presentation ch8 - fasteners, screws
TRANSCRIPT
Fasteners, Powers Screws, Connections
Helical thread screw was an important invention.
Power Screw, transmit angular motion to liner motion
Transmit large or produce large axial force
It is always desired to reduce number of screws
Typical cup screws
Other types of screw heads
Screw Designations
• United National Standard UNS
• International Standard Organization
Roots and crest can be either flat or round
Pitch diameter produce same width in the thread and space,
Single and Double threaded screws
Double threaded screws are stronger and moves faster
Multiple-threaded screws
Coarse thread Designated by UNC
• Fine Thread UNF, is more resistance to loosening, because of its small helix angle.
• They are used when Vibration is present• Class of screw, defines its fit, Class 1 fits have
widest tolerances, Class 2 is the most commonly used
• Class three for very precision application• Example:1in-12 UNRF-2A-LH, A for Ext. Thread
and B for Internal, R root radius• Metric M10x1.5 10 diameter mm major
diameter,1.5 pitch
Square and Acme Threads are used for the power screw
Preferred pitch for Acme Thread
1/51/51/61/61/81/101/121/141/16p,in
1 1/417/83/45/81/23/85/161/4d, in
Mechanics of Power Screws
Used in design to change the angular motion to linear motion, Could you recall recent failure of power screw leading to significant causalities
What is the relationship between the applied torque on power screw
and lifting force F
Torque for single flat thread
)sec
sec(
2
fld
fdlFdT
m
mmR
)(2
)(2
fld
lfdFdT
fld
fdlFdT
m
mmL
m
mmR
If the thread as an angle α, the torque will be
Wedging action, it increases friction
Stresses in the power Screw
pnd
F
A
V
pnd
F
pnd
Fd
T
tr
trb
tmB
3
2
3
6
2/
163
Shear stress in the base of the screw
Bearing stress
Bending stress at the root of the screw
Shear stress in the thread
nt number of engaged thread
Definition of important Terminologies
Major diameter d, Minor diameter dr Mean dia or pitch diameter dp
Lead l, distance the nut moves for one turn rotation
Bolt terminology
Main characteristics of Bolts
Some important properties
Load that a bolt can sustain
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Bolt Manufacturing Processes
Forging (upsetting)
Rolling
Thread-rolling processes: a) reciprocating flat dies; and b) two-roller dies.Threaded fasteners, such as bolts, are made economically by these processes at high rates of production
a) b)
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Manufacturing Processes -continued
Turning on screw machines
(a) Differences in the diameters of machined and rolled threads. (b) Grain flow in machined and rolled threads. Unlike machining, which cuts through the grains of the metal, rolled threads have improved strength because of cold working and favorable grain flow.
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Tightening Torque
It is typical on engines for bolts to have a specified tightening torque. Why?
It results in a quantified preload on the boltsInsures that parts never separate Maintains friction (no sliding to shear forces)Insures even distribution of loading
• prevent warpage of mating parts• uniform pressure distribution over seal or gasket
Prevents bolt from looseningReduces fatique effects
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Materials
Let’s look at the example of engine head bolts
The head isalumimum, so why are the head bolts made of steel ?
In what cases might you want to use an aluminum bolt?
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Bolt Grades
Grade indicates the tensile strength of the bolt
Determined by bolt material and heat treating
Loading to the fasteners and their Failure considerations
Bolts are used to clamp two or more parts
It causes pre tension in the bolt Grip length is the total thickness of parts and washers
l
ld
l t
t2
ldh
L’ effective grip= h+t2 if t2<d
=h=d/2 for t2 d
lt=L’- ld
Failure of bolted or riveted joints
Type of Joints
• Lap Joint (single Joint) But Joint
Example 1
Example 2
Example 2
Example 3
Weld
Weld under Bending
Brazing
• Joining process
• A filler metal is placed between two workpieces and heated until melted
• Two main types of Brazing– Ordinary– Braze welding
• Use of flux is very important
Filler Metals• Available in a wide range of brazing temperatures
• They come in a wide range of shapes
• Choice of the filler metal and its composition are important
• Diffusion of the filler metal in to the workpeice is an important consideration
Fig: a) Brazing b) Braze welding operation
Fluxes
• The use of flux in brazing is very important
• Generally made of:– Borax– Boric acid– Borates– Fluorides– Chlorides
• Wetting agents may also be added
Brazing Methods
• Torch Brazing
– Performed by heating the joint with a torch
– Depositing the filler metal in the joint
– Suitable part thickness (0.25 – 6.0)mm
– Not a automated process
– More than one torch can be used in this process
• Furnace Brazing
– Precleaned & Preloaded with brazing metal
– Heated in a furnace
Fig:An example of furnace brazing a)before b) after
Brazing Methods
Other Types Of Brazing
• Induction Brazing
• Resistance Brazing
• Dip Brazing
• Infrared Brazing
• Diffusion Brazing
Braze Welding
• Prepared like fusion welding
• Filler metal is deposited at the joint with the use of an oxyacetylene torch
• Considerably more filler is used
• Temperature is minimal compared to that of fusion welding; part distortion is minimal
Brazing Process Capabilities
• Dissimilar metals can be assembled with good joint strength
• Shear strength of brazed joints can reach 800Mpa
Fig:Joint Designs commonly used in brazing operations.The clearance between the two parts being brazed is an important factor in joint strength.If the clearance is to small, the molten braze metal will not fully penetrate the interface.
Good/Poor Designs
Soldering
• Different types of soldering
– Torch– Furnace– Iron– Induction– Resistance– Dip– Infrared– Ultrasonic– reflow (paste)– Wave
Reflow Soldering
• Solvents present in the paste are evaporated
• The Flux in the paste is activated and the fluxing action occurs
• The components are carefully preheated
• The solder particles are melted and wet the joint
• The assembly is cooled
Wave Soldering
• Popular approach to attaching circuits to circuit boards
Fig: a)Screening or stenciling paste onto a printed circuit board: 1) Stenciling process 2) a section of a typical stencil pattern b) wave soldering process
Types Of Fluxes
• Inorganic acids or salts – clean the surface rapidly
• Noncorrosive resin-based – used in electrical applications
• Soldering is used extensively in electronics industry
Adhesive Bonding
• Products are joined and assembled by the use of Adhesives
• Adhesives properties to be considered– Strength– Toughness– Resistance to various fluids– Ability to wet the surface to be bonded
Types of adhesives
Surface must be clean for joining parts
• Should avoid joints that might be subjected to peeling forces• Design for adhesive bonding
Adhesive Peeling Test
Fig : Characteristic behavior of (a) brittle (b) tough adhesive in a peeling test
Joint Design in Adhesive Bonding
Fig:Various joint design in adhesive bonding.
Configurations for adhesive bonds
Fig : (a) single lap (b) double lap (c) scarf (d) strap
Mechanical Fastening
• Threaded Fasters– Bolts– Screws– Nuts
• Other Fastening Methods– Stapling– Crimping– Snap-in Fasteners– Shrink and press fits
Rivets
Fig:a)solid b)tubular c) split (bifurcated) d) compression
Fig : Design guidelines for riveting (a)Exposed shank is too long; the result is buckling instead of upsetting (b)Rivets should be placed sufficiently far from edges to avoid stress concentrations (c)Joined sections should allow ample clearance for riveting tools (d) section curvature should not interfere with the riveting process
Joining Plastics
• Heat softens the plastic to a molten state
• Then pressure added & fusion takes place
• External Heat Sources– Hot air– Heated tools & dies– Electrical-Resistance– Lasers
• Internal Heat Sources– Ultrasonic welding– Friction welding