Fasteners, Powers Screws, Connections.ppt

8/10/2019 Fasteners, Powers Screws, Connections.ppt

1/31

Fasteners, Powers Screws,

ConnectionsHelical thread screw was an important invention.

Power Screw, transmit angular motion to liner motion

Transmit large or produce large axial forceIt is always desired to reduce number of screws


2/31

Definition of important

Terminologies

Major diameter d, Minor diameter dr Mean dia or pitch diameter dp

Lead l, distance the nut moves for one turn rotation


3/31

Single and Double threaded screws

Double threaded screws are stronger and moves faster


4/31

Bolts, Screws, and Studs

Does Anyone Know the Difference? A BOLT is Used with A NUT

A SCREW is Used with A TAPPED hole

The Hole May be SELF-Tapped e.g., Wood Screws

A STUD is a TREADED ROD that is

Inserted into a TAPPED HOLE to leaveExposed a Threaded STEM

A Stud May Also be WELDED to a Surface


5/31

Machine Screw Head Types

Slot Head Provides More Driving Torque, But Driver is Hard to Center

Not good for Power Driving


6/31

Threaded Inserts

Use With Soft Parent Materials Such as

Aluminum, Wood, Plastic, etc.


7/31

Insert Advantages Greater clamping pressure in softer materials - provides higher

strength, better seals. Better user of high-tensile-strength fasteners.

Resistance to vibration in assembly.

High-quality standard threads without tapping.

Permanent, wear-resistant threads.

Protective of expensive castings and moldings.

Reduced performance loss from cold flow of plastic.

Reduce risks in molding cycle.

Easy to install in a drilled or molded hole.

Suitable for automatic installation.

Wide variety of types to fit virtuallyany application


8/31

Screw Designations

United National

Standard UNS

International Standard

Organization

Roots and crest can be either flat or roundPitch diameter roduce same width in the thread and s ace


9/31

Common Tread forms

Joining

Joining

Lite-Duty (e.g., Light Bulbs)

Pwr Xmission


10/31

Coarse thread Designated by UNC

Fine Thread UNF, is more resistance toloosening, because of its small helix angle.

They are used when Vibration is present

Class of screw, defines its fit, Class 1 fits havewidest tolerances, Class 2 is the most commonlyused

Class three for very precision application

Example:1in-12 UNRF-2A-LH, A for Ext. Thread

and B for Internal, R root radius Metric M10x1.5 10 diameter mm major

diameter,1.5 pitch


11/31

Square and Acme Threads are

used for the power screw

Preferred pitch for Acme Thread

d, in 1/4 5/16 3/8 1/2 5/8 3/4 7/8 1 1 1/4

p,in 1/16 1/14 1/12 1/10 1/8 1/6 1/6 1/5 1/5


12/31

Mechanics of Power Screws


13/31

Used in design to change the angular motion to linear motion, Could

you recall recent failure of power screw leading to significant

causalities


14/31

What is the relationship between

the applied torque on power screw

and lifting force F


15/31

Torque for single flat thread

)secsec(

2

fldfdlFdT

m

mmR

)(

2

)(2

f ld

lfdFdT

f ldfdlFdT

m

mmL

m

mmR

If the thread as an angle , the torque will be

Wedging action, it

increases friction


16/31

Stresses in the power Screw

pnd

F

A

V

pnd

F

pndF

d

T

tr

tr

b

tm

B

3

2

3

6

2/

16

3

Shear stress in the base

of the screw

Bearing stress

Bending stress at the root

of the screw

Shear stress in the thread

ntnumber of engagedthread


17/31

Loading to the fasteners and their

Failure considerations


18/31

Bolts are used to clamp two or more parts

It causes pre tension in the boltGrip length is the total thickness of parts

and washersl

ld

lt


19/31

t2

ld

h

L effective grip= h+t2 if t2


20/31

Failure of bolted or riveted joints


21/31

Type of Joints

Lap Joint (single Joint) But Joint


22/31

Example 1


23/31

Example 2


24/31

Example 2


25/31

Example 3


26/31

Weld


27/31


28/31

Weld under Bending


29/31


30/31


31/31

Fasteners, Powers Screws, Connections.ppt

Documents

Transcript of Fasteners, Powers Screws, Connections.ppt