# OEM and Research lab interview series for mechanical engineers be it learning or practicing

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.

### Entrant Level- Typical OEM

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

### Candidate

Analysis: What is the difference between a plane disc and a bladed disc?

### Entrant Level- Typical Research

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

### Interviewer

Consider a wheel with three uniform rectangular blades supported on a horizontal shaft rotates at constant speed. Explain the role of gravity?

### Step1 - Comprehend the problem definition

Always analyse the graphical representations and loads. In many cases an FBD helps. This should be followed by right questions

Observation: Gravity could act axially and when the blade is horizontal it could cause bending. Nature of bending changes from left to right position casing fatigue. Of course axial load variation may not be significant. However quantification is important

However the gravity can affect the stress stiffening effect, by varying axial load

### Step 2: Candidate must quantify the problem understanding to conclude

As the blade rotates angle with the vertical of the left blade decreases and that of the right blade increases by ωt

The variation if significant not only causes fatigue but could change the natural frequency by varying stress stiffening effect

### Engineers' note:

• Transient loads (operating conditions significantly change)

### Fairly experienced

Expectation: Critical thinking skills for product design

### Interviewer

Consider a turbine wheel. Turbine speed becomes a limiting point in design. Based on centrifugal loading how do you decide the blade height?

### Step 1 - Comprehend the problem definition.

The blade’s centrifugal load determines rim loading for the wheel. Further, the blade height decides the flow annulus which is decided from fluid mechanics considerations. In our calculations blade height may be varied but the annulus area must be kept constant
System-level understanding helps!

Blade height can be larger or smaller but the annulus area must be kept constant. Now let us quantify!

### Step 2 - Candidate must quantify the problem understanding

The maximum RPM that produces permissible stress is given by:

Where r is the CG, distance of blade from the axis of rotation.

### Step 3 - Analysis

• Same annulus area but disc stresses are large in blades with smaller heights This is because the disc becomes larger hence experiences much higher centrifugal loading. However the natural frequency increases as blades become stiffer and hence vibration margin may increase
• Same annulus area but disc stresses are small in blades with larger heights This is because the disc becomes smaller hence experiences lesser centrifugal loading. However the natural frequency increases as blades become less- stiffer and hence vibration margin may decrease

### Engineers' note:

Turbine blades during take-off develop 800 to 900 HP. Loss of one turbine blade can also seriously hurt.

### Consider the challenge

If we operate in Mexico on a hot day the temperatures go up the density comes down speeds go up to make up for the thrust, but on a colder day speeds come down and gas would have higher density.

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