MECHANICAL
ASSEMBLY
Mechanical
assembly uses various methods to mechanically attach two (or more) parts
together. Inmost cases, themethod involves the use of discrete hardware
components, called fasteners, that
are added to the parts during the assembly operation. In other cases, the
method involves the shaping or reshaping
of one of the components being assembled, and no separate fasteners are
required.
Types
Mechanical
fastening methods can be divided into two major classes: (1) those that allow for disassembly, and (2) those that create a permanent joint. Threaded fasteners
(e.g., screws, bolts, and nuts) are examples of the first class, and rivets
illustrate the second.
Reasons
mechanical
assembly is often preferred over other joining processes discussed in previous chapters.
The main reasons are
(1)
ease of assembly and
(2)
ease of disassembly (for the fastening methods that permit disassembly).
(3)Mechanical
assembly is usually accomplished by unskilled workers with a minimum of special
tooling and in a relatively short time. The technology is simple, and the results
are easily inspected. Permanent joining techniques such as welding do not allow
for disassembly.
For
purposes of organization, we divide mechanical assembly methods into the
following
categories:
(1)
threaded fasteners,
(2)
rivets,
(3)
interference fits,
(4)
other mechanical fastening methods, and
(5)
molded-in inserts and integral fasteners.
THREADED FASTENERS
Threaded fasteners are discrete hardware
components that have external or internal
threads for assembly of parts. In nearly all
cases, they permit disassembly. Threaded fasteners are the most important
category of mechanical assembly; the common threaded fastener types are screws, bolts, and nuts.
SCREWS
Ascrew is an externally threaded fastener
that is generally assembled into a blind threaded hole. Some types, called self-tapping screws, possess geometries
that permit them to form or cut the matching threads in the hole.
The types include machine screws, capscrews, setscrews, and self-tapping screws.
Machine screws are the generic type, designed
for assembly into tapped holes. They are sometimes assembled to nuts, and in
this usage they overlap with bolts. Capscrews have the same geometry as machine
screws but are made of higher strength metals and to closer tolerances.
Setscrews are hardened and designed for assembly functions such as fastening collars,
gears, and pulleys to shafts as shown in Figure 32.3(a). They come in various geometries,
some of which are illustrated in Figure 32.3(b). A self-tapping screw (also called
a tapping screw) is designed to formor cut threads in a preexisting hole into
which it is being turned.
Bolt
A bolt is an externally threaded fastener
that is inserted through holes in the parts and ‘‘screwed’’ into a nut on the
opposite side.
Nut
A nut is an internally threaded fastener
having standard threads that match those on bolts of the same diameter, pitch,
and thread form.
Material
Avariety of materials are used to make
threaded fasteners, steels being the most common because of their good strength
and low cost. These include low and
medium carbon as well as alloy steels.
Fasteners made of steel are usually plated or coated for superficial resistance
to corrosion. Nickel, chromium, zinc, black oxide, and similar coatings are
used for this purpose.
Other threaded
Fastners & hardwares
Include studs,
screw thread inserts, captive threaded fasteners, and washers.
A stud (in the context of fasteners)
Is an externally threaded fastener, but
without the usual head possessed by a bolt. Studs can be used to assemble two
parts using two nuts as shown in FigureThey are available with threads on one
end or both .
Screw thread
inserts
Screw thread inserts are internally threaded
plugs or wire coils made to be inserted
into an unthreaded hole and to accept an
externally threaded fastener. They are assembled into weaker materials (e.g.,
plastic, wood, and light-weight metals such as magnesium) to provide strong
threads.
Captive threaded
fasteners
Captive threaded fasteners are threaded
fasteners that have been permanently preassembled to one of the parts to be
joined. Possible preassembly processes include welding, brazing, press fitting,
or cold forming.
Washer
A washer is a hardware component often used
with threaded fasteners to ensure
tightness of the mechanical joint; in its
simplest form, it is a flat thin ring of sheet metal.
Washers serve various functions. They (1)
distribute stresses that might otherwise beconcentrated at the bolt or screw
head and nut, (2) provide support for large clearance holes in the assembled
parts, (3) increase spring tension, (4) protect part surfaces, (5) seal the
joint, and (6) resist inadvertent unfastening
STRESSES AND STRENGTHS
IN BOLTED JOINTS
Typical
stresses acting on a bolted or screwed joint include both tensile and shear . Once tightened, the bolt is loaded in tension, and the
parts are loaded in compression . In addition, forces may be acting
in
opposite directions on the parts, which results in a shear stress on the bolt
cross section.
