A heat-bent polycarbonate tube acts as the intake shell. A 5.5W motor rotates it for fine orientation. A pneumatic cylinder pulls a string inside the tube to cinch around a held element. Inspired by Team 355Z's design from the recent Luke-Does-Robotics speedcad. This page documents the R24-legal fabrication process, orientation/cinch hardware layout, and the theoretical decision analysis for pairing this intake with the chain bar vs. swing bar lifts.
Per R24d, verbatim: "Plastic may be mechanically altered by cutting, drilling, bending, etc. It cannot be chemically treated, melted, cast, or bonded to another part. Heating non-shattering plastic to aid in bending is acceptable." Polycarbonate (Lexan) is on the legal-types list per R24e. The two binding constraints:
Score-and-snap or band-saw a 4 × 8 in rectangle. Verify dimensions with calipers — inspectors will measure. Sand all four edges with 220-grit until smooth. Sharp edges are stress concentrators that crack during bending.
Drilling a flat sheet on a drill press is straightforward. Drilling a curved tube is a recipe for cracks. Mark two 1/16 in holes, both on the long axis at the 4 in midpoint, spaced exactly 4 in apart along the 8 in length — they will end up 180° apart on the finished tube. Center-punch lightly, drill with a sharp bit at low RPM, deburr both sides.
Polycarbonate's glass transition is around 295 °F. Aim for 280–310 °F on the surface. Sweep the heat gun across the entire sheet for 60–90 seconds until it's pliable like leather — flexes 30° under light hand pressure with no cracking sound.
Lay the heated sheet flat, place the 2.4 in mandrel along one short edge, and roll the sheet around the mandrel. Aim for ~10° gap at the seam — do not try to close it. The gap is where the spring-back margin lives, and a closed seam invites a future student to bond it (illegal). Wrap clean cotton cloth tightly around the assembly. Apply two C-clamps with rubber pads at the ends. Wait 5 minutes for the polycarbonate to cool below glass transition.
Remove clamps and slide the mandrel out. The tube will spring open by 5–10° — that's the gap design. Final ID should be 2.4–2.55 in. If tighter, you over-bent (re-heat lightly, relax); if looser, you under-bent (re-heat, clamp again). Photograph the finished tube with the mandrel partially inserted (visual proof of single-piece formation, no bonding).
Cut two 1×1×3 VEX C-channel pieces. Drill the center hole sized for the shaft (1/4 in or 5mm hex — match what's already on the lift). Press a bearing flat into each end cap on the inside face. The bearing bore positions the tube precisely centered on the shaft. End caps DO NOT touch the polycarbonate directly — bolt them through the tube wall using #4-40 screws + washers on both sides. Washers spread the clamping load so the screw head doesn't crack the polycarb.
Mount a 60T high-strength sprocket on the hex shaft, just outboard of one end cap. Mount a 12T sprocket on the 5.5W motor's output shaft. Connect with #25 chain. Keep the chain run short (~3 in center-to-center) for tight tracking and minimal slap. Use a chain tensioner if needed — small idler sprocket on a slotted bracket.
Stock 5.5W cartridge gives 100 RPM. After 1:5 reduction → 20 RPM at the tube — about 3 seconds per quarter-turn. Fast enough for orientation changes between scoring sequences, slow enough that a held element doesn't fly out under angular momentum if the cinch hasn't pulled tight yet.
Two options:
The whole assembly — tube, end caps, axle, sprockets, motor — bolts to the end of the lift arm via a small mounting plate. The assembly's footprint (about 6 in wide × 3 in tall × 5 in deep) is well within the 18-in-wide chassis envelope at horizontal rest. Verify R3 fit before tightening the final bolts — the mounted intake must clear the chassis cavity at θ = 0° and θ = 180° (or the chain-bar level position).
| Option | Cylinder location | Air line routing | Verdict |
|---|---|---|---|
| A — Stationary | Mounted on the lift arm, opposite end from the motor. Aligned with shaft axis. | Air line runs along the lift arm to the cylinder. Stationary on the arm, flexes only with arm motion. | Recommended. Cleanest air-line routing. Swivel ferrule between rod and string decouples twist. |
| B — Rotating | Bolted to the rotating tube assembly between the end caps. | Air line rotates with the tube. Needs spool or rotary union. | Don't. Rotary unions aren't stock VEX parts. Fail mode is air leak under rotation. |
| C — Coaxial | Inside the tube, rod becomes the string anchor. | Air line runs through the hollow axle. | Compact but constrained. Cylinder body must fit inside 2.55 in ID. Only smallest VEX cylinders work. |
The cylinder body bolts to the lift arm at the opposite end from the motor, with its rod pointed in line with the rotating axle. The string runs from the rod, into the end of the hex shaft (drilled out to a 1/8 in through-bore for hollow operation), through the tube interior, anchors at the far end cap.
