3 screws fasteners spr part a

8/3/2019 3 Screws Fasteners SPR Part A

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ME C312: Desi n of Machine Elements

Chapter 8Screws, Fasteners, and the

-Part-A: Power ScrewsI/C: Regalla Srinivasa Prakash


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Scope: Introduction

Design of Power Screws

The Mechanics of Power Screws

Body stresses and failure prediction

Design of Non-permanent fasteners Threaded Fasteners (Bolts and Bolted Joints)

Joints Fastener Stiffness

Member Stiffness

Bolt Strength

Tension Joints External Load

Relating Bolt Torque to Bolt Tension Gasketed Joints

Tension Joints in Dynamic Loading

Adequacy Assessment, Specification Set, Decision Set and Design

Shear Joints

Non-threaded fasteners Pins and Keys

Omitted:

ec on - an s a s ca rea men an ormu ae roug ou e c ap er


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Introduction

single integral parts will not do

Joints may be permanent, semi-permanent and non-permanent joints

Helical screw is an non-permanent joint

Helical screw was a very important invention

The biggest challenge in designing a joint made up ofscrews is to enable the total product retain its shape andfunctionality without coming apart under external load


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Thread Standards and Definitions

2

1

3

4

7

85

6


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LEAD AND MULTI-THREADED SCREWS

Linear distance moved by the screw for onerotation of nut is the lead,

l = * multi licit of threadin

Double threaded, l=2p1 rotation

- ,Question:

A screw is to be used to advance the carriagep.

pitch (1.5 mm) but one having multiplicity of

threading of 2 (double threaded) and thesecond 3 (triple-threaded) are being used.

l=pAssuming the automated screw speed to besame in both cases, which screws use willintroduce greater surface roughness on

Answer: The lead for the first screw is 2*1.5 = 3 mm and that for second screw is

3*1.5=4.5 mm. Since the second screw gives higher feed rate, cut will be rougher insecond case.


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The Metric Thread:

1106

5 9

3

2

7

10

MJ Profile,

for fatigue

loading


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Tensile Stress Area

2

42/ ttprt dAddd =+=

t


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Diameters and Areas of Coarse-

Pitch and Fine-Pitch Metric

.

Question:

Between two screws having samemajor diameter and same pitch,

member strength?

Answer:

The one with fine-pitchedthreads. (Why?)

Because, from this table, youcan see that the tensile stressarea is more for fine-pitch

.


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TWO APPLICATIONS OF THREADED SCREWS

Square threads

cme rea s

Used in machine tools, screw compressors etc.

Fasteners

Metric threaded (triangular) Used for bolts, screws etc.


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POWER SCREW APPLICATIONS

(a) Square thread; (b) ACMEthread.


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Power screw or Screw jack mechanism

Electric screw jack operated lift table

The Joyce worm-gear screw jack.Bench Vice / Work Vise


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THE MECHANICS OF POWER SCREWSfor raising the load,

Helix angle

Lead anglefor lowering the load,

=

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

m


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+

f

lF

Raising:

sincos fPR

=

=m

R

dlf

P

.1

+= fdlFdT mmR

lF

mLowering:

sincos

ss

fPL

+

=

+

=m

Llf

P

.1

m

=lfdFd

T mm

+m


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Self locking of power screws

L

friction in order to lower the load

n cer a n ns ances un es ra e , e oa may

itself lower by causing the screw to spin; it may

In such cases, TL is either zero or negative (canyou exp a n w a nega ve L p ys ca y means

Whenever, the load does NOT lower by itself

unless a positive TL is applied, the screw is said tobe self-locking


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CONDITION FOR SELF-LOCKING OF POWER SCREWS

+

=fld

Tm

mmL

2

Thus the condition for self-locking is

mL

Divide both sides b . Since l/ =tan

tan>

Thus the screw is self locking whenever the coefficient ofr c on s grea er an e angen o e ea ang e.


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Accounting for collar friction

Normall a collar is em lo edto enable the power screwsystem to have sufficient

being raised

the component being raised,

additional torque needs to beapp e to ra se t e oa , t s scalled as collar friction torque Tc

ccdFfT =

c,

the collar is not too big, it isenough to use a mean diameter,

2c

T should be added todc, at which the collar friction

force is concentrated

thread friction torque to

calculate total TR.


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Power screw s raising efficiency

the raising torque with friction

friction

2

To =

o FlT ==RR TT 2

Use Tables for values of coefficient of fand fc.


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Coefficients of Friction ffor Threaded Pairs

Thrust-Collar Friction Coefficients


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Raising torque for ACME screws

+ secfdlFd mm

=

sec2 fldmR

The effect of the thread angle in ACME thread is to increasethe friction force between the screw and the nut due to the

For power screw application, though the ACME thread is not

su a e ue o g er r c ona orce resu ng rom we g ngaction, is invariably used because it is easier to manufacturethan the square threads.

It permits the use of split nut.


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Typical industrial nomenclature of Acme threads:


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(a) Effect of thread angle in ACME threads: Normal thread force isincreased due to thread angle

c


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Body stresses in power screws

, x Transverse shear (no contribution to von-Mises

is zero and is zero where bending stress ismaximum; hence needs to be only independentlychecked for)

Bearing pressure (no contribution to von-Misesstress because it is distributed over the thread andis maximum at the middle of thread and is zero at

e roo o e rea Torsional shear stress, xy x a compress ve stress, z


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Body stresses in the screw

pressure

Critical element at which

e von- ses s ress sevaluated


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Body stresses in the screw threads: those to be taken into accountfor estimation of the von-Mises stress at the critical element

Power screws are operated normally at low speeds and hence static designenough.

316R

xy T = 3L

xy =or 2r

zd

F

A

F

==

r r

FFpM 624 ==== pndpndcI trtrx

4 2

1

62'

1

2222

zxyzxyxzzyyx +++++=

'

;3': 22

y

DExyyyxx nstressPlane =++=


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Body stresses in the screw: those that need only to beindependently checked (no need to take into account in von-Mises

Buckling:lS

SF y

y

12

=

J. B. Johnsons

crit

fixed; (c) one end free and one end fixed; (d)

one end rounded and pivoted, and one endfixed.


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Body stresses in the screw: those that need only to beindependently checked (no need to take into account in von-Mises

Must be less than the safe

pndpnd tmtmB

2

==ear ng pressure g ven n

Table 8-3. Causes too much

wear and sometimes

crushin .

FFV

333===

It is at the centre of theroot area. Must be less

pnpn trtr than the shear yieldstrength of material.


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Loading taking by the successive threads

relative fractions of the total load being born by

We w ll cons er the ollow ng shar ng pr nc ple:

First engaged thread carries 38% Second engaged thread carries 25%

Third engaged thread carries 18%

Rest of the threads carry negligibly smaller loads andhence do not govern the design

The seventh threads is virtually free of load


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The tensile stress area of metric threadsonce more

-.various diameters and the tensile stress area forcoarse- itch and fine- itch metric threads as

follows. Minor diameter d = d 1.226869p

Pitch diameter dm = d 0.649519p

Diameter for tensile area and the tensile stress areacan be estimated as follows (see Table 8-1).

2rmtd =

2

4tt dA =


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Solved example 8-1:

kN.6.4Fmm;40dc0.08;fcf2p;lmm;4p;32 ======= mmd

lead.and

diameterminordiameter,pitchwidth,threaddepth,thradtheFind)(a

mm284-32p-ddr

mm304/2-32p/2-ddm:Sol

===

===

TLandTRFind(b)

mmp ===

3 screws fasteners spr part a

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