC threads. The only thing that really keeps a fastener tight is the correct pre-load, and this can be just as easily achieved with either thread. UNF threads are more prone to damage and thread fouling. Fine thread bolts are also more susceptible to stripping and require greater thread engagement for equivalent thread strength than the same size coarse thread fastener. Due to their higher tensile stress area UNF fasteners can be torqued more, and therefore develop greater clamping force than the equivalent size UNC fastener.

Why Torque?
The reason we torque fasteners to a given spec is because it is the most convenient, practical method for controlling the amount of pre-load or “stretch” in the bolt, which in turn provides the necessary clamping force for the assembly. Torque values are calculated considering the material of the nut and bolt, the surface finish (including lubricants or retention compounds), and other factors. In practice, the most common method is to use a table of pre-calculated torque values such as that shown in Table 2.

There is a pitfall to controlling pre-load by torque though. The majority of the torque used to tighten a fastener is not directly used in achieving the desired pre-load. Of the torque we apply to a fastener, approximately 45% is consumed to overcome friction in the threads, 40% consumed to overcome friction between the nut face and the joint, and another 5% is consumed by prevailing torque - the torque required to screw a locking-type nut down the threads of a bolt. Thus only 10% is available to produce bolt pre-load. This means that changes in either the friction of the threads (as in rusty or oily threads), or under the nut face (when flat washers are used or the nut embeds in the bracket) can have a huge impact on the pre-load. This is why top pro engine builders tend to use strain gauges or ultrasonic measurement to measure actual bolt stretch, rather than torque. These methods are not practical for most of us though, but there are some rules we can follow to minimize the pitfalls:

* Avoid using multiple flat washers, as the relative motion between them and the nut and the joint alters the friction under the nut face. It is difficult to avoid using flat washers altogether, as having the nut embed in the bracket does the same thing. The best solution is to use a flanged nut and/or flange head bolt when embedding is a problem.
* Always turn the nut with the torque wrench, rather than the bolt, to avoid further muddying the waters with bolt torsion and shank/bracket friction.
* Use a calibrated torque wrench to evenly and smoothly tighten nuts to spec.

The more a nut and bolt is tightened, the greater the pre-load in the bolt, and therefore the more external load it can sustain within material limits. As the bolt strains to return to its original length it “fights back” against any external tension load, until its pre-load clamping force is exceeded. In addition, the tighter the bolt and nut, the more friction in the threads, and the less it is susceptible to loosening. In summary – loose is useless and tight is right!

But how tight is tight enough? A good rule of thumb is to use an established table of recommended torque values or to tighten a fastener to about 70-80% of its maximum torque capacity. Note that almost all torque specifications published are for clean, dry threads. In calculating assembly torque for any threads that are not clean and dry, exact figures are difficult to determine – experience and judgement are the best tools, along with direct strain measurement in critical applications. Common compounds applied to threads such as grease and anti-seize normally reduce the required torque by 20-40% or more. It’s worth noting the reason critical fasteners such as ring gear bolts are never to be re-used. Such bolts are required to achieve extremely high clamping loads in order to do their job. This means they must be installed and torqued so highly that they approach their yield point, sometimes very closely. Add the stress they see in service, and we cannot be sure that they will retain all of their tensile strength if they have been removed and reinstalled.

Setting Torque
The proper technique for tightening a fastener to spec is as follows: Tighten the fastener a little at a time (3 or more steps), pausing to allow the stress in the threads to relax. Finish with an even pull until the torque wrench clicks or indicates final torque, pause, and then pull again to check.