VEX doesn't sell a hollow hex shaft as a stock part, but you can drill out a 1/4 in solid hex shaft on a lathe to a 1/8 in through-bore. Document the modification in the build log — it's a "fabricated part" derived from a legal raw stock part.
| Chain bar lift | Swing bar lift | |
|---|---|---|
| Manipulator orientation through arc | Stays level (1:1 chain on static sprocket) | Rotates 180° with arm |
| Tube axial axis through arc | Stays horizontal (perpendicular to arc plane) | Stays horizontal (perpendicular to arc plane) |
| Held element orientation through arc | Preserved — cup stays upright, pin stays oriented | Inverted at 180° unless 5.5W counter-rotates |
| Role of the 5.5W rotation motor | Fine alignment only — rotates tube to match goal stem orientation at deposit. Set-and-hold, no continuous control. | Compensation — must counter-rotate to keep cup upright through the arc, OR re-orient at deposit. Continuous coordination with lift angle. |
The chain bar's whole engineering virtue is that the 1:1 chain ratio keeps the manipulator level regardless of arm angle. When you mount the polycarb tube at the chain bar's end-platform, that level-preservation cascades to the held element — a cup picked up upright at the loader stays upright through the entire 180° sweep, arrives at the goal in the orientation it started in.
The 5.5W rotation motor is then doing one job: rotating the tube around its axial axis (the axis that runs left-right through the tube end caps, perpendicular to the lift's arc plane) to fine-align the held element with the goal stem at deposit. That's a set-and-hold operation:
The swing bar's defining issue is that the manipulator rotates 180° with the arm. When you mount the polycarb tube at the swing bar's end, the tube and any held element rotate with the arm too. The 5.5W rotation motor can compensate — this is exactly Mitigation B (active wrist motor) from /spartan-hero-swingbar-lift Section 2. The 5.5W has to do continuous coordination:
Take the same cup, the same tube, the same string tension. Run 100 cycles on each lift and count drops:
| Configuration | Theoretical drop rate (cup) | Theoretical drop rate (pin) | Notes |
|---|---|---|---|
| Chain bar + tube | < 2% | < 1% | Cup orientation never changes. Cinch only fights translation (≤ 1g acceleration). Drops are from cinch force decay (string fatigue) or pneumatic leak. |
| Swing bar + tube + Mit. B | 5–15% | < 1% | Cup drops are from rotation lag (tube falls behind arm) or coordinated-control tuning errors. Pin drops are rare because pin orientation flip is harmless. |
| Swing bar + tube + passive wrist (Mit. A) | < 3% | N/A (pin doesn't need wrist) | Pendulum settling at deposit is the failure mode. Add 0.5 s settle delay in code. Drop rate approaches chain bar's number but at the cost of the freed motor port (the 5.5W is now wasted — the passive wrist is gravity-driven). |
These are theoretical estimates pending build-and-test data. The point isn't the absolute numbers — it's the relative ratio. Chain bar + tube has the lowest cup-drop rate because it removes a class of failure mode (orientation lag) that the swing bar variants must actively manage.
| Dimension | V1.5 four-bar + V5 claw | Chain bar + polycarb tube | Swing bar + polycarb tube |
|---|---|---|---|
| Cup deposit reliability | 4 level + simple grip |
5 level through arc + cinch + fine-orient |
3 depends on Mit. B coordination |
| Pin deposit reliability | 4 standard |
5 cinch + level |
5 orientation flip OK for pins |
| Pin-in-cup combo | 3 grip width-limited |
5 cinch holds combo at cup waist |
3 pin shifts during rotation |
| Cycle time | 3 multi-stage lift+tilt |
4 one arc + one rotation |
4 one arc, simpler than chain bar |
| Build complexity (higher = simpler) | 4 well-known parts |
2 chain + tube + cinch + 5.5W = many systems |
3 no chain — one fewer subsystem |
| Control complexity (higher = simpler) | 4 single-axis PID |
4 decoupled PIDs (lift, rotate) |
2 coupled control (rotate tracks lift) |
| Failure modes (higher = fewer) | 4 claw, lift motor |
3 chain, polycarb, string, cyl, cinch |
3 polycarb, string, cyl, cinch (no chain) |
| Match flexibility (multi-goal) | 5 angle-tunable per goal |
2 tower-cut commits |
2 tower-cut commits |
| Notebook story | 3 familiar, less novel |
5 many decision matrices |
5 orientation analysis is rich |
| Total (out of 45) | 34 | 35 | 30 |