Page 37 - Analysis and Design of Machine Elements

An Overview of Machine Design
The actual mechanical system needs to be idealized and simplified to facilitate load 15
analysis. According to the loads and operating conditions, it is important to predict
potential failure modes and decide on the corresponding design criteria.
3. Design analysis and evaluation
According to the design criteria, perform strength or rigidity analyses to determine
basic geometrical dimensions of machine elements that meet performance require-
ments against failure. The variability in manufacturing, assembly and service are con-
sidered by introducing modifying factors in the analyses. The calculated results need
to be rounded up to standard or preferred dimensions.
The performance of machine elements need to be assessed against material limits.
A sufficient safety factor should be given to account for uncertainties and imprecision
in the calculations due to idealization and simplification in design equations and data
approximation. If the performance does not meet design requirements, which is the
usual case, revise the previous initial design, dimensions or materials and then iterate
the design process until the specified requirements, like desired design lifetime or
design safety factor, are satisfied.
4. Drawings and documentation
Once satisfactory results have been obtained, designers need to convert calculated
dimensions to detailed drawings, considering manufacturing, assembly, maintenance
and standardization. Design is implemented by a complete set of drawings and doc-
umentation with design specifications for all elements, including tolerances, fits and
finishes. Standards, codes or preferred dimensions are usually incorporated to facil-
itate purchase and manufacture.

1.4 Materials and Their Properties

The selection of materials is an integral part of machine element design. The material
selected for a specific machine element is determined by application requirements or,
more precisely, by the requirement of material properties, such as strength, stiffness,
reliability and durability. Additional factors, such as recyclability, environmental pollu-
tion and costs are also considered while selecting materials.

1.4.1 Types of Materials
Most machine elements are made from metals or metal alloys, such as steels, aluminium
alloys and bronzes. Different numbering systems are employed to identify metal mate-
rials [8, 9]. Other materials, such as polymers, ceramics and composite materials, are
also used in machine elements in various applications.

1.4.1.1 Steels and Alloys
Because of the properties of high strength, high stiffness, durability and relative ease of
fabrication, many types of steels are used in machine elements, including carbon steels,
alloy steels, stainless steels, structural steels, tool steels and so on.
Steels, including carbon steel and alloy steel, are the most extensively used material
for machine elements. Carbon steel is an alloy of iron and carbon with small amounts
of manganese, silicon, sulfur and phosphorus. According to the content of carbon, we