182 lines
5.0 KiB
Markdown
182 lines
5.0 KiB
Markdown
---
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title: "Choosing Gear Ratios for FRC Mechanisms"
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tags:
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- gear ratio
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- mechanism
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- design
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- motor
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- torque
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- speed
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- frc
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- 2890
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type: training-module
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owner: 2890
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status: active
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sources:
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- "https://docs.wcproducts.com/frc-build-system/belts-chain-and-gears/gears"
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- "https://www.frcdesign.org/learning-course/stage1/1B/torque-speed/"
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- "https://blog.thebluealliance.com/2013/06/24/behind-the-design-understanding-motor-and-gearbox-design/"
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growth: tree
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---
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# Choosing Gear Ratios for FRC Mechanisms
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## Core Principle
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**Speed and torque are inversely proportional.** A 4:1 gear reduction gives you 4× the torque but 1/4 the speed.
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```
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Gear Ratio = Driven Gear Teeth / Driver Gear Teeth
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If ratio = 4:1 (input:output = 1:4):
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Torque → multiplied by 4
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Speed → divided by 4
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```
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## The Design Process
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### Step 1: Define Your Goal
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Ask these questions:
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- What does this mechanism need to **do**?
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- How **fast** does it need to move?
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- How **heavy** is the load?
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- What's the **time constraint** (game-specific)?
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### Step 2: Calculate Required Output
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For each mechanism:
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- **Drivetrain** — Key metric: top speed. Typical: 10-15 ft/s
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- **Elevator** — Key metric: lift speed. Typical: 3-6 ft/s
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- **Arm** — Key metric: rotation speed. Typical: 90-180°/sec
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- **Intake** — Key metric: roll speed. Typical: fast (1-2 sec)
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- **Shooter** — Key metric: RPM. Typical: 3000-6000 RPM
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### Step 3: Match Motor to Load
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**NEO Vortex specs (Team 2890's standard):**
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- Free speed: 6784 RPM
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- Stall torque: 3.6 Nm (0.36 kg·m)
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- Peak output: 640W
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- Continuous (40A): ~375W
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**Work backward from desired output speed:**
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```
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Required output speed = X RPM
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↓
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Account for mechanism reduction (belts, chains, gears)
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↓
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Calculate motor speed needed
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↓
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Find gear ratio
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```
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## Drivetrain Gear Ratio Selection (Team 2890)
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Team 2890 uses **L1 and L3** on MK4i modules with NEO Vortex:
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- **L1 (8.14:1)** — ~14.4 ft/s, lower torque. Use when high speed needed, less pushing.
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- **L3 (6.12:1)** — ~12.8 ft/s, higher torque. Use when more pushing power, climbing.
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**L1 = faster, less torque. L3 = slower, more torque.** Swap based on game demands. Field-swappable.
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## Mechanism Design Guidelines
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### Elevators / Vertical Motion
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```
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Motor → Belt reduction → Sprocket → Chain → Carriage
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```
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- Target: 3-6 ft/s vertical
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- Common ratios: 4:1 to 10:1 per stage
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- Watch for staging — two-stage elevators need compounding reduction
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### Arms / Swing Motion
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```
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Motor → Gearbox → Arm
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```
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- Target: 90-180°/sec rotation
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- Consider start-up torque (gravity fight at low angle)
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- PID control essential for repeatable positioning
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### Shooters / Flywheels
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```
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Motor → Gearbox → Wheel
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```
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- Target: 3000-6000 RPM wheel speed
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- High reduction = more torque at wheel, faster spin-up
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- Common ratio: 3:1 to 5:1
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### Intake Rollers
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```
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Motor → Direct or light reduction → Roller
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```
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- Target: Fast roll-in (under 2 seconds)
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- Low reduction or direct drive works
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- Rubber roller = good grip, no gear reduction needed
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## Motor Sizing Rules of Thumb
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1. **Stall torque > Load torque at worst angle**
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2. **Free speed > Desired output speed by 2× minimum**
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3. **Current draw at stall < 80% of breaker rating**
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4. **Continuous torque > Average load torque**
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## Common FRC Motor/Gearbox Pairings
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- **NEO Vortex** — Built-in 4:1, 1696 RPM output. Drivetrain, mechanisms.
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- **NEO Vortex + planetary** — External reduction, variable output. Elevators, arms.
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- **Falcon 500** — Integrated, 1680 RPM output. Drivetrain, high torque.
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- **Kraken X60** — Integrated, ~1680 RPM output. Newer alternative to Falcon.
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## The Calculation
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```
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Desired output RPM = X
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Motor free RPM = 6784 (NEO Vortex)
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Total reduction needed = 6784 / X
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Example: Want 600 RPM output
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6784 / 600 = 11.3:1 reduction needed
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Split across stages:
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Stage 1 (belt): 3:1
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Stage 2 (gearbox): 4:1
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Total = 12:1 — close to target
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```
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## Signs You've Got It Wrong
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- **Too fast / not enough torque:** Robot stalls under load, wheels slip
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- **Too slow / too much torque:** Mechanism moves too slowly to be useful
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- **Motor overheating:** Too much load for continuous operation — need more reduction or bigger motor
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- **Brownouts:** Current spikes from stalling — check breaker sizing
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## For Team 2890 Students
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When designing a mechanism:
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1. **Define the task** — what does it need to do in the game?
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2. **Pick the motor** — NEO Vortex is standard on 2890
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3. **Calculate the speed you need** — game constraints (time limits, field size)
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4. **Work backward** — gear ratio = motor speed / desired output speed
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5. **Split across stages** — belt + gearbox is easier than single-stage high ratio
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6. **Test and tune** — PID tuning can fix some speed issues, but wrong gear ratio can't be fixed with software
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## Related
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- [[swerve-modules]] — MK4i gear options for drivetrain
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- [[neo-vortex-motor]] — motor specs
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- [[spark-flex]] — controller configuration
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- [[motor-basics]] — understanding motor curves
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---
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*Research from web search — gear ratio design for FRC mechanisms*
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*Queue: research complete — stored in wiki* |