ME 9: Machine Elements

Proof required

Link the public repo with shaft design examples from the NPTEL lectures, each solved by hand and then checked in CalculiX or FreeCAD FEM written up (a typeset PDF, LaTeX source or notebook), and show the self-check: which answers matched and how you fixed the ones that did not.

What gets checked

• Each shaft uses a stated failure theory (Tresca, von Mises or a fatigue criterion) and a safety factor.
• Stress concentration factors carry their source and geometry.
• The hand answer and the finite element run agree within 10%, or the gap is explained.

Common mistakes

• Answers copied from a solution with no working.
• Missing units, or a free-body diagram or sign convention left out.

Powstik Guide

A public write-up of shaft design examples from the NPTEL lectures, each solved by hand and then checked in CalculiX or FreeCAD FEM, with the self-check shown.

Steps

1. Study the linked course material.
2. Solve the problems on your own first.
3. Check them against the self-check and rework the misses.
4. Typeset the write-up (LaTeX or a notebook) in a public repo.
5. Ask someone who knows the subject to check it.

README
ME 9: Machine Elements, unit 1: Shafts: stress concentration, failure theories and fatigue
Problems:
Self-check result (matched / reworked):
What I got wrong first and why:

What gets sent back

• No working shown.
• The self-check result is missing.

Resources

What a verifier looks for

• Each shaft uses a stated failure theory (Tresca, von Mises or a fatigue criterion) and a safety factor.
• Stress concentration factors carry their source and geometry.
• The hand answer and the finite element run agree within 10%, or the gap is explained.
• Is the engineering right, not just the final number?

You'll sign in first, then come straight back here.

Proof required

Link the public repo with the gear questions of 2.007 problem set 3 and Exam 2, and the bearing-life examples in the NPTEL lectures written up (a typeset PDF, LaTeX source or notebook), and show the self-check: which answers matched and how you fixed the ones that did not.

What gets checked

• Gear ratios, pitch-line speeds and tooth loads carry units.
• Bearing life uses L10 = (C/P)^p million revolutions (p = 3 for ball, 10/3 for roller bearings), at 90% reliability or with a stated adjustment factor.
• Answers match the 2.007 solutions, and misses are reworked.

Common mistakes

• Answers copied from a solution with no working.
• Missing units, or a free-body diagram or sign convention left out.

Powstik Guide

A public write-up of the gear questions of 2.007 problem set 3 and Exam 2, and the bearing-life examples in the NPTEL lectures, with the self-check shown.

Steps

1. Study the linked course material.
2. Solve the problems on your own first.
3. Check them against the self-check and rework the misses.
4. Typeset the write-up (LaTeX or a notebook) in a public repo.
5. Ask someone who knows the subject to check it.

README
ME 9: Machine Elements, unit 2: Gears, belts and bearings
Problems:
Self-check result (matched / reworked):
What I got wrong first and why:

What gets sent back

• No working shown.
• The self-check result is missing.

Resources

What a verifier looks for

• Gear ratios, pitch-line speeds and tooth loads carry units.
• Bearing life uses L10 = (C/P)^p million revolutions (p = 3 for ball, 10/3 for roller bearings), at 90% reliability or with a stated adjustment factor.
• Answers match the 2.007 solutions, and misses are reworked.
• Is the engineering right, not just the final number?

You'll sign in first, then come straight back here.

Proof required

Link the public repo with the spring questions of 2.007 Exam 2 and the bolt and spring examples in the NPTEL lectures written up (a typeset PDF, LaTeX source or notebook), and show the self-check: which answers matched and how you fixed the ones that did not.

What gets checked

• Bolt preload and joint stiffness are shown before the external load.
• Spring rate uses k = Gd⁴/(8D³n) with the coil count and wire diameter stated.
• Answers match the 2.007 solutions, and misses are reworked.

Common mistakes

• Answers copied from a solution with no working.
• Missing units, or a free-body diagram or sign convention left out.

Powstik Guide

A public write-up of the spring questions of 2.007 Exam 2 and the bolt and spring examples in the NPTEL lectures, with the self-check shown.

Steps

1. Study the linked course material.
2. Solve the problems on your own first.
3. Check them against the self-check and rework the misses.
4. Typeset the write-up (LaTeX or a notebook) in a public repo.
5. Ask someone who knows the subject to check it.

README
ME 9: Machine Elements, unit 3: Bolted joints, springs and flexures
Problems:
Self-check result (matched / reworked):
What I got wrong first and why:

What gets sent back

• No working shown.
• The self-check result is missing.

Resources

What a verifier looks for

• Bolt preload and joint stiffness are shown before the external load.
• Spring rate uses k = Gd⁴/(8D³n) with the coil count and wire diameter stated.
• Answers match the 2.007 solutions, and misses are reworked.
• Is the engineering right, not just the final number?

You'll sign in first, then come straight back here.

Proof required

Link the public repo with the raw data (or links to it), photos of the set-up, the analysis notebook and the predicted-vs-measured table with uncertainties (under 18 without an adult: a simulation of the same set-up, compared with published values), and a README that says how to rerun it and what limits the result.

What gets checked

• The table shows d, D, n, the predicted and the measured rate for three springs, with units and uncertainties.
• The uncertainty in the prediction is propagated, showing the d⁴ term dominates.
• Every gap is explained with a cause the reviewer can check.

Common mistakes

• A result that nobody else can rerun from the repo.
• A table with no explanation of the gaps.

Powstik Guide

A measured experiment on helical springs (for example from old ballpoint pens or a hardware kit), in a public repo.

Steps

1. Write down what you expect before you start.
2. Measure each spring's wire diameter d, mean coil diameter D (outside diameter minus d) and active coils n (total minus about 2 for closed ends). Without calipers, find d from the solid length: squeeze the spring flat between two books and divide its length by the total coils. Find D from a zoomed phone photo against a ruler. Predict the rate with k = Gd⁴/(8D³n): G ≈ 79 GPa for carbon spring steel, about 69 GPa for stainless (a magnet sticks firmly only to carbon steel). Stand each spring over a skewer so it cannot buckle. Load it in steps with coins of known published mass or water measured in a jug (1 mL weighs 1 g). Measure the length each time from a phone photo against a ruler, and fit the measured rate. Use small springs only and keep your face away while loading. Do this for three springs. Fill a predicted-vs-measured table with uncertainties and explain the gaps (coil count, end coils, the d⁴ sensitivity).
3. Fill the predicted-vs-measured (or hand-vs-simulated) table.
4. Explain every gap with a cause someone can check.
5. Write the README and ask someone who knows the subject to check it.

README
ME 9: Machine Elements — Predict and measure a spring's stiffness
What I expected:
How to rerun it:
Predicted vs measured:
Why they differ:
Safety: hand tools and eye protection only

What gets sent back

• No predicted-vs-measured table, or no uncertainties.
• Machine tools, welding, pressure vessels, combustion, or springs and flywheels storing dangerous energy without a trained supervisor.

Resources

What a verifier looks for

• The table shows d, D, n, the predicted and the measured rate for three springs, with units and uncertainties.
• The uncertainty in the prediction is propagated, showing the d⁴ term dominates.
• Every gap is explained with a cause the reviewer can check.
• Check the engineering and the numbers, not just that it exists: do the units, the hand calculation and the table agree?
• Build photos show hand tools only and eye protection worn: no machine tools, welding, pressure vessels, combustion, or springs and flywheels storing dangerous energy (unless a trained supervisor and a proper workshop are shown).

You'll sign in first, then come straight back here.