17.1 FACTOR OF SAFETY
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When we do basic design work we typically use a process of the basic format,
In particular the design process often involves a factor of safety that allows for,
- variations in material properties
- variations in expectations load
- the non-uniform distribution of stress
The factor of safety is applied as,
The values vary for various systems. The values given below are reasonable for static systems.
For example let us consider a two member mechanism 1.44 from [Beer and Johnston]
17.1.1 Practice Problems
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1. The mechanism below is to be analyzed. Before answering any of the questions, read the notes, and study the diagram.
- The cable is 1/8 inch in diameter, and made of 6061-T6.
- The upper bracket is made of cold rolled steel.
- The pulley is made of nylon - assume it is frictionless.
- The beam is made of oak.
- The pulley is supported by a brass pin. There are brackets on both sides of the pulley.
a) Find the new length of the cable (assume the cable does not stretch on the pulley). Is the factor of safety high enough?
b) Is the factor of safety for stress failure in the beam high enough?
c) What is the minimum pin diameter for the pulley?
e) How thick should the upper brackets be for a factor of safety of 3?