There are stresses applied on the threads
throughout their engagement length with the nut in a direction parallel to the
axis of the bolt.These shear stresses can cause stripping of the threads. (This failure can also occur on the
internal threads of the nut.)
The strength of a threaded fastener is
generally specified by two measures:
(1) tensile strength
(2) Proof
strength is
roughly equivalent to yield strength; specifically, it is the maximum tensile
stress to which an externally threaded fastener can be subjected without permanent
deformation.
Problems
The problem that can arise during assembly is
that the threaded fasteners are
Over tightened, causing stresses that exceed
the strength of the fastener material.
failure can occur in one of the following ways:
(1) external threads (e.g., bolt or screw)
can strip
(2)
internal threads (e.g., nut) can strip
(3) the bolt can break because of excessive
tensile stresses on its cross-sectional area .
Solution
Thread
stripping, failures (1) and (2), is a shear failure and occurs when the length
of engagement is too short (less than about 60% of the nominal bolt diameter).
This can be avoided by providing adequate thread engagement in the fastener
design.
Tensile
failure (3) is the most common problem.The bolt breaks at about85% of its rated
tensile strength because of combined tensile and torsion stresses during
tightening .
Tensile sstress
The
tensile stress to which a bolt is subjected can be calculated as the tensile
load applied to the joint divided by the applicable area.
The
tensile stress area for a threaded fastener is the cross-sectional area of the
minor diameter.which is
1)A=
¼ pi(D - 0.9382P)^2 p=pitch per mm….British
2)
A= ¼ pi(D - 0.973/n)^2 n-pitch per
inch…American
TOOLS AND METHODS
FOR THREADED FASTENERS
The
basic function of the tools and methods for assembling threaded fasteners is to
provide relative rotation between the external and internal threads, and to
apply sufficient torque to secure the assembly.
Reason
It
is important that the tool match the screw or bolt and/or the nut in style and
size, since there are so many heads available. Hand
tools are usually made with a single point or blade, but powered tools are
generally designed to use interchangeable bits. The powered tools operate by
pneumatic, hydraulic, or electric power.
Dependance
Whether
a threaded fastener serves its intended purpose depends to a large degree on
the amount of torque applied to tighten it. Once the bolt or screw (or nut) has
been rotated until it is seated against the part surface, additional tightening
will increase the tension in the fastener (and simultaneously the compression
in the parts being held together)
And
tightening will be resisted by an increasing torque. Thus, there is a correlation between the torque required
to tighten the fastener and the tensile stress experienced by it.
Preload
To
achieve the desired function in the assembled joint (e.g., to improve fatigue
resistance) and to lock the threaded fasteners, the product designer will often
specify the tension force that should be applied.This force is called the preload.
T=CDF
where
T ¼ torque, N-mm (lb-in); Ct ¼ the torque coefficient whose value typically
ranges between 0.15 and 0.25,
depending on the thread surface conditions; D ¼ nominal bolt orscrew diameter,
mm (in); and F ¼ specified preload tension force, N (lb).
Various
methods are employed to apply the required torque, including (1) operator feel—not very accurate, but
adequate for most assemblies; (2) torque
wrenches, which measure the torque as the fastener is being turned; (3) stall-motors, which are motorized wrenches
designed to stall when the required torque is reached, and (4) torque-turn tightening, in which the
fastener is initially tightened to a low torque level and then rotated a
specified additional amount (e.g., a quarter turn).
RIVETS AND EYELETS
Rivets
are widely used for achieving a permanent mechanically fastened joint. A rivet is an unthreaded, headed pin used to join two (or
more) parts by passing the pin through holes in the parts and then forming
(upsetting) a second head in the pin on the opposite side. Riveting is a
fastening method that offers high production
rates, simplicity, dependability, and low cost.
The
deforming operation can be performed hot or cold (hot working or
cold
working), and by hammering or steady pressing. Once the rivet has been
deformed, it cannot be removed except by breaking one of the heads.
Application
Riveting
is one of the primary fastening processes in the aircraft and aerospace
industries for joining skins to channels and other structural members.
Rivets
are used primarily for lap joints. The clearance hole into which the rivet is inserted
must be close to the diameter of the rivet. If the hole is too small, rivet
insertion will be difficult, thus reducing production rate. If the hole is too
large, the rivet will not fill the hole and may bend or compress during formation
of the opposite head. Rivet design tables are available to specify the optimum
hole sizes.
