Interviews at OEMs and Research labs – Strength/Stress Calculations

An OEM or a research interview is designed to test the ability of the candidate to solve a problem by applying physics-driven thinking skills. Unless the interviews are for people who would develop mechanical methods/mathematical models for high-end research interviews mostly check fundamentals, application and thinking skills.

Let us choose the topic "Strength/Stress Calculations" for the following three levels of interview

Entrant Level- Typical OEM

Expectation: Understanding of basic loads and ability to calculate stress in simple components

Interviewer

Consider the unsupported joint as shown in the figure. Find the ratio of maximum stress to minimum stress in the sheet across the bolt hole.

Candidate

Analysis: The sheet metal is subjected to both bending and axial load. Since both stresses have the axial direction these stresses could be added.

Let the area of the plate be A and plate thickness be h. The bending moment caused is, Fh and maximum stress caused is given by

The ratio of maximum stress to minimum stress

Entrant Level- Typical Research

Expectation: Understanding of basic loads and ability to observe unobvious facets

Interviewer

Find the torsional stiffness for the beam considering only the thrust loading experienced by the driven helical gear and hence the moment produced by it

Candidate

Step1 - Comprehend the problem definition

Understand that the helical gears have three components of loading. Namely Separating load, tangential load and axial load. Out of which axial load causes the bending moment whose vector direction is perpendicular to shaft axis

Observation: Torsional stiffness can be expressed as M/θ. Here, we consider the moment M due to axial load, and by finding the slope owing to M, torsional stiffness could be computed.

Step2 - Candidate must quantify the problem understanding to conclude

Elastic deformation of the beam is shown below

Engineers' note:

Axially the shaft is very stiff, however the flexural stiffness must be designed such that deflection and slope of the shaft do not affect transmission.

Fairly experienced

Expectation: Critical thinking skills for product design

Interviewer

Explain under what situations, the centrifugal loading leads to significant stress in shafts.

Candidate

Step 1 - Comprehend the problem definition.

Shafts in many product lines run at very high RPMs. The question is does this high RPM, lead to significant loading causing high hoop and radial stresses?

Observation: The critical fact is if the shaft is hollow or solid. Also, the shaft being a long cylinder it falls under plane strain conditions. Therefore, the equations used for hoop and radial stresses must be those derived assuming plane strain conditions for an axisymmetric cylinder. Generally, shafts are hollow as weight consideration is very critical to performance, and of course shaft integrity is of utmost significance.

Engineers' note:

Apart from centrifugal loading, in gas turbine situation, gyroscopic loading is significant for shafts. This is brought about by both yaw and pitch manoeuvres coupled with spin rate leading to gyro couple

Step 2 - Candidate must quantify the problem understanding

The following stress equations are applicable for hollow shafts

Note: Since the shaft is thin-walled same result is got even if we use, plane stress conditions!

Engineers' note:

Stress caused by CF loading is around 8Mpa.This compared with yield strength of a typical high strength structural steel (1400Mpa) is of very small value and is just 0.5% of the yield strength.

Step 3 - Analysis

The observation is very simple and straight forward the stress increases 4 times if speed is doubled, so is the case with radius. In the case of steam turbine shaft the radius is much higher compared to gasturbine and hence CF loading may become considerable!

Did you know?

  • Discontinuity stresses in rotating components?
  • What is free hoop radius?

Learn with us most complex and absolutely industry relevant facets of stress/strength calculations!

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