Basic
Mechanical Fasteners
part II
TED126
Types
mechanical fasteners… Threaded
Two very basic types of machine-threaded fasteners
are
– nuts & bolts and screws.
Their great advantages include the
– ease of assembly, which generally requires no special
equipment, as well as
– the possibility to de- and re-assemble without (much)
damage to the joint.
– They may be used for most materials.
Types
mechanical fasteners… Threaded
Screws and bolts are made in a wide range of materials, with
steel being perhaps the most common, in many varieties.
Where great resistance to weather or corrosion is required,
stainless steel, titanium, brass or bronze may be used, or a
coating such as brass, zinc or chromium applied.
Electrolytic action from dissimilar metals can be prevented
with aluminum screws for double-glazing tracks...
– Some types of plastic, such as nylon or teflon, can be threaded and
used for fastening requiring moderate strength and great resistance
to corrosion or for the purpose of electrical insulation.
Types
mechanical fasteners… Threaded
Screws and bolts are usually in tension when properly fitted.
In most applications they are not designed to bear large shear
forces.
– for example, when two overlapping metal bars joined by a
bolt are likely to be pulled apart longitudinally,
– the bolt must be tight enough so that the friction between the
two bars can overcome the longitudinal force.
– If the bars slip, then the bolt may be sheared in half, or
friction between the bolt and slipping bars may erode and
weaken the bolt (called fretting).
– For this type of application, high-strength steel bolts are
used and these should be tightened with a torque wrench.
Types
mechanical fasteners… Threaded
High-strength bolts usually have a hexagonal head with an
ISO strength rating (called property class) stamped on the
head.
– The property classes most often used are 8.8 and 10.9.
– The number before the point is the tensile ultimate strength in MPa
divided by 100.
• The number after the point is 10 times the ratio of tensile yield strength
to tensile ultimate strength.
• For example, a property class 5.8 bolt has a nominal (minimum) tensile
ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times
tensile ultimate strength or 0.8(500) = 400 MPa.
Tensile yield strength is M10, property class 8.8 bolt can very
safely hold a static tensile load of about 15 kN
Types
mechanical fasteners… Threaded
A screw is a shaft with a helical groove or thread formed on
its surface.
Its main uses are as a threaded fastener used to hold objects
together, and
– as a simple machine used to translate torque into linear
force.
– It can also be defined as an inclined plane wrapped around a
shaft.
– Screws can normally be removed and reinserted without
reducing their effectiveness.
– They have greater holding power than nails and permit
disassembly and reuse.
Types
mechanical fasteners…
When screws and bolts cannot be used,
–
–
–
–
–
–
–
–
nailing,
riveting,
roll pins,
pinned shafts,
welding,
soldering,
brazing, and
gluing are some alternatives.
Types
mechanical fasteners… Threaded
Nuts & bolts are generally used with
– pre-drilled holes.
In the case of a nut and bolt, a simple hole will do.
Access from both sides of the component during
assembly is generally required,
– although this may not be necessary if captive or welded nuts
can be used.
Because of their relatively high cost, nuts & bolts
are mainly used for thicker sections, where they are a
well-established technique.
Types
mechanical fasteners… Threaded
Nuts & bolts
Typical use is for structural assembly in a wide
range of engineering applications, particularly where
high strength is required.
They can also be found in smaller form in for
example components where de- and re-assembly
may be required.
Types
mechanical fasteners… Threaded
Screws
Self-tapping or thread-forming screws, on the other hand,
require no nuts or tapped holes.
Mostly used with pre-drilled holes although self-drilling screws
are available,
– the screw forms a thread in the materials being joined when
inserted, avoiding the need for tapping of the hole or for access to
both sides.
Flow drilling (causing the material around the hole to be
extended beyond the normal material thickness) usually
provides enough material for thread engagement, although if
required an additional nut or clip may be used.
Types
mechanical fasteners… Threaded
Screws
Self-drilling screws may be used without the need for
pre-drilled holes.
In thin materials, a screw with a special tip can be
used to flow drill the hole in the material, providing
additional thread engagement.
Basic Types
mechanical fasteners… Threaded
Machine Bolt - a bolt with a square or hexagonal head on one end
and a threaded shaft on the other end; tightened with a wrench; used to
connect metal parts.
Lag bolt / screw - refers to a large wood screw with a hexagonal
head, driven by a wrench rather than a screwdriver.
Hanger Bolts - have a machine thread on one end, wood
threads on the other. These bolts are frequently found in
furniture.
Stove Bolts - similar to a carriage bolt, but usually used in metal. It
requires a square hole in the metal being bolted to prevent the bolt from
turning.
Basic Types
mechanical fasteners… Threaded
Carriage Bolts - has a domed or countersunk head, and the shaft is
topped by a short square section under the head. The rib neck carriage
bolt has several longitudinal ribs instead of the square section, to grip
into a metal part being fixed.
Machine screws - has a cylindrical shaft, threaded its entire length,
and fits into a nut or a tapped hole.
Traditional Wood Screw - has a tapered shaft, allowing it to
penetrate un-drilled wood.
Self-tapping Screw - Sheet metal - have sharp threads that
cut into a material such as sheet metal or plastic. They are sometimes
notched at the tip to aid in chip removal during thread cutting.
