1 Screw Rivet Weld

FASTENERS - THREADED

d = major diameterdr = minor diameterdp = pitch diameterp = pitch

√H = √3 / 2 p

1Element of Machine Dynamics and Design / Erum Khan

TERMINOLOGIES

The pitch “p” is the distance between adjacent thread forms measured parallel to the thread axis.parallel to the thread axis.

The pitch is the reciprocal of the number of thread forms per inch N.

The major diameter “d” is the largest diameter of a screw threadThe major diameter d is the largest diameter of a screw thread.

The minor (or root) diameter “dr” is the smallest diameter of a screw thread.

The pitch diameter “dp” is a theoretical diameter between the major and minor diameters.

The lead “l” is the distance the nut moves parallel to the screw axis pwhen the nut is given one turn.

For a single thread, the lead is the same as the pitch.


For a single thread, the lead is the same as the pitch.

THREAD STANDARD

American National (Unified) thread standardThread size is specified by giving the # of threads per inch NThread size is specified by giving the # of threads per inch N.

Thread angle is 60◦ and the crests of the thread may be either flat or roundedrounded.

Two major series are: UN and UNR.

The difference is that a root radius must be used in the UNR series, which reduced thread stress-concentration factors. UNR series threads have improved fatigue strengths.

Unified threads are specified by stating the nominal major diameter, the number of threads per inch, and the thread series, for example, 5/8 in-18 UNRF or 0.625 in-18 UNRF.


THREAD STANDARD

Metric thread standardMetric threads are specified by writing the nominal major diameter andMetric threads are specified by writing the nominal major diameter and pitch in millimeters (M12 × 1.75).

Two major series are: M and MJTwo major series are: M and MJ.

The MJ profile has a rounded fillet at the root of the external thread ( f l h hi h f ti t th i i d)(useful where high fatigue strength is required).

Square and Acme threads, are used on screws when power is to be transmitted.


POWER SCREW

A power screw is a device used in machinery to change angular motion into linear motion, and, usually, to transmit power. Familiar applications include the lead screws of lathes, and the screws for vises, presses, and jacks.screws for vises, presses, and jacks.


POWER SCREW

A square-threaded power screw with single thread having a mean diameter dm, a pitch p, a lead angle λ, and a helix angle ψ is m, p p, g , g ψloaded by the axial compressive force F. We wish to find an expression for the torque required to raise this load, and anotherexpression for the torque required to raise this load, and another expression for the torque required to lower the load.


Force diagrams: (a) lifting the load; (b) lowering the load.

THREADED FASTENERS

ular

fa

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regu

d on

bot

h

) was

her-

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s;ch

amfe

red

ener

al; (

b)n

both

sid

) jam

nut

c

nd v

iew

, ge

amfe

red

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face

; (e)

uts:

(a) e

nar

nut

cha

with

was

he

xago

nal n

u; (

c) re

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jam

nut

wes

.


Hex

nut

(d) j

side

THREADED FASTENERS STIFFNESS

Pretension or Bolt Preload Twisting the nut stretches the bolt to produce the clamping force This clamping force is called theproduce the clamping force. This clamping force is called the pretension or bolt preload.

Spring Rate is the ratio between the force applied to the member and the deflection produced by that force.p y

Grip The grip l of a connection is

th t t l thi k f th l dthe total thickness of the clamped

material. OR grip is the sum of the

thicknesses of both members

and both washers9

Element of Machine Dynamics and Design / Erum Khan

and both washers.

THREADED FASTENERS STIFFNESS

At = tensile-stress areawhere

lt = length of threaded portion of gripAd = major-diameter area of fastener


Ad major diameter area of fastenerld = length of unthreaded portion in grip

TENSION JOINTS – THE EXTERNAL LOAD

Fi = preloadP = external tensile loadPb = portion of P taken by boltPm = portion of P taken by membersPm portion of P taken by membersFb = Pb + Fi = resultant bolt loadF = P − Fi = resultant load on membersFm Pm Fi resultant load on membersC = fraction of external load P carried by bolt1 − C = fraction of external load P carried by members or Bolt1 C = fraction of external load P carried by members or Bolt

The load P is tension, and it causes the connection to stretch, or elongate, through some distance δ. We can relate this elongation to the stiffnesses by recalling that k is the force divided by the


deflection. Thus

TENSION JOINTS – THE EXTERNAL LOAD


RELATING BOLT TORQUE TO BOLT TENSION

The diameter of a hexagonal nut is equal to 1-1/2 times the nominal size. Therefore the mean collar diameter is dc = (d + 1.5d)/2 = 1.25d.

We now define a torque coefficient K as the term in brackets, and so


Or it can be written as

PROBLEM 8–15A 3/4 in-16 UNF series SAE grade 5 bolt has a 3/4 -in ID tube 13 in long, clamped between washer faces of bolt and nut by turning the nut snug and adding one-third of a turn. The tube OD is the washer-face diameter dw = 1.5d = 1.5(0.75) = 1.125 in = OD.