Categories
The
tooling andmethods used in riveting can be divided into the following
categories:
(1)
impact, in which a pneumatic hammer delivers a succession of blows to upset the
rivet;
(2)
steady compression, in which the riveting tool applies a continuous squeezing
pressure
to
upset the rivet; and (3) a combination of impact and compression. Much of the equipmentusedinriveting
is portableandmanually operated.Automaticdrilling-and-riveting machines are
available for drilling the holes and then inserting and upsetting the rivets.
Eyelets
Eyelets
are thin-walled tubular fasteners with a flange on one end, usuallymade from sheet
metal. They are used to produce a permanent lap joint between two
(or more) flat parts. Eyelets are substituted for rivets in low-stress
applications to save material, weight, and cost.
During
fastening, the eyelet is inserted through the part holes,
and
the straight end is formed over to secure the assembly. The forming operation
is called setting and is performed
by opposing tools that hold the eyelet in position and curl the extended
portion of its barrel.
ASSEMBLY METHODS
BASED ON INTERFERENCE FITS
Several
assembly methods are based on mechanical interference between the two mating parts
being joined. This interference, which occurs either during assembly or after
the parts are joined, holds the parts together.
Press Fitting
A
press fit assembly is one in which the two components have an interference fit
between them. The typical case is where a pin (e.g., a straight cylindrical
Fastening
with an eyelet:
(a)
the eyelet, and
(b)
assembly sequence:
(1)
inserting the eyelet
through
the hole and
(2)
setting operation.
Standard
pinsizes are commercially available to accomplish a variety of functions, such
as (1) locating andlocking the components—used to augment threaded fasteners by
holding two (or more)parts in fixed alignment with each other; (2) pivot
points, to permit rotation of one
component
about the other; and (3) shear pins. Except for (3), the pins are normally hardened.
Various
pin geometries are available for interference fits.
The
basic type is a straight pin,
usually made from cold-drawn carbon steel wire or bar stock, ranging in
diameter from 1.6 to 25mm(1/16 to 1.0 in
Dowel
pins are manufactured to more precise
specifications
than
straight pins, and can be ground and hardened. They are used to fix the
alignment of assembled components in dies, fixtures, and machinery.
Taper pins
possess a taper (reduce in thickness )
of 6.4mm
(0.25
in) per foot and are driven into the hole to establish a fixed relative
position between the parts. Their advantage is that they can readily be driven
back out of the hole.
grooved pins—
solid
straight pins with three longitudinal grooves in which the metal is raised on
either side of each groove to cause interference when the pin is pressed into a
hole;
knurled pins, pins with a knurled pattern that causes interference in
the mating hole
coiled pins,
also called spiral pins, which are made by rolling strip stock into a coiled
spring.
Shrink and
Expansion Fits
These
terms refer to the assembly of two parts that have an
interference
fit at roomtemperature. The typical case is a cylindrical pin or shaft
assembled into a collar.
To
assemble by shrink fitting, the
external part is heated to enlarge it by
thermal
expansion, and the internal part either remains at room temperature or is
cooled to contract its size. The parts are then assembled and brought back to
room temperature, so that the external part shrinks, and if previously cooled
the internal part expands, to form a
strong
interference fit.
Expansion fit is when only the internal part is cooled to contract
it
for assembly; once inserted into the mating component, it warms to room
temperature, expanding to create the interference assembly. These assembly
methods are used to fit gears, pulleys, sleeves, and other components onto
solid and hollow shafts.
Snap Fits and
Retaining Rings
Snap
fits are a variation of interference fits. A snap fit
involves
joining two parts in which the mating elements possess a temporary interference
while being pressed together, but once assembled they interlock to maintain the
assembly.
Advantages
of snap fit assembly include (1) the parts can be designed with self aligning features,
(2) no special tooling is required, and (3) assembly can be accomplished very
quickly. Snap fitting was originally conceived as a method that would be
ideally suited to industrial robotics applications; however, it is no surprise
that assembly techniques that are easier for robots are also easier for human
assembly workers.
A retaining ring, also known as a snap ring, is a fastener that snaps
into a circumferential groove on a shaft or tube to form a shoulder.
OTHER MECHANICAL
FASTENING METHODS
Stitching
is a
fastening operation in which a stitching machine is used to form the U-shaped
stitches one at a time from steel wire and immediately drive them through the
two parts to be joined.
