6.design of fasteners

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Design of Fasteners


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Design of Screw

Fasteners


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Introduction A screw thread is formed by cutting a

continuous helical groove on cylindricalsurface.

A continuous single helical groove is known as

single threaded or single start. If second groove is cut into the space between

the groove of first then it is double threaded ordouble start.

Screw joint are formed by bolt and nut usedfor joining machine parts or for fastening,adjustment, assembly, inspection, replacement.


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Advantages

1) These are convenient to assemble anddisassemble.

2) Highly reliable in operation.

3) Screw joint are adopted in various operatingconditions.

4) Screws are relatively cheap to produce due tostandardization.

Disadvantages

The main disadvantage of this joint is thestress concentration in the thread portion and

strength is less than welded or riveted joint.


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Types of Screw Fastening Bolts - They are basically threaded fasteners

normally used with nuts.

Screws - They engage either with a preformed ora self made internal threads.

Studs -They are externally threaded headless

fasteners. One end usually meets a tappedcomponent and the other with a standard nut.

Tapping screws -These are one piece fastenerswhich cut or form a mating thread when driveninto a preformed hole. These allow rapidinstallation since nuts are not used.

Set Screws -These are semi permanent fastenerswhich hold collars, pulleys, gears etc on a shaft.Different heads and point styles are available.


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Examples where screw joints are preferred

over welded joint.1) Assembly of crank shaft and connecting rod.

2) In braking system of an automobile because

screw joints are convenient to assemble anddisassemble and relatively cheap to produce

due to standardization.


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Forms of Thread

There are mainly two types of thread

1) Triangular or V type

2) Square thread


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Advantages of V thread

1) V threads offers greater frictional resistance

of motion than square thread and are thusbetter suited for fastening purpose.

2) These are stronger than square thread.

3) These are cheaper because of easy to cut by

die or on machine.

4) These are used to tighten the parts together in

bolts, nuts, stud and nut, tap bolts etc. becausethey prevent the nut from slacking back due

to high frictional resistance.


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Disadvantages

1) V threads are not suitable for power

transmission.

2) They have a component of force which acts

perpendicular to the axis causing bursting

action on the nut and increasing friction.


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Forms of Threads

1) With worth Thread (BSW) -


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2) Acme thread -


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3) Square Thread -


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4) V Thread -


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Stresses in screw fastenings

It is necessary to determine the

stresses in screw fastening due to

both static and dynamic loading in

order to determine their dimensions.In order to design for static loading

both initial tightening and external

loadings need be known.


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A) Initial tightening load

When a nut is tightened over a screw followingstresses are induced:

(a) Tensile stresses due to stretching of the bolt

(b) Torsional shear stress due to frictionalresistance at the threads.

(c) Shear stress across threads

(d) Compressive or crushing stress on the threads(e) Bending stress if the surfaces under the bolt

head or nut are not perfectly normal to the bolt

axis.


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a) Tensile Stress

Since none of the above mentioned stressescan be accurately determined bolts are usually

designed on the basis of direct tensile stress

with a large factor of safety.

boltsintensionInitialP

ddiameterCored

screwofpitchordiameterMeandWhere

dd

d

A

P

i

oc

c

oit

84.0

)2

(4

2840


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b) Torsional shear stress -

Due to twisting moment, the bolt is subjected totorsional shear stress.

3

16cs dT


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roottheatthreadofwidthb

contactinthreadofNumbern

nouofdiameteralnod

bnd

P

isnutforstressshearaverageThe

bnd

P

isscrewforstressshearaverageThe

o

o

n

c

s

min

c) Shear stress across the threads -


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d) Crushing stress on threads

The compression or crushing stress between thethread of screw nut is given by

ndd

P

co

crc )( 22


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e) Bending Stress

Let, Xdifference in height between the extremecorner of the nut or head.

EModulus of elasticity

llength of shank of the bolt

The bending stress induced in the shank of the bolt is

given by

lXE

b2


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2. Stresses due to external Forcesa)Tensile stress

oc

c

t

c

t

dd

And

nd

P

boltsofnumbernarethereiffoundbecandiametercoreThe

d

P

84.0

4

''

4

2

2


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b) Shear Stress

The shear stress induced in the bolt is given by

nd

P

o

s

2

4


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c) Combine tension and shear stress

22max

22

max

42

1

42

1

2)(

t

t

t

t

stressshearMaximum

stresstensileprincipleMaximum


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3. Stresses due to combine forces

Bolts are subjected to direct tensile and twisting

moment due to screwing up and external forces.

E.g. steam engine cover plate subjected to initial

tightening load and external load of steam

pressure, then the resultant load on the bolt isapproximately equal to the sum of initial tension

and the external load.


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The resultant load on nut is

boltofelasticitythetopartconnectedofelasticityofRatioa

bolttheonloadExternalP

bolttheoftighteningtoduetensionInitialP

ka

aWhere

Pa

aPP

i

i

2

2

1

1


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Standard values of k

Type Of Joint K=(a/1+a)

Metal to metal joint 0.00 to 0.10

Hard Copper Gasket 0.25 to 0.50

Soft copper gasket with long

through bolt

0.50 to 0.75

Soft packing with through bolts 0.75 to 1.00

Soft packing with studs 1.00


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Bolts with Uniform Strength When a bolt is subjected to shock load i.e. power

hammer, connecting rod, presses etc. In such cases, the torque is designed to absorb impact

load and to resist the torque to prevent the breakage of

thread.

In ordinary bolt, the effect of load concentrated on the

weakest part of the bolt i.e. the c/s area of the root of the

thread.

The stress in the threaded part will be more as comparedto the shank hence the maximum portion of energy will

be absorbed at the region of the threaded part which

may fracture the threaded portion.


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If the diameter of shank of the bolt is turned to

the core diameter of the thread then the shank of

the bolt will undergo a higher stress.

This means that shank will absorb large portion

of energy thus relieving the material at the

threaded portion

The bolt in this way become stronger and lighter

and it increases the impact load carrying

capacity.

This gives us bolts with uniform strength.


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Another method, an axial hole is drilled through

the head of the bolt as far as threaded portion,

such the area of the shank become equal to the

root area of the thread.

threadofdianeterCored

threadofdiameterouterd

holeofdiameterD

ddD

ddD

c

o

co

co

22

222

44


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6.design of fasteners

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