(a) What is the spring rate of the bolt and the tube, if the tube is made of steel? What is the joint constant C?

(b) When the one-third turn-of-nut is applied, what is the initial tension Fi in the bolt?

(c) What is the bolt tension at opening if additional

tension is applied to the bolt external to the joint?


PROBLEM 8–8The C clamp shown in the figure, uses a 5/8 in-6 Acme thread. The frictional coefficients are 0.15 for the threads and for the collar. The collar, which in this case is the anvil striker’s swivel joint, has a friction diameter of 7/16 in. Calculations are to be based on a maximum force of 6 lbf applied to the handle at a radius of 2-3/4 in from the screw centerline. Find the clamping force.p g


RIVETED JOINTS

Modes of failure in shearloading of a bolted or rivetedloading of a bolted or riveted connection: (a) Shear loading(a) Shear loading(b) Bending of rivet(c) Shear of rivet(c) Shear of rivet

(d) Tensile failure of members.(e) Bearing of rivet on members or(e) Bearing of rivet on members or bearing of members on rivet.


RIVETED JOINTS

Modes of failure in shear loading of a bolted or riveted connection:

(a) shear loading.

(b) bending of rivetPt

=σ(b) bending of rivet.

(c) Shear of rivet.

m2Z=σ

S 0.5774FF y2 ===

Dτ(c) Shear of rivet.

(d) Tensile failure of members.

ndA 2D

( ) nS

tdNbF

AF y

===σ

(e) Bearing of rivet on members or bearing of members on rivet.

( ) ntdNbA mcr−

nS

tdF

AF y

m c===σ


EXAMPLE 8–6Two 1- by 4-in 1018 cold-rolled steel bars are butt-spliced with two 1/ 2 - by 4-in 1018 cold-rolled splice plates using four 3/4 in-16 UNF grade 5 bolts as depicted in Fig. For a design factor of nd = 1.5 estimate the static load F that can be carried if the bolts lose preload.p


PROBLEM 8–43The figure shows a connection that employs three SAE grade 5 bolts. The tensile shear load on the joint is 5400 lbf. The members are cold-drawn bars of AISI 1020 steel. Find the factor of safety for each possible mode of failure.p


WELDED JOINTS

A welded joint is a permanent joint which is obtained by the fusion of the edges of the two parts to be joined together, withfusion of the edges of the two parts to be joined together, with or without the application of pressure and a filler material.

A li tiApplication:

Used in fabrication as an alternative method for casting or forging and as a replacement for bolted and riveted joints.

Also used as a repair medium e.g. to reunite metal at a crack, to build up a small part that has broken off such as gear tooth or to repair a worn surface such as a bearing surface.


ADVANTAGES & DISADVANTAGES OF WELDED JOINTS OVER RIVETED JOINTS

Advantages:

lighter than riveted structures

Disadvantages:

Because of uneven heatinglighter than riveted structures

greater strength

Because of uneven heating and cooling during fabrication, th b tprovides very rigid joints

possible to weld any part of a

the members may get distorted or additional

possible to weld any part of a structure at any point

t k l ti

stresses may develop.

requires a highly skilled labourtakes less time

inexpensive

q g y

inspection of welding work is more difficult

no joint looseningmore difficult

metallurgical changes occur


disassembly is a problem

TYPES OF WELDED JOINTS

1. Butt Joint

The butt joint is obtained by placing the plates edge to edgeThe butt joint is obtained by placing the plates edge to edge.

The butt joints may be:

1. Square butt joint 4. Double V-butt joint Corner joint

2. Single V-butt joint

Edge joint

3 Si l U b tt j i t5. Double U-butt joint

Edge joint

3. Single U-butt joint

T- joint


SUPPLEMENTARY WELD SYMBOLS

Spot Welding Weld all round

Seam Welding

Projection WeldingGrinding Finish

Machining Finish

B tt R i t

Machining Finish

Butt Resistance


BUTT WELD

Consider single V-groove weld loaded by the tensile force F. For either tension orthe tensile force F. For either tension or compression loading, the average normal stress is:stress is:

Where;h = weld throat (without reinforcement)l l th f th ldl = length of the weld

The average stress in a butt weld due to shear loading is:


LAP JOINT

It is obtained by overlapping the plates and then welding the edges of the plates.edges of the plates.

The cross-section of the fillet is approximately triangular. The fillet joints may be:joints may be:

1. Single transverse fillet are designed for tensile strength

2. Double transverse fillet

3. Parallel fillet joints

g g

are designed for shear strength


LAP OR FILLET WELD

A fillet weld is made with equal legs. The thinnest section is at the throat of the weld, at 45º from the legs.the throat of the weld, at 45 from the legs.

Throat thinnest section of the weld is called throat.L th l th f h id i k l i f thLeg the length of each side is known as leg or size of the weld.For parallel fillet weld the shear stress is:For parallel fillet weld, the shear stress is:


1 Screw Rivet Weld

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