Applications
of industrial stitching include light sheet metal assembly,metal hinges,
electrical connections, magazine binding, corrugated boxes, and final product
packaging. Conditions that favor stitching in these applications are (1)
high-speed operation, (2) elimination of the need for prefabricated holes in
the parts, and (3) desirability of using fasteners that encircle the parts.
Stapling,
preformed U-shaped staples are punched
through the two parts to be
attached.
The staples are supplied in convenient strips.The individual staples are
lightly stuck together to form the strip ,but they can be separated by the stapling
tool for driving.
Sewing is a
common joining method for soft, flexible parts such as cloth and leather. The
method involves the use of a long thread or cord interwoven with the parts so
as to produce a continuous seam between them.
Cotter pins
are fasteners formed from half-round wire into a single two-stem
pin,
MOLDING INSERTS AND
INTEGRAL FASTENERS
These
assembly methods form a permanent joint between parts by shaping or reshaping one
of the components through a manufacturing process such as casting, molding, or sheet-metal
forming.
Inserts in Moldings
and Castings This method involves the placement of a component into a
mold before plastic molding or metal casting, so that it becomes a permanent
and integral part of the molding or casting. Inserting a separate component is
preferable to
molding or casting its shape if the superior
properties (e.g., strength) of the insert material
are required, or the geometry achieved
through the use of the insert is too complex or
intricate to incorporate into the mold.
Integral fasteners involve deformation of component parts so
they
interlock and create a mechanically fastened
joint. This assembly method is most common
for sheetmetal parts.
DESIGN FOR ASSEMBLY
Design
for assembly (DFA) has received much attention in recent years because assembly
operations constitute a high labor cost for many manufacturing companies.
The
key to successful design for assembly is
(1)
design the product with as few
parts
as possible
(2) design the remaining parts so they are
easy to assemble. The cost
of
assembly is determined largely during product design
GENERAL PRINCIPLES
OF DFA
Most
of the general principles apply to both manual and automated assembly. Their
goal is to achieve the required design function by the simplest and lowest cost
means.
1)Use
the fewest number of parts possible to reduce the amount of assembly required.This
principle is implemented by combining functions within the same part that might
otherwise be accomplished by separate components (e.g., using a plastic molded
partinstead of an assembly of sheet metal parts).
2)
Reduce the number of threaded fasteners required. Instead of using separate
threaded fasteners, design the component to utilize snap fits, retaining rings,
integral fasteners, and similar fasteningmechanisms that can be
accomplishedmore rapidly.Use threaded fasteners only where justified (e.g.,
where disassembly or adjustment is required).
3)
Standardize fasteners. This is intended to reduce the number of sizes and
styles of fasteners required in the product. Ordering and inventory problems
are reduced, the assembly worker does not have to distinguish between so many
separate fasteners, the workstation is simplified, and the variety of separate
fastening tools is reduced.
4)Reduce
parts orientation difficulties. Orientation problems are generally reduced by designing
a part to be symmetrical and minimizing the number of asymmetric features. This
allows easier handling and insertion during assembly.
5)
Avoid parts that tangle. Certain part configurations are more likely to become
entangled
in parts bins, frustrating assembly workers or jamming automatic feeders.Parts
with hooks, holes, slots, and curls exhibitmore of this tendency than parts
without
DESIGN FOR
AUTOMATED ASSEMBLY
Methods
suitable for manual assembly are not necessarily the best methods for automated
assembly.
1)Use
modularity in product design. Increasing the number of separate tasks that areaccomplished
by an automated assembly system will reduce the reliability of the system.To
alleviate the reliability problem, Riley suggests that the design of the
product be modular in which each module or subassembly has a maximum of 12 or
13 parts to be produced on a single assembly system. Also, the subassembly
should be designed around a base part to which other components are added.
2)
Reduce the need for multiple components to be handled at once. The preferred
practice
for automated assembly is to separate the operations at different stations
rather than to simultaneously handle and fastenmultiple components at the
sameworkstation.
3)Limit
the required directions of access. This means that the number of directions in which
new components are added to the existing subassembly should be minimized. Ideally,
all components should be added vertically from above, if possible.
3)High-quality
components. High performance of an automated assembly system
requires
that consistently good-quality components are added at each workstation.
Poor
quality components cause jams in feeding and assembly mechanisms that result in
downtime.
4)Use
of snap fit assembly. This eliminates the need for threaded fasteners; assembly
is by simple insertion, usually from above. It requires that the parts be
designed with special positive and negative features to facilitate insertion
and fastening.
