Mechanical Assembly

Question 1

What are fasteners

Answer 1

Mechanical components that attach two components together

Question 2

What are the two main types of fastener

Answer 2

Those that allow disassembly and those that do not

Question 3

Why may mechanical assembly be preferred over other joining processes

Answer 3

Ease of assembly and ease of disassembly

Question 4

What are the four main types of mechanical assembly

Answer 4

Threaded Fasteners
Rivets
Interference Fits
Molded-in Inserts

Question 5

What is a screw

Answer 5

A screw is an externally threaded fastener that is generally assembled into a blind threaded hole

Question 6

What is a self tapping screw

Answer 6

A screw that has a geometry that permits it to cut the matching thread in the hole.

Question 7

What is a bolt

Answer 7

A bolt is an externally threaded fastener that is inserted through holes in the parts and “screwed” into a nut on the opposite side

Question 8

What is a nut

Answer 8

A nut is an internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form

Question 9

What is a machine screw

Answer 9

Machine screws are the generic type, designed for assembly into tapped holes

Question 10

What is a cap screw

Answer 10

Cap screws have the same geometry as machine screws but are made of higher strength metals and to closer tolerances

Question 11

How are most threaded fasteners produced?

Answer 11

Cold forming

Question 12

What is the most common material for fasteners and why?

Answer 12

Steels are most commonly used because of their good strength and low cost

Question 13

What failure may be caused by shear stresses in the threads of a fastener

Answer 13

Stripping of the threads.

Question 14

What are the two measures that determine the strength of a thread

Answer 14

Tensile strength and proof strength

Question 15

What is proof strength?

Answer 15

Proof strength is roughly equivalent to yield strength; specifically, it is the maximum tensile stress to which an externally threaded fastener can be subjected without permanent deformation.

Question 16

What are the three types of failure of a nut and bolt

Answer 16

Threading of the external threads of the bolt
Threading of the internal threads of the nut
Snapping of the bolt due to excessive tensile stress across the cross sectional area

Question 17

What is the pre-load

Answer 17

The specified tension force that should be applied to a threaded fastener

Question 18

What methods may be used to tighten a fastener to the required torque

Answer 18

(1) operator feel-not very accurate, but adequate for most assemblies
(2) torque wrenches, which measure the torque as the fastener is being turned
(3) stall-motors, which are motorized wrenches designed to stall when the required torque is reached
(4) torque-turn tightening, in which the fastener is initially tightened to a low torque level and then rotated a specified additional amount (e.g., a quarter turn)

Question 19

What are advantages of rivets?

Answer 19

High production rates, simplicity, dependability, and low cost

Question 20

What type of joint are rivets best suited for?

Answer 20

Lap joints

Question 21

What are the five main types of rivet?

Answer 21

Solid, turbular, semi-turbular, bifurcated and compression

Question 22

What are the two tooling methods for riveting?

Answer 22

(1) impact, in which a pneumatic hammer delivers a succession of blows to upset the rivet
(2) steady compression, in which the riveting tool applies a continuous squeezing pressure to upset the rivet

Question 23

What are eyelets?

Answer 23

Thin walled tubular fasteners that function like rivets, they are used for low stress applications to save material, weight and cost.

Question 24

What is a press fit?

Answer 24

A press fit assembly is one in which the two components have an interference fit between them. The typical case is where a pin of a certain diameter is pressed into a hole of a slightly smaller diameter.

Question 25

What is shrink fitting?

Answer 25

An external part is heated to enlarge it by thermal expansion, and the internal part either remains at room temperature or is cooled to contract its size. The parts are then assembled and brought back to room temperature, so that the external part shrinks, and if previously cooled the internal part expands, to form a strong interference fit.

Question 26

What is an expansion fit?

Answer 26

An expansion fit is when only the internal part is cooled to contract it for assembly; once inserted into the mating component, it warms to room temperature, expanding to create the interference assembly.

Question 27

What is a snap fit?

Answer 27

A snap fit involves joining two parts in which the mating elements possess a temporary interference while being pressed together, but once assembled they interlock to maintain the assembly. As the parts are pressed together, the mating elements elastically deform to accommodate the interference, subsequently allowing the parts to snap together

Question 28

What are advantages of a snap fit?

Answer 28

(1) the parts can be designed with self-aligning features
(2) no special tooling is required
(3) assembly can be accomplished very quickly

Question 29

What is a retaining ring?

Answer 29

A retaining ring, also known as a snap ring, is a fastener that snaps into a circumferential groove on a shaft or tube to form a shoulder

Question 30

What are the main principles of design for assembly DFA?

Answer 30

(1) design the product with as few parts as possible
(2) design the remaining parts so they are easy to assemble
(3) minimise use of threaded fasteners, only use when justified, e.g. disassembly is required
(4) standardize fasteners
(5) design for the factory worker, parts should be easily orientated and shouldn’t become tangled with each other.

Question 31

What are the main principles of designing for automated assembly DFAA?

Answer 31

1) use modularity, with a guide of about 12-13 parts per module
2) reduce need for multiple parts to be handled at once
3) limit the variety of directions the product must be accessed to add new components, ideally all components can be added vertically from above.
4) ensure high quality components are used
5) use snap fits

Question 32

Define the term proof strength

Answer 32

Proof strength can be defined as the maximum tensile stress that an externally
threaded fastener can sustain without permanent deformation.