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Designing Weldment'sThe designer or engineer faces many
questions when designing welded structures, at one time the
approach was to design from past experience, it's easy or so it
might seem. Using a design that already exists has great merit,
much of the work may be done leaving you to modify or tweak the
design, but invariably you will also inherit all of the designs
flaws, an old design may be out dated, overbuilt, expensive.Today
these methods are largely discarded, replaced by a more systematic
approach to design. Most modern designs rely on calculations and
mathematical equations to factor in the forces at work in order to
come up with an efficient plan.
For the bulk of our study we will use steel as our material in
question, although there is wide array of other materials steel
remains the most common and cost effective, commercial availability
is great and its mechanical properties make it a good choice. When
beginning a design you must select an approach to your plan, much
of this is done without thinking too much, the choice may be
assigned to you or your gut feeling and investigations may lead you
along. The biggest decision may be to choose a redesign of an
entire product or possibly" a tune up cleaning up a section or
adding a new feature for example.
Below I've compiled a list of guidelines to assist you in the
efficient use of Steel, these 15 or so elements are a systematic
approach to designing by either section or complete assembly.
Recognize the problem (why are we doing this? To improve or invent
Analyze a present design (how did it perform? Warranty, customer
feedback, sales people & market demand) Determine Load
criteria(whats its purpose) Major design considerations(Aesthetics,
Weight, Material, Cost) Layout of fabricated components Plate and
stock considerations and preparation Special sections and Forming
options Weld joint design The size and amount of weld filler
material Use of Sub-Assemblies Assembly Considerations Control
& correction of distortion Cleaning and Inspection We will
investigate and discuss each of these topics in order to get a
basic understanding of what the designer must do in order to
fulfill his or her obligation.
Recognizing the reason:As we begin to plan our design we must
ask what the goal is for the project, ether there is a product in
place already that served its purpose well and needs a facelift
such as a diet, new look, or perhaps beefing up to increase
rigidity or load carrying capability. It may be that weve been
asked to build a completely new product from the ground up,
although considerably more work, this has advantages in that there
are no predisposed notions, it's a clean slate for the designer to
use his or her imagination.
The Existing Design:If we are asked to modify or improve a
design already in production how should we approach this in a
logical manner? Investigate all we can about its past performance!
In the original incarnation was the product larger, heavier or more
ridged than its service life really required. Could we check
service records for warranty claims, failures, customer feedback,
sales and marketing data for clues as to what is really necessary
for effective redesign? Ask what features must be retained and what
features must be added?The Load Factors
Every designed part has necessity, but what is its job? What
work will it be asked to perform, and in what type of conditions.
Understanding the service environment is crucial in design of
weldment's, Loads are perhaps the principal factor in design, terms
like static and dynamic or impact give clues to the physical stress
the parts must withstand without failure. Five types of stress must
be considered when designing welded structure, Torsional, Tensile,
Compressive, Bending &Shear stress as well as variables such as
safety factors, vibration, temperature and overloads. When
searching for a place to start look to loads for ideas, an engines
speed can determine torque on a shaft, weight on a structure, wind
shear on a building, if the factors aren't clear use an assumed
load and test as you go.
Major Design Factors:In order to achieve a goal of producing a
part at the lowest overall cost it's necessary to evaluate the
design to make certain optimum use is made of metallurgical and
physical properties of the materials. Factors include: material and
labor Safety factor's should be sufficient but too large a margin
again adds cost Appearance should be pleasing but in hidden areas
consider cheaper grades of steel( hot rolled metal is just as
strong as cold rolled for a fraction of the cost) Analyze the use
of stiffeners to replace material thickness, this reduces weight
and cost Using the most cost effective material, remember higher
carbon content steels or addition of alloying makes forming harder
and preheating necessary Use as much stock steel sizes as possible,
specialty materials slow down delivery and increase cost. Always
keep in mind accessibility for welders and mechanics for part
replacement or service.
Layout of parts:Whenever possible make your designs as easy to
handle with standard tooling, use as few individual parts as
possible to reduce welding and machine time When producing parts
for weldmts try and nest parts in a series in order to take
advantage of every inch of the raw material as possible, as little
scrap as possible. Keep section shapes round ,rectangular, this
makes it easier to program for machining and robotic welding. Plate
Preparation:Plate preparation is the basis for all fabrication with
steel; many methods are available with economy being determined by
several factors, material selection, quantity, equipment available,
but cost is normally the deciding factor. We should decide on the
best method of producing the blank parts.Flame cutting, Shearing,
Punching, Laser, Plasma, Water Jet and machining will prepare edges
but we must consider fit up which ones can produce beveled edges
where necessary and cost of each.
Forming and Sections:The second major consideration in
fabricating is generally forming; forming parts can greatly reduce
the cost of a fabrication when welds are replaced by bends at
direction changes.When designing consider:Break press capacityRoll
formingMaking rings from tubeUse breaks to bend stiffeners in large
surfacesCastings where complex shapes are present
Weld joint design;As discussed weld joint is selected primarily
on load requirements, however variables in your design may result
in startling cost overrun.Consider joint requiring the least amount
of filler metal , use minimum root opening and bevel angle to
accomplish this goal. On thick plate sections use double V instead
of single V bevels to increase penetration and improve access for
welders, specify proper reinforcement on fillet welds.
The actual joints we choose fall into two distinctive groups
classified as grooved welds and fillet welds, each group
incorporates several variations for service requirements.
Groove Welded Butt Joints:
The single V butt joint used on plate 3/8 or lighter, requires
full fusion. Strong in static tension, not well suited for bending
concentrated at the root and shouldnt be subjected to fatigue,
impact loads or low temperature.A very low cost to produce.The
double V butt joint is the best for all loading conditions,
specified for heavy plate, penetration must be complete and
alternate welding must be done in order to keep the joint
symmetrical. Cost is higher than a single V but less filler is
ultimately required.
Single U butt joint readily meets ordinary load requirements;
used for work with high precision fit up, used on plate -
thickDouble U butt joint for plate over or heavier must have access
from both sides, costly to machine but uses less filler than the
single U meets all load requirements.
Square T joint Fillet weldsCan be made on one side or two and
light to reasonably thick materials where load subjects the weld to
longitudinal shear. Care must be taken to specifying this joint
where severe impact or transverse loads are encountered.The Single
bevel tee joint withstands severe loads compared to square tee
joints, confined to or greater, or less where one side welding can
only be done.Double bevel Tee joint used where transverse and
longitudinal shear are present, welded on both sidesSingle J tee
joint used in very heavy plate where welding one side is only
possible, for severe loadsDouble J tee joint weld both sides, 11/2
and thicker, unusually severe loads
Double fillet lap joint withstands more severe loads than single
fillet lap joints, widely used joint.
Flush corner Joint primarily used on 12 gauge and lighter
supports only moderate loads.Half open corner joint used more on
materials 12 gauge and up, more penetration than flush cornerFull
open corner joint general use joint where welding can be done from
both sides, used on all thickness, capable of carrying heavy loads,
good for fatigue and impact Edge joint suitable for plate or less
sustains light loads