Fasteners are some of the most important components in any machine, but they are often taken for
granted. It is vital to correctly choose and use them, or else the mechanical integrity
of a product is at risk and there can be devastating consequences. Thread type, preload, material
choice, coating spec, drive selection, and more, all need to be understood and applied
properly. In this video, we are going to go over all these fastener topics and many more,
so that you know the nuts and bolts of nuts and bolts.
Fasteners come in a wide range of shapes and sizes, from the tiny microscrews used in electronics
to the large structural bolts that hold buildings together. The vast variety available can be
a bit overwhelming, but we’ll show you a few of the most common types and talk about
where they’re best used.
These are hex head screws, which most people informally call bolts. Because they’re cheap,
strong, and easy to work with, they’re probably the most common fasteners you’ll encounter.
The only trade-off is that because these require a wrench or socket to tighten, the designer
has to leave a fair amount of room around the fastener to accommodate the tool. If you’re
designing with these, you can reference Machinery’s Handbook for a helpful table on tool clearance
requirements.
In situations where space is at a premium, you might consider a socket head screw. Since
they’re driven from the top, you can place them in tight spots, or bury them in a counter
bore. Low-head screws and button heads have a lower profile and are often used in electronics.
A flat head screw can be recessed inside a countersink and is great when you need the
head of the fastener to sit below the surface of thinner materials.
Screws are most commonly sold with a hex drive, either external in the case of hex head cap
screws, or internal in the case of socket head cap screws, and this is perfectly fine
for larger fasteners. For smaller fasteners, below maybe #6, hex drives strip out very
easily, and you should consider a more robust drive like Torx. High-strength nuts and screws
are sometimes manufactured with an external 12-point drive, which offers more points of
contact than the traditional 6-point hex.
It’s best to avoid Phillips and slotted drives if you can. While being widely available,
they are subject to “cam-out” and can strip easily. Even the construction industry
is moving to Torx drive because of the many advantages.
There’s also a lot of really odd-ball drive types that were designed for very specific
applications, or to purposely be obscure. Unless your design fits into these categories,
you should probably consider something more mainstream.
You may be wondering what the difference is between a “screw” and a “bolt”. Technically,
the difference depends on how the fastener is used, not the actual fastener itself. Bolts
are used with thru-holes and are fastened with a nut. Screws are used in tapped holes.
Screws have a Design-for-Assembly advantage since the technician only needs to use a single
tool to fasten the joint. Screws are most commonly made from either
low carbon or alloy steel, but you will also find them made from stainless steel, titanium,
Inconel, brass, and aluminum, or even plastics like nylon or PEEK.
The primary considerations for selecting a material is strength and corrosion resistance.
The strongest fasteners are usually made from heat treated alloy steel, but steel will corrode
when exposed to moisture. Coatings are commonly applied to alloy steel fasteners to make them
more resistant to corrosion.
Zinc plating is economical and is the most common coating on low and medium strength
fasteners. It is sufficient for dry environments that receive only occasional contact with
moisture. Grade-8 screws usually receive a yellowish chromate conversion coating which
has slightly better corrosion resistance than electroplated zinc.
Fasteners that will be exposed to the elements are often galvanized. This means that the
raw bolt is dipped in a pot of molten zinc, which adheres to the base metal. This process
is sometimes nicknamed “hot-dipped” and the resulting coating is much thicker than
the electroplated zinc. Galvanized fasteners are typically acceptable for exterior installations.
Aerospace fasteners are usually plated with cadmium. This is a very effective coating,
but cadmium is an extraordinarily dangerous material, and these hazards are compelling
manufacturers to find safer alternatives.
Some fasteners, particularly socket head cap screws, are coated with black oxide, and while
it looks nice, this doesn’t significantly improve the corrosion resistance over bare
steel.
Coatings are generally not sufficient for applications in which the fastener will be
immersed for extended periods. Switching to a stainless steel fastener made from an alloy
like 304 or 316 improves the corrosion resistance, but 300-series stainless is not nearly as
strong as alloy steel. If you need both high strength and corrosion resistance, you may
need to use a super alloy like A286 or 17-4PH. Nickel-based alloys like Inconel and Hastelloy
are used in the most demanding applications.
