combined chapters

combined chapters - Machine Component Design New Chapter...

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1 Machine Component Design New Chapter Fundamentals of Dimension Determination A Combined Chapters of 4, 6, 8 and 17 Mainly use the Teaching Notes Chapter 6 (Reference book) Fundamentals of Dimension Determination Linkages of machine parts need to have adequate strength and rigidity to support the forces acting on the parts. There are several considerations that apply in the design of machine elements. 6-1 Strength and Failure Strength: It means to make part strong enough so that it will not fail in service. Failure: It not only means breaking of a part but also related with wear, noise, plastic and elastic deformation, and creep of materials. The designer, in considering all the possible modes of failure, must consider calculations of the followings: 1. Allowable Stress 2. Rigidity 3. Stability 4. Wear Usually, the calculations will give the minimum permissible dimensions. In practical usage, these need to be modified. 6-2 Service Life and Reliability This will consider how long a mechanism or machine can run. This is a statistical problem. 6-3 Factor of Safety If life is to be infinite load service normal load safe imum s f max . . = ------------ (1) If life is to be finite life service desired life service useful s f = . . ------------- (2) If stress is main concern, from (1) by dividing area
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2 stress design stress damaging s f = . . ------------- (3) f.s. is always larger than one. 6-4 Factor of Safety with Static Loading Static Loading: the load varies relatively few times and slowly enough so that neither fatigue nor impact needs to be considered. There are two kinds of materials: ductile and brittle materials. Ductile materials can sustain over 5 percent elongation before rupture. Ductile materials: steels, aluminums, brasses. Brittle materials: cast iron For ductile material S S s f y = . . because it has a yielding stage, the stress concentration is not considered in the factor of safety. Where S y = yield strength, S = design stress A part made from a brittle material would give no warning in the form of yielding before breaking, nor would there be any local yielding in the regions of high stresses due to stress concentrations. Theoretical Stress-Concentration factor σ a σ n stress al no stress actual K t min = P P σ a > σ n K t > 1 For brittle material: S K S s f st u = . . Where S u = ultimate strength and K st = Static notch factor. K t is purely theoretical whereas K st refers to the effect of the stress concentration on failure of a real part.
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