Jennifer Marrs September, 2011
Contents
Tables
1-1Alternative Design Decision Matrix
1-2Common General Factors of Safety
1-6Useful Equations and Values
1-7Right Triangle Relationships
1-12Beam Deflections and Forces
1-14Critical Speed of Rotating Shafts
1-17Neuber’s Constant for Notch Sensitivity
1-18Linear Motion with Constant Acceleration
1-19Rotary Motion with Constant Acceleration
As part of the design process, the machine designer must consider safety, capacity, function, material, method of manufacture, form, cost, assembly, adjustment, reliability, and many other factors. The design process is iterative, and the steps can be revisited in any order as needed to refine the design and resolve problems. This section addresses the typical stages of a machine design project, conducted in this order with iterations: design specification, research, synthesis and conceptual design, then detail design and analysis. Factors of safety should always be considered and should be appropriately applied.
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FUNCTIONAL DESIGN SPECIFICATION
The first step in any new design process is to define what the device or part must do, with what frequency, under what conditions, and according to what requirements. It is helpful to prioritize these specifications to aid in evaluating trade-offs. Design specifications drive basic decisions like component or system type, structure, function, and allowable failures. When developing design specifications, it can be helpful to answer the following questions about the device or part:
General Considerations:
•Are there any domestic laws, codes, or standards governing the device?
•Must the device comply with laws and standards of any other countries?
•Is there a cost target or limit for the device?
•Is there a size or weight limit?
•Will there be shipping requirements?
•How many devices are likely to be made?
Functional Considerations:
•What does the device do?
•What is the input to the device?
•What is the output of the device?
•What loads/forces are involved?
•What movements are required and in what time period?
•What sort of contact is expected between the device and the target work piece or user?
•What sorts of intelligence or detection must the device include?
•Can the device’s function be broken into discrete functional elements?
•What physical principles are involved?
•What is the life expectancy of the device?
•Should the device be adjustable and/or repairable?
Ergonomic Considerations:
•What sort of access is required to service or install the device?
•Will parts of the device be lifted by