Design of Machine Elements - short answer type questions from AMIE exams (Summer 2019)
Explain the following terms:
Stress concentration factor.
In order to consider the effect of stress concentration and find out localized stresses, a factor called the stress concentration factor is used. It is denoted by Kt and defined as,
where σ0 and τ0 are stresses determined by elementary equations and σmax and τmax are localized stresses at the discontinuities. The subscript t denotes the ‘theoretical’ stress concentration factor.
Notch sensitivity factor
Notch sensitivity is defined as the susceptibility of a material to succumb to the damaging effects of
stress raising notches in fatigue loading. The notch sensitivity factor q is defined as
Fatigue strength reduction factor
This factor accounts for geometrical stress concentration factors due to thickness variations; weld geometries, surface notches or environmental conditions. The range of factor Kg is between 1 and 4, with its minimum value for smooth geometrical shapes and its maximum for 90 degree welded corners and fillet welds.
State and explain the efficiency of the screw.
The efficiency of the power screw is given by
See the following figure of square thread screw
Different types of rolling contact bearing.
Following are the two types of rolling contact bearings:
- Ball bearings
- Roller bearings.
The ball and roller bearings consist of an inner race which is mounted on the shaft or journal
and an outer race which is carried by the housing or casing.
The rolling contact bearings, depending upon the load to be carried, are classified as
- Radial bearings
- Thrust bearings.
What are the design criteria of brakes?
Write the formula for the efficiency of the screw jack.
The formula for the efficiency of the screw jack is
For maximum efficiency,
Maximum efficiency is
The malleability of a material is its ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium, copper, tin, lead, steel, etc. are malleable metals.
Lead can be readily rolled and hammered into thin sheets but can not be drawn into wire. Ductility is a tensile property, whereas malleability is a compressive property. Malleability increases with an increase in temperature.
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.
|A creep curve|
What do you understand by stress concentration?
In practice, discontinuities and abrupt changes in cross-section are unavoidable due to certain features of the component such as oil holes and grooves, keyways and splines, screw threads and shoulders. Therefore, it cannot be assumed that the cross-section of the machine component is uniform. Under these circumstances, the ‘elementary’ equations do not give correct results.
Let us consider a plate with a small circular hole.
When the plate is loaded at the edges, It is observed that there is a sudden rise in the magnitude of stresses in the vicinity of the hole. The localized stresses in the neighbourhood of the hole are far greater than the stresses obtained by elementary equations.
Stress concentration is defined as the localization of high stresses due to the irregularities present in the component and abrupt changes of the crosssection.
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