MECHANICAL ASSEMBLY Flashcards
How does mechanical assembly differ from the other methods of assembly discussed in previous chapters (e.g., welding, brazing, etc.)?
Mechanical assembly uses a mechanical fastening method for joining two (or more) parts, whereas welding, brazing, soldering, and adhesive bonding use heat and/or pressure, sometimes combined with a filler material to permanently join parts. Also, many of the mechanical fastening methods allow for disassembly - not possible with welding and brazing.
What are some of the reasons why assemblies must be sometimes disassembled?
For maintenance and repair service, to replace worn-out components, and to make adjustments.
What is the technical difference between a screw and a bolt?
Both are externally threaded fasteners. A screw is generally assembled into a blind threaded hole, whereas a bolt is assembled using a nut.
What is a stud (in the context of threaded fasteners)?
A stud is an externally threaded fastener that does not have the usual head possessed by a bolt.
What is torque-turn tightening?
Torque-turn tightening involves the tightening of the threaded fastener to a certain low torque level, and then advancing the fastener by a specified additional amount of turn (e.g., a quarter turn).
Define proof strength as the term applies in threaded fasteners.
Proof strength can be defined as the maximum tensile stress that an externally threaded fastener can sustain without permanent deformation.
What are the three ways in which a threaded fastener can fail during tightening?
(1) Stripping of the bolt or screw threads, (2) stripping of the internal fastener threads, and (3) excessive tensile load on the cross-sectional area of the bolt or screw.
What is a rivet?
A rivet is an unthreaded headed pin used to join two parts by inserting the pin through holes in the parts and deforming the unheaded portion over the opposite side.
What is the difference between a shrink fit and expansion fit in assembly?
In a shrink fit, the outside part is expanded by heating to fit over the mating component. Then cooling causes an interference fit with the component. In an expansion fit, the internal part is cooled so that it can be readily inserted into the mating component. Then, upon warming to room temperature, it expands to cause an interference fit with its mating part.
What are the advantages of snap fitting?
Advantages of snap fitting include (1) the method is fast, (2) no tooling is required, and (3) the parts can be designed with self-aligning features for ease of mating.
What is the difference between industrial stitching and stapling?
In stitching the U-shaped fasteners are formed during the assembly process. In stapling, the fasteners are preformed.
What are integral fasteners?
Integral fasteners make use of a forming operation on one of the parts to be joined to interlock the components and create a mechanically fastened joint.
Identify some of the general principles and guidelines for design for assembly.
Some of the general principles and guidelines in design for assembly include the following: (1) Use the fewest number of parts possible to reduce assembly required. (2) Reduce the number of threaded fasteners; instead use snap fits, retaining rings, integral fasteners, and similar fastening mechanisms that can be accomplished more rapidly. Use threaded fasteners only where justified, e.g., where disassembly or adjustment is required. (3) Standardize fasteners in order to reduce the number of sizes and styles in the product. (4) Design parts to be symmetrical and minimize asymmetric features. (5) Avoid parts that tangle.
Identify some of the general principles and guidelines that apply specifically to automated assembly.
Some of the principles and guidelines that apply specifically to automated assembly include the following: (1) Use modularity in product design. Each module to be produced on a single assembly system should have a maximum of 12 or 13 parts and should be designed around a base part to which other components are added. (2) Reduce the need for multiple components to be handled at once. (3) Limit the required directions of access. The ideal is for all components to be added vertically from above. (4) Use only high quality components. Poor quality components cause jams in feeding and assembly mechanisms. (5) Use snap fit assembly to eliminate the need for threaded fasteners.