Checking Torque
When checking an assembled joint, such as wheel lugs or steering-arm-to-knuckle joints, the best procedure is to loosen the fasteners and torque evenly to spec, as above. When one needs to know if the fastener had loosened in service, one can simply place the socket over the nut, make an alignment mark between the socket and a part of the joint that is stationary, back the nut off a quarter turn, and then re-torque to spec – how close the alignment marks line up will give an indication as to the degree of loosening in service. The snag is: this method is problematic for checking fasteners that use chemical thread-locking compounds. Breaking the chemical bond in checking the torque defeats the purpose of the thread locker, and the resulting cured compound in the threads increases thread friction, resulting in less torque available for pre-loading the fastener – meaning the fastener will now be looser and weaker if torqued to the same spec again. However, since the cured thread locker will add to the friction in the threads, it stands to reason that it would take more than the original assembly torque used when it was not cured, to break the fastener free either by tightening or loosening. Therefore, torque can be checked by holding the bolt head stationary, and applying assembly torque to the nut, while checking to make sure there is no relative movement between nut and bolt. If the torque wrench indicates assembly torque and the nut and bolt have not moved relative to one another, the fastener is still tight.

Washers
If a washer is necessary, there is really only one type that should be considered in a structural bolted joint, and that is the flat washer. Its purpose is to act as an increased load-bearing surface for either the head of the bolt and/or the face of the nut. This use should only be considered when using a nut or bolt with insufficient bearing area resulting in it digging into the surface of the joint (embedding) if a washer were not used. Embedding is to be avoided. Not only does it damage the surface, but the unpredictable stress that occurs when fasteners embed into the joint destroys any chance we have of achieving proper pre-load by torquing. The only other purpose a flat washer serves is to act as a shim to either position the threads more favourably, or to adjust the position of a castle nut so that the slots better line up with the hole in the bolt. Use of washers as shims is dubious at best, and should be avoided if possible by using the correct length bolt.

Preventing Loosening
As we have seen, loose fasteners are weak and quickly lead to failure. The best way to prevent a fastener from loosening is to do it up tightly enough that there is sufficient clamping force across the joint to prevent relative motion between the bolt head/nut and the joint, as well as sufficient inter-thread friction to prevent any relative motion between the threads. If a fastener is new, clean, dry, torqued to the proper spec with a calibrated wrench and it is properly sized and used in a sufficiently rigid joint - it will stay tight. Of course, there are a lot of “ifs” in that statement, and we off-roaders live in an imperfect world at best, so there are several methods available to assist in preventing the loosening of fasteners. Which is best for the application depends partly on the root cause of the loosening, and partly on the characteristics of the locking device. Root causes of loosening are usually one of:

* Overloading of the joint causing clamping force and friction in the joint to be overcome, leading to slippage in the joint, bending of parts, and ultimately slippage of the bolt head and/or nut face which will lead to loosening. Undersized fasteners, improperly torqued fasteners, and insufficiently rigid joints are culprits here.
* If the parts of a bolted joint are subjected to different amounts of heating and cooling, or if they are made from different materials subject to the same thermal cycle, the resulting differences in thermal expansion and contraction in the joint can lead to loosening. Effects are cumulative and can combine with other forms of loosening. The difficulty of keeping alluminium wheels tightly fastened to steel hubs with steel lugs and nuts is a classic example.
* Severe vibration in a joint can lead to bolt loosening. Again, effects are cumulative and can combine with other causes.

The following are the most effective methods of helping to control loosening – but none will replace a properly tightened fastener. There are many other methods not listed (such as split beam nuts, star washers, Bellville washers and lock wiring), simply because they are uncommon, largely ineffective or too complex and expensive for the majority of our uses.

Lock Nuts
There are many types and brands of lock nuts available (Figure 9). There are also countless proprietary types available, but most use some variation, or combination, of the following basics:

Nylon Collar Lock Nuts
The most common type of locking nut, they have a small nylon insert at the top of the nut, the ID of which is slightly less than the major diameter of the bolt’s thread. As the bolt threads into the nylon area it impresses its own threads into the nylon and the friction bond achieved resists loosening. Nylon collar lock nuts can be re-used up to about 10 times, but are only good up to temperatures of about 250 degrees Fahrenheit.