Material and coating specifications are defined by standards, such as ASTM, NAS, AS and SAE,
or the former “MIL” and “AN” specs. Metric fasteners are defined by ISO or DIN
specifications. Various grades of fasteners are sometimes defined by the standards, and
for hex head screws, the grade is usually designated by marks on the heads. Metric fasteners
use a system of numbers stamped into the head. In either case, the higher the grade, the
higher the tensile strength of the fastener. Alloy steel socket head cap screws generally
have a strength similar to grade 8 bolts.
It is important to ensure that the fasteners you are purchasing conform to the required
specifications for your product or industry. For example, you may have seen screws like
this at the hardware store, where they are typically labeled as “pan head” screws.
Dimensionally, they are very similar to button head screws, and both are made from steel
and zinc plated. However, the hardware store screws are made from soft carbon steel, while
the button heads are made from heat treated alloy steel per an ASTM specification, and
are over three times as strong. For some projects, hardware store fasteners are fine, but for
critical applications, purchase fasteners from a company like McMaster-Carr that lists
exactly what specifications they conform to.
Fastener materials and coatings is a hugely important area, especially in industries like
aerospace, marine, and energy. A particular point of concern is hydrogen embrittlement.
During the plating process, hydrogen is often produced which can become trapped inside the
metal’s crystal structure. This causes the steel to become brittle and susceptible to
spontaneous failure. There was a rash of aviation accidents that occurred when MS jet nuts that
had hydrogen embrittlement failed in service. In the corrosive environments common in the
marine and energy industries, stress corrosion cracking can cause similar, spontaneous failures.
Another consideration is galvanic corrosion. When materials that are far apart on the galvanic
series are in contact in the presence of an electrolyte, a galvanic cell is formed. In
this case, accelerated corrosion occurs on the more anodic material. If the anodic material
happens to be small or thin, this corrosion can be extremely severe.
Fasteners are available in both coarse and fine threads. Coarse thread fasteners, conforming
to the UNC thread specification, have fewer threads per inch and are generally the first
choice. However, fine thread fasteners are used in high-performance applications like
aerospace and automotive because they have several advantages. Size-for-size, fine thread
bolts are about 10% stronger than coarse thread bolts, and the lower helix angle makes them
more resistant to vibration. However, they are easier to cross thread and don’t tolerate
debris or thread damage well.
A slight variation to the UNF and UNC standards is the J thread type. Instead of the root
of the external thread being a sharp V, the J thread type has a prescribed root radius.
The radius helps improve fatigue performance for cyclically loaded screws. You typically
only see this type of thread in aerospace. When designing tapped holes, you need to have
enough thread engagement to allow the bolt to be fully tightened to its tensile limit.
For threads in most metals, a thread engagement length of around two times the nominal size
of the screw is sufficient. An interesting nuance of threaded holes is that they don’t
share the tensile load evenly along their length. In this FEA, we’re simulating tightening
a steel screw into an aluminum tapped hole, and you can see that the first few threads
do almost all the work. Once you get beyond about the eighth thread, there is almost no
load being carried. Hence, there is little benefit to additional thread engagement, and
it simply adds to the assembly time.
If the tapped threads in a workpiece ever become damaged, you can use a component called
a helical insert (often sold under the tradename Helicoil) to repair them. You drill out the
original thread with a drill that is larger than major diameter, and use a special tap
to cut new threads. Then, you install the insert with a threaded mandrel, and break
off the tang at the bottom.
Sometimes people will install Helicoils in brand new parts, especially those made of
soft materials like aluminum or magnesium. This won’t necessarily make the thread any
stronger, but it will make it a lot more durable if you will be removing and replacing the
fastener frequently.
If you need to thread machine screws into plastic, especially 3D-printed parts, heat-set
inserts create a strong, durable thread. There is typically a special tip that you install
on a standard soldering iron, which slips inside the insert and allows you to press
it into the plastic.
You should do your absolute best to avoid subjecting screw threads to significant shear
loads. The decreased area at the root of the thread makes this the weak part of the fastener,
and the threads bearing on the mating pieces will lead to fretting. Instead, it’s best
to transmit shear loads with dowel pins or shear bosses.
But if you look at any building built from steel, you’ll find thousands of fasteners
in shear. Why is it okay there? Structural bolts have an unthreaded section of a controlled
length that acts as a dowel. The shearing plane intersects this dowel section, instead
of the screw threads, creating a very simple but sound connection.
