Mechanical Assembly Flashcards
What are fasteners
Mechanical components that attach two components together
What are the two main types of fastener
Those that allow disassembly and those that do not
Why may mechanical assembly be preferred over other joining processes
Ease of assembly and ease of disassembly
What are the four main types of mechanical assembly
Threaded Fasteners
Rivets
Interference Fits
Molded-in Inserts
What is a screw
A screw is an externally threaded fastener that is generally assembled into a blind threaded hole
What is a self tapping screw
A screw that has a geometry that permits it to cut the matching thread in the hole.
What is a bolt
A bolt is an externally threaded fastener that is inserted through holes in the parts and “screwed” into a nut on the opposite side
What is a nut
A nut is an internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form
What is a machine screw
Machine screws are the generic type, designed for assembly into tapped holes
What is a cap screw
Cap screws have the same geometry as machine screws but are made of higher strength metals and to closer tolerances
How are most threaded fasteners produced?
Cold forming
What is the most common material for fasteners and why?
Steels are most commonly used because of their good strength and low cost
What failure may be caused by shear stresses in the threads of a fastener
Stripping of the threads.
What are the two measures that determine the strength of a thread
Tensile strength and proof strength
What is proof strength?
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.
What are the three types of failure of a nut and bolt
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
What is the pre-load
The specified tension force that should be applied to a threaded fastener
What methods may be used to tighten a fastener to the required torque
(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)
What are advantages of rivets?
High production rates, simplicity, dependability, and low cost
What type of joint are rivets best suited for?
Lap joints
What are the five main types of rivet?
Solid, turbular, semi-turbular, bifurcated and compression
What are the two tooling methods for riveting?
(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
What are eyelets?
Thin walled tubular fasteners that function like rivets, they are used for low stress applications to save material, weight and cost.
What is a press fit?
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.
What is shrink fitting?
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.
What is an expansion fit?
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.
What is a snap fit?
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
What are advantages of a snap fit?
(1) the parts can be designed with self-aligning features
(2) no special tooling is required
(3) assembly can be accomplished very quickly
What is a retaining ring?
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
What are the main principles of design for assembly DFA?
(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.
What are the main principles of designing for automated assembly DFAA?
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
Define the term proof strength
Proof strength can be defined as the maximum tensile stress that an externally
threaded fastener can sustain without permanent deformation.