Deformed Thread (Elliptically Offset) Lock Nut
This all-metal lock nut is my personal favorite. It has no practical temperature limit and can be reused many, many times. The top threads of this nut are deformed (usually elliptical or triangular in shape) so that they tightly grip the male threads of the bolt, creating a very secure locking action but without damaging the male threads. Examples include Torquenut®, Stover®, and Clevloc® nuts.

Castellated Nut
The castellated nut has slots cut in the top and is used with a bolt that has a single hole through its threaded end. In use, the nut is installed and torqued to spec and then rotated so that the nearest slot aligns with the hole in the bolt. A cotter pin is then installed through the slots and the hole, to lock the nut in place. The disadvantage to this type is that, because of the clearance required between the slots to allow for cotter pin insertion, it is difficult to achieve a precise torque setting and simultaneously line up the hole and slots. For the same reason, the cotter pin prevents the nut from backing off, but due to the clearances involved, does not hold the nut tightly to prevent any loosening. The castellated nut is best suited for low-torque applications such as holding a wheel bearing in place.

Figure 9 – From left: nylon collar lock nuts, castellated nuts, spring lock washers, flanged and non-flanged deformed thread lock nuts

Spring (Split) Lock Washers
I mention this so-called locking device only in an effort to turn you off them forever! I can’t stand the things and believe they are next to completely useless. The typical spring washer is made of slightly trapezoidal wire formed into a helix of one coil. It is supposed to work by acting as a compressed spring – presumably to add to bolt pre-load and prevent loosening. However, when we combine our knowledge of bolt stretch and pre-load with the fact that the split washer is always compressed completely flat under any properly tightened bolt, we can see that the idea that this thing would effectively contribute to bolt pre-load is ridiculous. The only other way it could possibly help is that the sharp trapezoidal ends dig in slightly to the bolt’s bearing surface and the face of the joint (but only if the washer were harder than the bolt’s bearing surface, which is extremely unlikely). However, when we remember the pitfalls of inaccurate pre-load caused by excessive/unpredictable friction under the bolt head/nut face consuming too much of the tightening torque, we can see that this is hardly a good idea. Not only that, but experience teaches us that the damn things invariably squish and splay out under any decent amount of torque anyway. I think they are useless junk that should be avoided on extreme off-road machines!

Thread Locking Compounds
The final method to consider is the family of chemical thread locking compounds such as Loctite™. A thread locking compound is an anaerobic adhesive, meaning it is applied to threads in a liquid form, and when the fasteners are joined and oxygen is excluded, they cure into a solid, plastic-like compound “locking” the threads together. They are available in a wide variety of strengths to suit different applications, from those that can be disassembled by hand to those that require the application of heat and power tools. The manufacturer’s application directions should be carefully followed and it is advisable to avoid using too much – usually a drop or two will do. They provide excellent resistance to loosening but can be messy and expensive. They also make tightening to spec, torque checking, and disassembly more complicated. Most thread locking adhesives actually create more friction in the threads than clean, dry threads so that assembly torque will have to be adjusted accordingly.

Conclusion
Fasteners are an essential and integral part of every one of our machines. Getting them right is critical, as the consequences of their all-too-common failure, ranges from embarrassing and frustrating to down-right scary. Let’s face it – nothing particularly good happens when fasteners or bolted joints fail! As with any other tech topic, the wise fabricator/builder must apply this or any other technical information with great care, at his/her own risk, and always seek competent professional help when required. May you experience great success in your fastening endeavours and keep safe out there!

Smith, Carroll. Engineer to Win: The Essential Guide to Racing Car Materials Technology, 1985 (Motorbooks International)
Gren, Robert E., Oberg, E., Jones, F.D., Horton, H.L., Ryffel, H. H. (Editors). Machinery’s Handbook, 24th Edition, 1992, (Industrial Press, Inc.)
Aird, Forbes. High Performance Hardware: Fastener Technology for Auto Racers and Enthusiasts, 1999, (Berkley Pub Group)
Smith, Carroll. Nuts, Bolts, Fasteners & Plumbing Handbook, 1990, (Motorbooks International)

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