ME 307 Machine Design I Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I 8-1 8-2 8-3 8-4 8-5 8-6 8-7 8-8 8-9 8-10 8-11 8-12 8-13 8-14 8-15 Dr. A. Aziz Bazoune Thread Standards and Definitions The Mechanics of Power Screws Strength Constraints Joints-Fasteners Stiffness Joints-Member Stiffness Bolt Strength Tension Joints-The External Load Relating Bolt Torque to Bolt Tension Statically Loaded Tension Joint with Preload Gasketed Joints Fatigue Loading of Tension Joints Shear Joints Setscrews Keys and Pins Stochastic Considerations Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 2

ME 307 Machine Design I

Announcements

  

HW #5 Ch 18, on WebCT Due Date for HW #5 is Mon. DEC. 31, 2007 Quiz on Ch. 18, Mon. DEC. 31, 2007 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 3

ME 307 Machine Design I 8-1 Thread Standards and Definitions Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 4

ME 307 Machine Design I

Introduction

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.

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 5

ME 307 Machine Design I Introduction

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

power screw

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 such as

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 6

ME 307 Machine Design I

Fasteners

•a

Threaded Fastener

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

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 7

ME 307 Machine Design I

Thread Profiles

(c)

Thread profiles. a) Square (b) ACME; (c) UN, ISO Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 8

ME 307 Machine Design I

Thread Profile Parameters

Figure 8-1

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

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 9

ME 307 Machine Design I

Threads

(a) Single (

STANDARD

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

Dr. A. Aziz Bazoune CH-8 LEC 33 Slide 10

ME 307 Machine Design I

Thread 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

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 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 d 2 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 d 1 . It follows that d 1 = d - 5 h/4 = d - 1.08 p This leads to the rule of thumb for suitable tapping size .

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 12

ME 307 Machine Design I Thread Profile For Metric System (M, MJ) major H= 0.5(3) 1/2 p pitch US N= # threads/in minor a ISO 68= a Dr. A. Aziz Bazoune American National (Unified) thread standard= 60° Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 13

ME 307 Machine Design I

Thread Standards

1) ISO Metric thread system

: Table 8-1 Major diameter (mm), 2

a

= 60° Standard thread is RH Specifications: e.g.: M12x1.75

or MJ12x1.75

M = Basic Metric, J = round root; 12 = nominal major diameter (mm); 1.75 = pitch (mm) Tensile stress area A t = p /4(d m +d r )/2 (see footnote T.8.1)

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 14

ME 307 Machine Design I Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 15

ME 307 Machine Design I

Thread Standards

2) The American National (Unified) Thread standards

is used mainly in the US: Table-8-2 (Size designation) use d UN=regular thread, UNR=round root (use root radius)

Specifications

: 5/8”-18 UN, UNC, UNF UNR, UNRC, UNRF 5/8”=d 18 = N (thread size) UN = Unified, F=fine, C=Coarse, R =Round Root

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 16

ME 307 Machine Design I

Thread Standards

3. Square (a) and The ACME Threads

(b)-

used mainly in power screws Table 8.3 gives preferred pitches for ACME threads

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 17

ME 307 Machine Design I Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut);

d m

is the mean diameter of the thread contact and is approximately equal to (

d

+

d r

)/2.] Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 18

ME 307 Machine Design I Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 19

ME 307 Machine Design I

ACME Thread Profile

Figure 15.5 Details of ACME thread profile. (All dimensions are in inches.) Dr. A. Aziz Bazoune CH-8 LEC 33 Slide 20

ME 307 Machine Design I

ACME Thread Properties

Cres t diameter,

d c

, in.

1/4 5/16 3/8 7/16 1/2 5/8 3/4 7/8 1 1 1/8 1 1/4 1 3/8 1 1/2 1 3/4 2 2 1/4 2 1/2 2 3/4 3 3 1/2 4 4 1/2 5

N umber of threads per inch,

N

n

16 14 12 12 10 8 6 6 5 5 5 4 4 4 4 3 3 3 2 2 2 2 2

a Tens ile s tres s area,

A t

, in

0.02663

0.04438

0.06589

0.09720

0.1225

0.1955

0.2732

0.4003

0.5175

0.6881

0.8831

1.030

1.266

1.811

2.454

2.982

3.802

4.711

5.181

7.338

9.985

12.972

16.351

2 Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints S hear s tres s area,

A s

, in 2

0.3355

0.4344

0.5276

0.6396

0.7278

0.9180

1.084

1.313

1.493

1.722

1.952

2.110

2.341

2.803

3.262

3.610

4.075

4.538

4.757

5.700

6.640

7.577

8.511

ME 307 Machine Design I

Mechanics of Power Screws

Dr. A. Aziz Bazoune



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

The joyce

applications include

clamps or warm-gear vises, presses, lathes lead screw jack screws, and jacks.

Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 22

ME 307 Machine Design I Mechanics of Power Screws Weight supported by three screw jacks. In each screw jack, only the shaded member rotates.

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 23

ME 307 Machine Design I

Mechanics of Power Screws

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

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 24

ME 307 Machine Design I

Vehicle Screw Jack

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 25

ME 307 Machine Design I Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 26

ME 307 Machine Design I

Power Screw with

collar

Different types of collars

Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 33 Slide 27