Engineering Analysis: Detailed Design Phase Multidisciplinary Senior Design Rochester Institute of Technology Mechanical Engineering Department Rochester, NY USA

Try this: In the next 2 minutes, estimate the distance defined by “shoulder-height with respect to the ground outside”. ~1 minute: develop analysis plan ~1 minute: analyze You MUST have an answer in 2 minutes, but it doesn’t have to be right – just a decent guess.

Before we go into answers… Who was unhappy about having to come up with a solution in 2 minutes? Who came up with a solution in 2 minutes?

Before we go into answers… What approach did you take? What questions did you ask yourself?

Before we go into answers… What approach did you take? (estimate floor height x 2) + shoulder height? (estimate step height x # steps) + shoulder height? Envision # people you could stack off the balcony? Something else? What questions did you ask yourself? Whose shoulders? Which ground? Does it matter?

The most important questions: Why do you ask? or What do you want to learn from my answer?

This is first-order analysis! Dr. Amuso discussed this already. Appropriate for systems-level design phase “Does this solution even remotely make sense?” “Is there any feasible solution to the problem before us?” “Will it cost $1, $10, or $100?” Maybe doesn’t take 2 minutes, but is usually not complete, or heavy on the assumptions.

Today: higher-order analysis Appropriate for detailed design phase Bigger time investment Analysis involves more parameters Don’t want to do this for ALL designs, just the most promising one (or maybe two). (Potentially) better results Include more effects -> more accurate Need to know more about the problem

What type of analysis, and when? Look at a few possible scenarios. How in-depth should the analysis be? What information is important to the analysis? What will you learn by doing each analysis?

Possible Scenarios: I am designing a system to drop a quarter, from my outstretched arms, off the 4th floor balcony. My friend needs to do laundry and asked to borrow a quarter.

Possible Scenarios: I am designing a system to drop a quarter, from my outstretched arms, off the 4th floor balcony. It has to reach the ground in less than 0.5 sec and land within 1 m of my friend. (Gotta dress up for an interview in 20 minutes!) Terminal velocity of a quarter ~28mph = 28*5280/3600 ft/sec = 41 ft/sec So the height is really important – we are right on the edge of making the 0.5 sec cutoff, to where being off by a couple of feet could be a big deal

Possible Scenarios: I am designing a system to drop a quarter, from my outstretched arms, off the 4th floor balcony. It has to reach the ground in less than 0.5 sec and land within 1 cm of my friend. (No time to waste hunting around for it – that shirt is wrinkling in the dryer as we speak!) Now the landing area is critical. What is the wind speed? What is the local ground profile?

“My friend needs to do laundry and asked to borrow a quarter “My friend needs to do laundry and asked to borrow a quarter.” The analysis we’ve already done is fine. More than enough.

“It has to reach the ground in less than 0 “It has to reach the ground in less than 0.5 sec and land within 1 m of my friend.” Need to know: terminal velocity of a quarter, height to within ~1 ft, amount of time consumed by release mechanism

It has to reach the ground in less than 0 It has to reach the ground in less than 0.5 sec and land within 1 cm of my friend.” Need to know: terminal velocity of a quarter, height to within ~1 ft, amount of time consumed by release mechanism And local terrain variation, wind speed, orientation of quarter @ drop…

Lesson learned: Not all analysis require you to know average daily wind speed and the terminal velocity of a quarter at sea level. Figure out how accurate your analysis must be before you start. You may not know all the variables before you start. Iterate, or use model to evaluate different design options

Example: one student’s work on one subsystem of a design System weight: 44 lb Footplates: 8.7 lb! Customer: “Reduce weight, reduce cost, increase functionality”

Problem Definition Footplates fall under “support patient” function. (Component #3). Three Engineering Requirements will be met in whole or in part by the footplates.

Systems-Level Design Simple spreadsheet analysis of representative loading scenario allows the team to evaluate different materials and thicknesses. This is generous on the assumptions, but lets us compare options in an educated manner.

Detailed Design Weight distribution across a footprint??? Collect some benchmark data on deflection under load: dial indicator, corner supports, and a willing 250 lb volunteer. Use this information to calibrate a finite element model.

Detailed Design THEN, evaluate different material removal options using ANSYS. FINALLY, predict performance vs. spec for the proposed design.

Lessons Learned Assumptions/boundary conditions matter A little bit of judicious prototyping or experimentation can be a huge help Use your analysis to make design decisions and to predict performance vs. spec.

Questions?