Nine Thread Types
mechanical fasteners… Threaded
Sharp V thread
not used much as a thread design.
American National
similar as a sharp but stronger.
Unified Thread Standard UTS (1949)
current standard….unified with the U.S., United Kingdom and
Canada. UNF, UNC, UNEF etc.
Square
this form is used for power/force transmission i.e. linear jacks,
clamps. The friction is low and there is no radial forces
imposed on the mating nuts. The square thread is the most
efficient conventional power screw form. It is the most difficult
form to machine. It is not very compatible for using split nutsas used on certain machine tool system for withdrawing the
tool carriers. Wood vise.
Nine Thread Types
mechanical fasteners… Threaded
ACME
Used for power transmission i.e. lathe lead screws. Is easier
to manufacture compared to a square thread. It has superior
root strength characteristics compared to a square thread.
The acme screw thread has been developed for machine tool
drives. They are easy to machine and can be used with split
nuts.
Whitworth
Most, if not all, British cars, motorcycles, airplanes and
machinery up to the 1970s – and possibly beyond – used
Whitworth thread forms.
Worm
…simply a screw, captured in place and running in bearings,
that accepts the input motion. Its threads engage the teeth of
the worm wheel which is basically a disk with partial screw
threads running around its circumference. Worm is widely
used for speed reduction or increase, torque multiplication
and resolution, and accuracy enhancement for positioning
systems.
Nine Thread Types
mechanical fasteners… Threaded
Knuckle
…is usually rolled from sheet metal or cast, and it is used for
light-bulb bases, bottle caps, and glass jars.
Buttress
A strong low friction thread. However it is designed only to
take large loads in on direction. For a given size this is the
strongest of the thread forms. When taking heavy loads on
the near vertical thread face this thread is almost as efficient
as a square thread form. Car jacks.
Basic Types of Heads
mechanical fasteners…
FLAT AND OVAL HEADS (UNDERCUT): The standard flat or oval head 80° to
82° counter sunk screw will fit a standard counterbored hole and is particularly
adaptable to flush assemblies in thin stock.
ROUND HEAD: Not recommended for new design (see pan head). This head was
the most universally used design in the past.
BINDING HEAD (Straight Side): Most generally used in electrical and radio work
because of its identifying undercut beneath the head, which binds and
eliminates fraying of stranded wire. A medium - low head with ordinarily
sufficient bearing surface. Not recommended as a Phillips Recessed head – see
Pan Head for better functional design.
Basic Types of Heads
mechanical fasteners…
HEXAGON HEAD (TRIMMED): This is the standard type of wrench-applied
hexagon head, characterized by clean, sharp corners trimmed to close
tolerances. Recommended for general commercial applications.
SQUARE SHOULDER SCREWS: An adaptation of the standard carriage bolt
design. Possesses a truss head on a square shank, which resists rotation when
located or driven into place.
Basic Types of Drives
mechanical fasteners…
Standard / Straight / Slot head has a single slot, and is driven by a flat-bladed
screwdriver. The slotted screw is common in woodworking applications, but is
not often seen in applications where a power driver would be used, due to the
tendency of a power driver to slip out of the head and potentially damage the
surrounding material.
Phillips screw drive has slightly rounded corners in the tool recess, and was
designed so the driver will slip out, or cam out, under high torque to prevent
over-tightening.
Basic Types of Drives
mechanical fasteners…
Torx is a star-shaped "hexalobular" drive with six rounded points. It was designed to
permit increased torque transfer from the driver to the bit compared to other
drive systems.
TORX is very popular in the automotive and electronics industries due to resistance
to cam out and extended bit life, as well as reduced operator fatigue by
minimizing the need to bear down on the drive tool to prevent cam out.
Square drive prevents slippage even under maximum torque, and the screws have
positive holding power in a variety of materials.
Basic Types of Drives
mechanical fasteners…
Allen / Hex Key… There are six contact surfaces between bolt and driver and
torque is constrained by the length and thickness of the key.
Clutch Type A or standard clutch head resembles a bow tie. These were
common in GM automobiles of the 1940s and '50s, particularly for body
panels.
Clutch Type G head resembles a butterfly. This type of screw head is
commonly used in the manufacture of mobile homes and recreational
vehicles
Spax Screws
Patented MULTIhead:
Countersinks screw head flush
with material.
Patented serrations:
Quicker and easier fastening.
Patented 4CUT™ Point:
No pre-drilling, no splitting.
Square end divides fibers.
mechanical fasteners… Installing a wood screw
Two holes are drilled
Pilot hole
– First hole drilled through completely through first piece
and into the second.
Shank hole
– Larger drill bit - drilled completely through the first piece
of wood.
If needed a countersink is used to put the flat head screws
flush with the material’s surface.
If using a #4 screw, pilot hole for hard wood is 1/16” and shank hole would
be 7/64”. Softwood pilot would be 3/64” and shank hole is the same as the
hard wood.
mechanical fasteners… Sizes
Machine Screws and Bolts
– 1/4” x 20
– First number is the diameter
– Second is the number of threads per inch
Wood Screws
– #6 x 3
– First number is the diameter
– Second is the length of the screw
After the size you will have three letters indicating the head.
THE END