threaded fasteners ch 10

7/23/2019 Threaded Fasteners Ch 10

1/37

Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.

Threaded Fasteners andPower Screws


2/37


Figure 10.1 (p. 386)Helical threads of pitch p, lead L and lead angle .

Some worm and power screws

have multiple threads

Lead L = pitch p for single thread


3/37


Figure 10.2 (p. 386)Unified and ISO thread geometry. The basic profile of the external thread isshown.


4/37


Table 10.1b (cont.)


5/37


Table 10.2 (p. 388)Basic Dimensions of ISO Metric Screw Threads


6/37


3 Classes of Fit for Screw Threads

Class 1

Loosest fit and largest tolerances

Class 3

Tightest fit and smallest tolerances


7/37


Figure 10.4 (p. 389)Power screw thread forms. [Note: All threads shown are external (i.e., on thescrew, not on the nut); dm is the mean diameter of the thread contact and is

approximately equal to (d+ dr)/2.]

Difficult tomanufacture

High axial loadcarrying capacity


8/37


Figure 10.5 (p. 391)Weight supported by three screw jacks. In each screw jack, only the shaded

member rotates.


9/37


Figure 10.6 (p. 391)Screw thread forces.

Kinematics:: tanm

L

d

=

Infinitesimal portion of nut

q = tangential force, w = portion of total axial force

W, n = normal force, f = friction coefficient,fn = friction force, q dm/2 = torque applied to nutsegment


10/37


Equilibrium Yields Torque Eq. In tangential direction

In axial direction

Combine and solve for q

Torque

cos cos sin 0nq fn n =

sin cos cos 0nw fn n + =

cos cos sin

cos cos sin

n

n

fq w

f

+=

cos cos sin

2 2 cos cos sin

m m n

n

d Wd f T Q

f

+= =


11/37


Torque (cont.) Using kinematic relationship

If application has bearing surface or thrust collar

For square thread or ACME thread cosn is nearly 1:

For lower load, reverse signs on q and friction force fn

cos

2 cos 2m m n c c

m n

Wd f d L Wf d T

d fL

+= +

2 2m m c c

m

Wd f d L Wf d Td fL

+= +

cos2 cos

m m n

m n

Wd f d LTd fL

+=

cos

2 cos 2 2 2

m m n c c m m c c

m n m

Wd f d L Wf d Wd f d L Wf d T

d fL d fL

= + = +

+ + S uare


12/37


Figure 10.7 (p. 394)Comparison of thread angles measured in axial and normal planes ( and n).


13/37


Overhauling and Self-Locking Self-locking screw: sufficient friction so that

positive torque required to lower the load Overhauling screw: low enough friction so that

axial load will cause screw to turn

Neglecting collar friction

Beware of vibration that can reduce f, and selflocking screw no longer locks!

cos= tan (square thread)n

m m

L Lf

d d

=


14/37


Efficiency of Power Screws

Output work by power screw for one turn =

WL

Input work = T = 2

Efficiency = output work/input work

cos cos tan

cos cos cot

m n n

m m n n

d fL f Le

d fd L f

= =

+ +


15/37


Figure 10.8 (p. 396)

Efficiency of Acme screwthreads when collar friction isnegligible. (Note: Values for

square threads are higher by

less than 1 percent.)


16/37


Figure 10.9 (p. 397)Ball bearing screw assembly with a portion of the nut cut away to show

construction. (Courtesy Saginaw Steering Gear Division, General Motors

Corporation.)

Very low coefficient of friction due to mostly rolling contact-

usually overhauling screw


17/37


Figure 10.11 (p. 400)Force flow for a bolt in tension.

Bearing stress on thread = P/A where

( )2 2 / 4iA d d=

t/p = no. of threads in contact

( )2 24

i

P p

td d

=

This is average value of

Bearing stress for the threads

Thread 1 carries more load. Why?


18/37


To distribute load more evenly overthreads

Make the nut out of softer material than

the thread Manufacture threads of nut with slightly

greater pitch than bolts

Modify nut design


19/37


Figure 10.12 (p. 402)A special nut provides more nearly equal distribution of load amount threads incontact.


20/37


Thread stripping stress

Stripping due to shear failure. Shear area for ISOthread:

where t = nut height, d = diameter of shear fracturesurface

For balance between bolt tensile failure and nut

thread shear failure

If made of same material Ssy = 0.58 Sy, so

Nut is usually softer, so t = (7/8) d is standard

( )0.75A d t=

( ) ( )2

0.9 S = d 0.75 S4

t y y syF A S d d

= =

0.47t d=


21/37


Figure 10.15 (p. 404)Basic threaded fastener types.


22/37


Figure 10.16 (p. 405)Some common screw (and bolt) head types.


23/37


Figure 10.17 (p. 405)"Tamper-resistant" screw heads.


24/37


Table 10.4 (p. 407)Specifications for Steel Used in Inch Series Screws and Bolts


25/37


Table 10.5 (p. 408)Specifications for Steel Used in Millimeter Series Screws and Bolts


26/37


Figure 10.18 (p. 409)Bolt loads and stresses that are due to initial tightening of a nut. M= 0 for the

bolt and nut assembly shown, that is, T1 = T2 + T3 + T4 (where T1 = nut wrench

torque). T2 = nut face friction torque = fFira(where ra is the effective radius of nutface friction forces). T3 = bolt head friction torque fFirh(where rh is the effectiveradius of bolt head friction forces). T4 = wrench torque required to keep bolt headfrom turning. Note that T4 = 0 if fFirh> T1T2.


27/37


Figure 10.19 (p. 409)Bolt tension versus elongation, resulting from tightening by torquing versus directtensioning, and for black oxide versus galvanized surfaces [5]. Note: Directtension is produced by hydraulic loading; hence, no torsional stresses are

produced.)


28/37


Figure 10.20 (p. 412)Common types of lock washers.


29/37


Figure 10.21 (p. 412)(a) Slotted and (b) castle nuts. Each is also shown with a drilled bolt and cotter pin.


30/37


Figure 10.22 (p. 412)Examples of free-spinning locknuts.


31/37


Figure 10.23 (p. 413)Examples of prevailing-torquelocknuts. ( Courtesy SPSTechnologies, Inc.)


32/37


Figure 10.24 (p. 414)Free-body study of bolt tensile loading.


33/37


Figure 10.25 (p. 414)Fband Fcversus Fe per bolt for soft clamped membersrigid bolt.


34/37


Figure 10.26 (p. 415)Fband Fcversus Feper bolt for

rigid clamped memberssoft bolt.


35/37


Figure 10.27 (p. 416)Force relationships for bolted connections.


36/37


Figure 10.28 (p. 417)One method for estimating the effective area of clamped members (forcalculating kc). Effective area Ac is approximately equal to the average area of

the dark grey section.


37/37


Figure 10.29 (p. 418)Examples of nonintended bolt bending.

threaded fasteners ch 10

Documents