dr. a. aziz bazounechapter 8: screws, fasteners and the design of nonpermanent joints me 307 machine...

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I



8-1 8-1 Thread Standards and DefinitionsThread Standards and Definitions8-28-2 The Mechanics of Power ScrewsThe Mechanics of Power Screws8-38-3 Strength Constraints Strength Constraints 8-48-4 Joints-Fasteners StiffnessJoints-Fasteners Stiffness8-58-5 Joints-Member Stiffness Joints-Member Stiffness 8-68-6 Bolt Strength Bolt Strength 8-78-7 Tension Joints-The External LoadTension Joints-The External Load8-88-8 Relating Bolt Torque to Bolt TensionRelating Bolt Torque to Bolt Tension8-98-9 Statically Loaded Tension Joint with PreloadStatically Loaded Tension Joint with Preload8-108-10 Gasketed JointsGasketed Joints8-118-11 Fatigue Loading of Tension JointsFatigue Loading of Tension Joints8-128-12 Shear JointsShear Joints8-138-13 SetscrewsSetscrews8-148-14 Keys and PinsKeys and Pins8-158-15 Stochastic ConsiderationsStochastic Considerations

CH-8 LEC 33 Slide 2


ME 307 Machine Design I AnnouncementsAnnouncements

HW #5 Ch 18, on WebCT Due Date for HW #5 is Mon. DEC. 31, 2007 Quiz on Ch. 18, Mon. DEC. 31, 2007

CH-8 LEC 33 Slide 3



CH-8 LEC 33 Slide 4

8-1 8-1 Thread Standards and DefinitionsThread Standards and Definitions


ME 307 Machine Design I IntroductionIntroduction

The fundamental operation in manufacture is the creation of

shape - this includes assembly, where a number of

components are fastened or joined together either

permanently by welding (Ch9) for example or detachably

(nonpermanent) by screws, nuts and bolts and so on. Since

there is such a variety of shapes in engineering to be

assembled, it is hardly surprising that there is more variety

in demountable fasteners than in any other machine

element. Fasteners based upon screw threads are the most

common, so it is important that their performance is

understood, and the limitations of the fastened assemblies

appreciated.

CH-8 LEC 33 Slide 5


ME 307 Machine Design I IntroductionIntroduction

There are two distinct uses for screw threads and they usually demand different behavior from the threads :

•a power screw such as a lathe leadscrew or the screw in a car lifting jack which transforms rotary motion into substantial linear motion (or vice versa in certain applications), and

CH-8 LEC 33 Slide 6


ME 307 Machine Design I FastenersFasteners

•a Threaded Fastener similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small.

CH-8 LEC 33 Slide 7


ME 307 Machine Design I Thread ProfilesThread Profiles

Thread profiles.

a) Square (b) ACME; (c) UN, ISO

(c)

CH-8 LEC 33 Slide 8


ME 307 Machine Design I Thread Profile ParametersThread Profile Parameters

Lead=L=n pFigure 8-1Terminology of screw threads. Sharp Vee Threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation

CH-8 LEC 33 Slide 9


ME 307 Machine Design I ThreadsThreads

(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.

Text Reference: Figure 15.2, page 667 CH-8 LEC 33 Slide 10


ME 307 Machine Design I Thread SystemsThread Systems

A thread 'system' is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Whitworth, Sellers, British Standard Pipe (BSP) are just three of the many systems which proliferated before the adoption of the

1.ISO Metric thread system. 2.The American National (Unified) Thread standards is used mainly in the US.3.Square and ACME

CH-8 LEC 33 Slide 11


ME 307 Machine Design I Thread geometry

The basic profile of ISO Metric threads is built up from contiguous equiangular triangles of height H disposed symmetrically about a pitch line which becomes the pitch cylinder of diameter d2 when the profile is rotated about the

axis to form the thread. The distance between adjacent triangles - the pitch - is p = 2 H /√3. The tips of the triangles are truncated by h/8 to form the major diameter ( size ) d of the thread, and the bases are truncated by h/4 to form the minor diameter d1 . It follows that d1 = d - 5 h/4 = d - 1.08 p.

This leads to the rule of thumb for suitable tapping size




Thread Profile For Metric System (M, Thread Profile For Metric System (M, MJ)MJ)

pitch

minor

major

H= 0.5(3)1/2 p

US N= # threads/in

a ISO 68= a American National (Unified) thread standard= 60°CH-8 LEC 33 Slide 13


ME 307 Machine Design I Thread Standards

1)1) ISO Metric thread systemISO Metric thread system: Table 8-1 Major diameter (mm),

2a= 60° Standard thread is RHSpecifications: e.g.: M12x1.75 or M = Basic Metric, J = round root; 12 = nominal major diameter (mm); 1.75 = pitch (mm)

Tensile stress area At =p/4(dm+dr)/2 (see footnote T.8.1)

MJ12x1.75



ME 307 Machine Design I Thread StandardsThread Standards

2)2) The American National (Unified) The American National (Unified) Thread standards Thread standards is used mainly in the

US: Table-8-2 (Size designation) use d

UN=regular thread, UNR=round root (use root radius)

SpecificationsSpecifications: 5/8”-18 UN, UNC, UNF UNR, UNRC, UNRF

5/8”=d 18 = N (thread size) UN = Unified, F=fine, C=Coarse, R =Round RootCH-8 LEC 33 Slide

16


ME 307 Machine Design I Thread StandardsThread Standards

3. Square (a) and The ACME Threads (b)-used mainly in power screws Table 8.3 gives preferred pitches for ACME threads




Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut); dm is the mean diameter of the thread contact and is approximately equal to (d + dr)/2.]



ME 307 Machine Design I ACME Thread Profile

Figure 15.5 Details of ACME thread profile. (All dimensions are in inches.)

Text Reference: Figure 15.5, page 670 CH-8 LEC 33 Slide 20


ME 307 Machine Design I ACME Thread Properties

Crest diameter,d c, in.

Number ofthreads per inch, a

n

Tensile s tressarea, A t, in2

Shear s tressarea, A s, in2

1/45/163/87/161/25/83/47/81

1 1/81 1/41 3/81 1/21 3/4

22 1/42 1/22 3/4

33 1/2

44 1/2

5

1614121210866555444433322222

0.026630.044380.065890.097200.12250.19550.27320.40030.51750.68810.88311.0301.2661.8112.4542.9823.8024.7115.1817.3389.98512.97216.351

0.33550.43440.52760.63960.72780.91801.0841.3131.4931.7221.9522.1102.3412.8033.2623.6104.0754.5384.7575.7006.6407.5778.511

Crest diameters, threads per inch, and stresses for Acme thread.

N



ME 307 Machine Design I Mechanics of Power Screws

A power screw is a device that is common to tools or machinery that are used to change angular motion into translation. It is also capable of developing a large amount of mechanical advantage. Familiar applications include clamps or vises, presses, lathes lead screws, and jacks.

The joyce warm-gear screw jack



ME 307 Machine Design I Mechanics of Power ScrewsMechanics of Power Screws

Weight supported by three screw jacks. In each screw jack, only the shaded member rotates.




Example of catalogue:http://www.roton.com/web/quick.power_screws.jsp

Mechanics of Power ScrewsMechanics of Power Screws



ME 307 Machine Design I Vehicle Screw Jack




Different types of collars

Power Screw with collar


dr. a. aziz bazounechapter 8: screws, fasteners and the design of nonpermanent joints me 307 machine...

Documents

design of nonpermanent

machine design i fasteners

machine design i ch

machine design i introduction

jointsfasteners stiffness

gasketed joints

shear joints

jointsmember stiffness