Add MAXPlanetary gearbox documentation

- Cartridge-based planetary (2:1 to 125:1)
- 27 gear ratios via 5 cartridges + brake
- Brake cartridge specs (sprag clutch, 16.95Nm working torque)
- 90-degree gearbox option
- Connection to Team 2890 mechanism builds

sources/2890/maxplanetary-gearbox.md

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+---
+title: "MAXPlanetary Gearbox System — REV-21-2100"
+tags:
+  - hardware
+  - gearbox
+  - planetary
+  - REV
+  - 2890
+  - ion
+type: hardware-spec
+owner: 2890
+status: active
+sources:
+  - "https://www.revrobotics.com/maxplanetary-system-kit/"
+  - "https://docs.revrobotics.com/ion-build/motion/maxplanetary-system"
+---
+
+# MAXPlanetary Gearbox System — REV-21-2100
+
+## Overview
+
+Team 2890 uses the **MAXPlanetary** as their standard cartridge-based modular planetary gearbox. It's designed specifically for NEO-class motors (NEO, NEO 550, Falcon 500, 775) and allows rapid gear ratio changes without redesigning the entire mechanism.
+
+## Key Features
+
+- **Cartridge-based design** — swap gear ratios on the fly without rebuilding
+- **27 gear ratios available** — 2:1 to 125:1 by stacking cartridges
+- **Self-contained lubricated cartridges** — no re-greasing on swap
+- **Three mounting options** — face mount, side mount, or stacked
+- **1/2in hex output shaft** — compatible with standard FRC hex bearings
+- **MAXSpline output** — integrates with REV ION system
+- **Assembly front-to-back** — change ratios while mounted on robot
+
+## Available Cartridges
+
+| Cartridge | Ratio | Notes |
+|-----------|-------|-------|
+| 2:1 | 2:1 | **Reverses direction** — invert motor in code |
+| 3:1 | 3:1 | |
+| 4:1 | 4:1 | |
+| 5:1 | 5:1 | |
+| 9:1 | 9:1 | |
+| 1:1 Brake | 1:1 | Sprag clutch — holds position, prevents backdriving |
+
+**Combo math:** Stack cartridges for compound ratios. Example: 5:1 × 4:1 × 3:1 = 60:1 total reduction.
+
+## Brake Cartridge
+
+The **1:1 Brake Cartridge** uses a sprag clutch (torque diode) to mechanically lock output when motor is stationary. Automatically engages when output exceeds motor velocity.
+
+**Specs:**
+- Working torque: 16.95 Nm (150 in-lb)
+- Static torque (no failure): 1,200 Nm (10,621 in-lb)
+- Recommended as first cartridge (closest to motor) for maximum brake life
+
+## 90 Degree Gearbox
+
+The **MAX 90 Degree Gearbox** provides right-angle power transmission. Attaches to MAXPlanetary in 90° orientation for compact mechanisms in tight spaces. Through-bore design allows easy mounting.
+
+## Connection to Training
+
+**For students:** The MAXPlanetary teaches gear reduction fundamentals:
+- **Why reduction?** — trade speed for torque. Motor has high RPM, low torque. Gearbox slows it down but multiplies force.
+- ** planetary stages** — each cartridge adds one stage of reduction. More stages = more torque, less speed.
+- **Ratio math** — multiply cartridge ratios to get total. 3:1 × 5:1 = 15:1.
+- **Brake cartridge** — mechanical holding, not motor braking. Independent of motor state.
+
+**Related training modules:**
+- [[motor-basics]] — understanding motor output before gearbox
+- [[swerving-modules]] — MK4i doesn't use MAXPlanetary (direct drive), but mechanisms do
+- [[ion-build-system]] — integrates with REV ION structure
+- [[2890-codebase-index]] — mechanism controllers that use gearbox output
+
+## Source Links
+
+- Product: https://www.revrobotics.com/maxplanetary-system-kit/
+- Docs: https://docs.revrobotics.com/ion-build/motion/maxplanetary-system
+
+## Notes
+
+Chris typically uses MAXPlanetary on mechanisms (intake, shooter, climber) rather than drivetrain. Swerve modules have built-in belt reduction in the MK4i. For any mechanism that needs high torque at low speed — that's where MAXPlanetary lives.
+
+Key concept for students: **gear reduction multiplies torque while dividing speed**. A 5:1 gearbox means the output turns 5× slower but the motor can push 5× harder.