Mechanical Assembly

Question 1

Fasteners

Answer 1

Hard wear components used to mechanically attach two components

Question 2

Why are mechanical assemblies often preferred

Answer 2

-ease of assembly and sometimes disassembly
-can be performed by unskilled workers in short time
-simple technology
-results easily inspected
-also useful in field installation

Question 3

Threaded fasteners

Answer 3

Have external or internal threads

Question 4

Screws

Answer 4

Externally theeaded fastner assembled into a blind threaded hole

Question 5

Self tapping screws

Answer 5

Posses geometries that permit them to form of cut the matching threads in the hole

Question 6

Bolt

Answer 6

Externally threaded faster that is inserted through holes in the parts and screwed into a nut on the opposite side

Question 7

Nut

Answer 7

Internally threaded fastened having standard threads that Mach those of bolt.

Question 8

Cap screws

Answer 8

Same geometry as machine screws but are made of higher strength metals and to closer tolerances

Question 9

How are most threaded fasteners manufactured

Answer 9

Cold forming

Question 10

Forces on a bolt

Answer 10

Bolt in tentjon and parts in compression
Forces may act in opposite direction to parts causing shear stress
Stresses applied on threads throughout their engagement length. These shear stresses are called stripping of threads

Question 11

In what measures determines the strength of a fastner

Answer 11

Tensile strength
Proof strength

Question 12

Prof strength

Answer 12

Kinda like yield strength. It is the maximum tensile stress to which an externally threaded fastener can be subjected without permanent deformation

Question 13

Ways a bolt and nut assembly can fail

Answer 13

External threads can strip
Internal threads can strip
Bolt can break for excessive tensile stresses

Question 14

Preload

Answer 14

Tension force that should be applied to lock threaded fastener

Question 15

Methods to apply required torque to fasteners

Answer 15

-operator feel
-torque wrenches- measures torque as fastener is turned
-stall motors- motorized wrenches designed to stall when desired torque reached
-fourgue turn tightening- fastner is initially tightened to a low torque level and then rotated a specific required amount

Question 16

Benefits of rivets

Answer 16

-high production rates
-simplicity
-dependability
-low cost

Question 17

Rivet

Answer 17

An unthreaded, headed pin used to join two or more parts (usually lap joint) by passing the pin through holes in the parts and then forming a second head in the pin on the other side

Question 18

Why does rivet hole need to be an optimum size

Answer 18

Too small and rivet can’t be inserted easily, decreasing production rates
Too large and the diced will not fill hole and may bend or compress during formation of the opposite head

Question 19

Solid rivet

Answer 19

Solid

Question 20

Tubular rivet

Answer 20

Hollow to end

Question 21

Semi tubular rivet

Answer 21

Hollow at end

Question 22

Bifurcated rivet

Answer 22

Hollow to end with pointy ends

Question 23

Compression rivet

Answer 23

Two parts to river

Question 24

Methods used in riveting

Answer 24

impact- hammer used in blows to upset rivet
-steady compression- rivet applies a continuous squeezing pressure to upset tool
-impact and compression combination.

Question 25

Eyelet.

Answer 25

Thin walled tubular fasteners with a flange on one end. Makes permanent lap joint.

Question 26

When is an eyelet used instead of a rivet

Answer 26

Low stress application to save material, weight and cost

Question 27

How are eyelet fastened

Answer 27

Eyelet is inserted through the part go,es, and the straight end is formed over to secure assembly. Performed by opposing tools that hold the eyelet in position and curl extended part over its barrel

Question 28

Press fitting

Answer 28

Two components have an interference fit, a pin is pressed into a hole of a slightly smaller diameter.

Question 29

Pin functions in press fitting

Answer 29

Locating and locking components Pivot points - permit rotation Sometimes Shear pins- break under a sudden or severe shearing to shave rest of assembly

Question 30

Maximum effective stress in terms of radial fit pressure

Answer 30

It is two times the radial fit pressure

Question 31

Maximum effective stress in terms of yield strength of material

Answer 31

Yield strength divided by safety factor

Question 32

Dowel pins

Answer 32

Straight rods used for precise alignment of components

Question 33

Straight pins

Answer 33

Long and this and pointed

Question 34

Taper pins

Answer 34

Like dowel but with taper

Question 35

Shrink fitting

Answer 35

External part heated and enlarged by thermal expansion. Parts assembled then cool to contract

Question 36

Expansion fit

Answer 36

When only internal part is cooled to contract it for assembly

Question 37

Snap fit

Answer 37

Joining two parts in which the mating elements possess a temporary interference while being pressed together, but once interlocked maintain the assembly. Elastic deformation occurs,

Question 38

Expansion fit

Answer 38

When only internal part is cooled to contract it for assembly

Question 39

Advantages of snap fit

Answer 39

parts can be designed with self aligning features -no special tooling required -assembly can be accomplished Very quickly

Question 40

Retaining (snap) ring

Answer 40

A fastner that snaps into a circumference groove on a shaft or tube to form a shoulder. Can be used to locate or restrict movement of parts mounted on the shaft

Question 41

How to assemble as retaining ring

Answer 41

Special pliers tool used to elastically deform ring so that it fits over shaft and then released into groove

Question 42

Key to a successful design

Answer 42

design for as few parts possible -design so parts are easy to assemble

Question 43

How best to design for assembly

Answer 43

use fewest number of parts possible -reduce number of threaded fasteners required (eg snap fits instead) -standardise fasteners- reduce number of sizes and styles to simplify process -reduce part orientation difficulties-easier handing in production -avoid parts that tangle

Question 44

How to design for automated assembly

Answer 44

use modularity in product design-more separate tasks decrease reliability -reduce need for multiple components handled at once -limit the required directions of accessx- ideally all parts added vertically from above -high quality components- poor causes problems with assembly -use snap fit assembly-eliminates need for threaded fasteners