WOBBLarm.py
Is this printed arm strong enough at full throttle, with a safety factor of 3?
HOLDS
Peak 9.46 MPa on face 2 at (25.8, -8, 0), safety factor 5.07 against yield (target 3). Confidence is medium.
peak stress
9.46 MPa
safety factor
5.07
max deflection
4.22 mm
target
3
Confidence medium
- singularities: excluded from the answer: 14.4 MPa at a sharp inside corner (face 17) (a sharp corner: singular in theory, so the value depends on the mesh: 11.5 -> 14.6 -> 14.4 MPa); 9.94 MPa at a sharp inside corner (face 2) (a sharp corner: singular in theory, so the value depends on the mesh: 9.75 -> 9.89 -> 9.94 MPa); 9.7 MPa at a sharp inside corner (face 21) (a sharp corner: singular in theory, so the value depends on the mesh: 9.17 -> 9.38 -> 9.7 MPa). Wobbl reports the stress 4.12 mm or more away from these spots
Stress
Checks
| status | check | result |
|---|---|---|
| pass | mesh convergence | von Mises stress settled: 9.44 -> 9.53 -> 9.46 MPa over 3 meshes, last change 0.7% |
| pass | force balance | reactions balance the loads to 0.00022% (forces) and 2.2e-05% (moments) |
| pass | hand calculation | beam theory through the critical section gives 6.69 MPa nominal; FEA peak is 1.42x that, a stress concentration of about 1.4 from the local shape |
| warn | singularities | excluded from the answer: 14.4 MPa at a sharp inside corner (face 17) (a sharp corner: singular in theory, so the value depends on the mesh: 11.5 -> 14.6 -> 14.4 MPa); 9.94 MPa at a sharp inside corner (face 2) (a sharp corner: singular in theory, so the value depends on the mesh: 9.75 -> 9.89 -> 9.94 MPa); 9.7 MPa at a sharp inside corner (face 21) (a sharp corner: singular in theory, so the value depends on the mesh: 9.17 -> 9.38 -> 9.7 MPa). Wobbl reports the stress 4.12 mm or more away from these spots |
| pass | bolt bearing | worst hole of fix1 (d 3.2 mm) carries 35.9 N sideways and 41.8 N along its axis; bearing stress 0.561 MPa, safety factor 85.60 on bearing |
| pass | units and sanity | part 188 mm across, peak strain 0.0056, max deflection 4.22 mm (2.2% of size): all plausible |
| pass | large deflection | deflection is large next to the wall thickness, so Wobbl solved the first mesh with large deflection theory too: the peak moves +2.3% and the deflection +0.1%; linear theory holds |
| pass | load introduction | the off part loads enter through a rigid attachment (the attached part does not flex). With a distributing coupling (the attached part adds no stiffness) instead, the governing stress is 9.46 MPa (1x, safety factor 5.07) at the same spot as the main answer, on the mesh already converged there. The verdict is the same either way |
How to fix it
Round the sharp inside corner to a 1.71 mm radius
Stress piles up in the sharp inside corner. A fillet of 1.71 mm (about half the 3.42 mm wall) spreads it out. With that radius the peak is usually about 1.4x the 6.69 MPa beam stress, roughly 9.36 MPa (safety factor 5.13).
Switch to PLA (3D printed)
Stress stays about the same for the same shape and load, so PLA (3D printed) (50 MPa) gives safety factor 5.28. It is 2.1x as stiff, so deflection scales by 0.486. Other options: PETG (3D printed) (4.97), ABS (3D printed) (3.49).
Mesh convergence
| level | nodes | global size mm | finest mm | governing MPa | change | max deflection mm | solve |
|---|---|---|---|---|---|---|---|
| 0 | 18,653 | 8.57 | 1.05 | 9.44 | 4.19 | 3.7 s | |
| 1 | 21,671 | 7.28 | 1.05 | 9.53 | +1.0% | 4.2 | 4.9 s |
| 2 | 22,730 | 6.19 | 1.05 | 9.46 | -0.7% | 4.22 | 5.5 s |
Stress peaks
Every peak Wobbl found, with its value at each mesh. Peaks at sharp corners and idealized supports are excluded from the answer because their value depends on the idealization, not the part.
| # | where | x, y, z (mm) | MPa by level | status |
|---|---|---|---|---|
| 1 | at a sharp inside corner (face 17) | 46.4, 8, 15.1 | 11.5 → 14.6 → 14.4 | excluded |
| 2 | at a sharp inside corner (face 2) | 22.1, -8, 0 | 9.75 → 9.89 → 9.94 | excluded |
| 3 | at a sharp inside corner (face 15) | 47, -7.04, 3 | 9.51 → 9.18 → 9.42 | excluded |
| 4 | at a sharp inside corner (face 21) | 78.8, 6.97, 13.2 | 9.17 → 9.38 → 9.7 | excluded |
| 5 | at a sharp inside corner (face 14) | 26.1, 8, 18.6 | 9.12 → 9.09 → 9.16 | excluded |
Setup
| material | Nylon PA12 (SLS or MJF) |
| limit | 48 MPa |
| target safety factor | 3 |
| mass | 40.43 g |
| load | 25 N up at 150,0,15 @ top and within 15 of 150,0,15 rigid |
| load | 0.5 N*m +z at 150,0,15 @ top and within 15 of 150,0,15 rigid |
| fixture | bolt holes d=3.2 and x<0 |
| part | 7 holes, 0 inside fillets, 15 sharp inside corners, walls about 13.8 mm |
Assumptions
- Linear elastic material and small deflections (no yielding redistribution, no buckling).
- Nylon PA12 (SLS or MJF): E 1700 MPa, Poisson 0.39, von Mises vs yield 48 MPa (HP 3D High Reusability PA 12 datasheet; EOS PA 2200).
- Loads are static and applied all at once; no impact, vibration or fatigue.
- Printed part treated as isotropic at 100% infill; across the layers it is roughly 90% as strong. Pass --print-direction to check that.
- Bolted holes are rigidly held over the hole and a washer sized ring (twice the hole radius).
- Remote loads reach the part through a rigid attachment (the attached part does not flex).
- Contact between separate parts, friction and preload are not modeled.
Notes
- Built from arm.py with length=150, width=16, root_height=20, tip_height=12, window=3, pad=30.
- 4 curved elements folded on thin walls, so this mesh uses straight sided second order elements.
- 3 curved elements folded on thin walls, so this mesh uses straight sided second order elements.