In cases where you need a fastener to also precisely align components, you might consider
a shoulder screw. They have a precision ground shank under the head that can be used as a
pin, shaft, or linear guide, and they’re a common component in punch and die assemblies.
Shoulder screws usually have a much smaller threaded section than other screws, so you
can’t use them to generate a lot of clamping force. Also note that they’re not nearly
as strong in shear as a dowel pin, so you shouldn’t subject them to heavy loads.
Washers are used to help spread the bearing stress of bolts and nuts over a wider area.
This is important if you have a loose clearance hole where only a small portion of the bolt
head would be in contact with the joint members. Washers also reduce marring of the parts and
make torque values more reliable.
It’s generally best practice to use a washer under the head of a hex head screw, as well
as the nut. Washers are not often not used on socket head cap screws, but to get away
with this, it is important to specify a close fit on the clearance hole.
An alternative to a free washer is a flanged screw or nut. These are preferred on mass-produced
products to eliminate the handling time of a separate washer.
In practice, vibration is the biggest problem for bolted connections in machines and vehicles,
and without careful consideration, the machine will literally shake itself to pieces!
Applying the proper preload goes a huge way in improving the resistance to vibration.
The general guideline is to apply between 70 and 90 percent of the bolt’s proof load
as preload for static joints.
Preload is usually controlled by the applied torque. The only trouble with this method
is that most of the applied torque is lost due to friction in the threads and between
the head of the screw and the workpiece. The materials, coatings, and lubricants all play
a role, and a correction factor, called the k-factor, is used in the torque formula to
account for this. However, even when the k-factor is chosen correctly, the torque method is
only accurate to within plus or minus 25%.
For statically loaded joints, there is no reason not to apply the maximum preload to
the fastener. However, if a joint is loaded cyclically, the fastener may be subject to
fatigue, and the preload and fastener material may need to be adjusted to account for the
cyclic stress.
Another closely related parameter is the grip length of the screw. This is the distance
between the head of the bolt and the start of the thread engagement, and the longer this
distance is, the more resistant the screw will be to loosening.
Sometimes preload is not enough to prevent a fastener from loosening, and additional
provisions must be made. For bolted connections, lock nuts are extremely effective. The most
basic style has a nylon element that is deformed by the screw threads and holds the two together.
These are called “Nylocs.”
In applications subject to higher temperatures, above about 250 degrees F, the nylon element
will soften and not be effective. Instead, deformed thread locknuts are used. These come
in a few different styles, but in all cases, the threads are specifically made slightly
out of round so that they tightly grip the screw threads.
You may have seen these bent lock washers at the hardware store. These don’t do anything
and really have no place in engineering applications.
There are, however, other washer designs that are quite effective. These are called Nord-Lock
washers and they come as a two-piece assembly that is glued together. An interlocking wedge
feature causes the two washers to separate as the screw is loosened, which actually increases
the tensile load in the fastener.
Liquid thread lockers like Loctite are another good option. There are scores of formulations
on the market that are designed for different holding strengths, operating temperatures,
materials, and thread sizes. Its important to select the correct formula for your application,
and fully read the manufacturer’s datasheet. Some compounds require special primers and
curing procedures, or only work properly for certain-sized fasteners.
Loctite works extremely well on clean parts and screws, but it is somewhat sensitive to
residual grease and cutting oil. It is also difficult to visually confirm after assembly
that it has been applied correctly, or even at all.
Mass-produced products often use fasteners with a nylon “patch” that is pre-applied
to the threads. This isn’t as effective as Loctite, but is a lot faster to assemble.
There are also proprietary taps that cut a special thread form that is compatible with
standard fasteners. The modified thread geometry creates a locking effect that makes the fastener
more difficult to remove.
When all else fails, the most effective way to prevent loosening is to physically prevent
the fastener from turning after it’s tightened. Aircraft mechanics use safety wire for this,
and it’s extremely effective as long as it’s installed properly.
One thing to watch out for is that a fastener that has been properly torqued looks the same
as one that is only finger tight. A good practice is to mark the fasteners one-by-one as they
are torqued. Then, a visual inspection will reveal which ones were missed. This special
marking wax makes it clear when a fastener has been loosened, either by a person or by
vibration. We really appreciate you taking the time to
watch our video. We couldn’t possibly cover every type of fastener or design, so if you
have a question or something to add, be sure to leave a comment. In the companion blog
post, we’ve listed a bunch of additional resources, as well as all the charts we featured
in this video.
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