# Interviews at OEMs and Research labs – Engineering Failures

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

### Interviewer

Consider a long thin cylinder subjected to internal pressure. Does it suffer a compressive strain?

### Candidate

Analysis: The cylinder is assumed to be thin and is suffers both hoop and longitudinal stresses which are tensile in nature. If the cylinder is open then it suffers only hoop stress.
If the cylinder has to suffer the compressive strain, it is the Poisson’s effect that is responsible

Where v is the poisson′s ratio and E is the Young′s modulus

### Entrant Level- Typical Research

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

### Interviewer

Develop expression for contact pressure for an interference fit subject to rotation and temperature. Assume different materials for inner and outer cylinders

### Step1 - Comprehend the problem definition

We must first conceive the effect of temperature and RPM loading. What is to be noted here is the CF loading is determined by not only the radius but also density

Observation: We must observe that E, Young’s modulus at elevated temperature decreases significantly hence the actual E at that temperature must be assumed. Also capturing the worst operating temperature becomes very important.

### Engineers' note:

In order to understand the effect of temperature, one must also consider the unobvious effect also that is overall stiffness of the system decreases and hence the contact pressure reduces!

### Step2 - Candidate must quantify the problem understanding to conclude

Temperature and effect of rotation effect

One must observe the following

• ± sign indicates that depending on thermal coefficient of expansion and density either inner or outer cylinder could expand more meaning pressure could increase or decrease.
• The suffix T for Young’s modulus indicates that it is the effect of temperature.
• Thermal growth is dominant generally centrifugal growth is less dominant.

### Fairly experienced

Expectation: Critical thinking skills for product design

### Interviewer

Discuss the effect of redesigning the bearing support structure?

### Step 1 - Comprehend the problem definition.

First, understand all the factors that are influenced by BSS

• Vibration margin
• Damping
• Clearances of the supported elements

Observation: The bearing support structure again is influenced by temperature.

### Engineers' note:

Damping must come from the bearing support structure and damping coming from shaft is detrimental

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

Consider an idealised shaft and bearing support structure

### Step 3 - Analysis

• So bearing support structure reduces the natural frequency as it is in series connection with the shaft stiffness.
• The hysteretic damping is limited to 3 to 4% from a BSS which is quite sufficient
• Stiffness must be a choice between vibration margin and clearance requirement

### Did you know?

• What is numerical damping?
• How numerical damping depends on a particular integration scheme?
• Could we employ hysteretic damping time domain?

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