Power Summary
STAGE 1B · POWER — Choosing gears vs belt vs chain, ratio recap, and the design checklist
What We Built
You sized a reduction from motor to wheel.
- Spun up SpurGear add-in and HTD belt pulleys.
- Constrained spacing with User Parameters.
- Joined moving parts with revolute joints.
- Now: pick the RIGHT transmission for the job.
Quick reframe of Stage 1B. Ask the room: 'Who used gears? Who used belt?' Remind them every drivetrain and mechanism on a 7558 robot is a reduction problem — motor spins fast and weak, we trade speed for torque. This summary is the decision framework they'll reuse all season.
Ratio In One Line
Ratio = driven teeth ÷ driving teeth.
- Bigger output gear = more torque, less speed.
- Multiply stages: 4:1 then 3:1 = 12:1.
- Output RPM = motor RPM ÷ total ratio.
- Belt/chain ratio uses sprocket/pulley teeth too.
Do this live on the whiteboard, not slides. Kraken free speed ~6000 RPM, NEO ~5676 RPM. Work one example: NEO at 5676 through 12:1 = ~473 RPM at the wheel. Common mistake: students flip driving/driven and get an overdrive. Sanity check — output should almost always be SLOWER than the motor.
Center Distance Matters
Gear center distance = (pitch dia 1 + pitch dia 2) ÷ 2.
- 20DP: pitch dia = teeth ÷ 20 (inches).
- Too close = binding; too far = skipping teeth.
- Belts and chain need correct CC + tension.
- Drive it from a User Parameter, not a typed number.
Reinforce why we parameterized: change one motor or gear and the whole layout updates. For chain, remind them tensioning exists (slot the motor mount or use a tensioner) because chain stretches. For HTD belt there's NO tension adjustment in CAD — center distance must be exact or the belt won't fit on real hardware.
Gears Vs Belt
- Most compact, highest torque density
- Best for big reductions in tight space
- Need lube, make noise, precise CC
- Use: drivetrain gearboxes, arm joints
- Quiet, clean, no lubrication
- Spans long distances, light
- Can't transmit huge torque; can skip
- Use: elevators, intakes, light reductions
Frame it as tradeoffs, not winners. Gears win when torque is high and space is tight (swerve modules use gears). Belt wins for long, clean, quiet runs like elevator stages. Mention 5mm HTD for light loads, 9mm/15mm wide for heavier. Skipping a belt tooth under load is the classic failure — that's why belt is not for the final high-torque stage.
Chain Vs Belt
- Handles high torque, takes abuse
- Tolerant of dirt and rough CC
- Heavier, needs tension + lube
- Use: drivetrain, heavy arms, climbers
- Lighter, silent, maintenance-free
- Tight CC tolerance, no slack
- Lower torque ceiling
- Use: fast light mechanisms
#25 chain is the FRC workhorse for drivetrain and high-load joints — forgiving and strong. #35 for very heavy climbers. Downside: weight and you must tension it. Rule of thumb for the team: torque + abuse = chain or gears; light + clean + quiet = belt. Have them name a mechanism on last year's robot for each.
PICK FOR THE LOAD, NOT FOR THE HABIT
High torque and tight space → gears. Long and clean → belt. High torque and abuse → chain.
Real Parts, Real Sizes
Everything bolts to 1/2in hex or 2x1 tube.
- 20DP gears: WCP, AndyMark, REV.
- 5mm/9mm HTD belt + pulleys for light loads.
- #25 chain + 1/2in hex sprockets for power.
- Insert vendor STEP/F3D — don't model from scratch.
Remind them: Insert > Insert Derive or drag a downloaded STEP/F3D from REV, WCP, AndyMark, MAXSwerve. Don't hand-model a COTS gear — you'll get the tooth profile wrong and it won't mesh. The 1/2in hex shaft is the team standard; bores and bearings all reference it. 2x1 0.1in-wall tube is the structural backbone.
Before You Commit
Does output RPM/torque hit your target?
- Did you use real vendor parts (STEP/F3D)?
- Is center distance a User Parameter?
- Are moving parts on revolute/slider joints?
- Did you leave room for chain/belt tension?
Walk this list slowly — it's their pre-flight check for any power mechanism this season. Most common misses: forgetting tensioning clearance, typing center distance as a raw number instead of a parameter, and using rigid joints where a revolute is needed so the mechanism can't move in a motion study.
Common Mistakes
Flipping driving/driven — accidental overdrive.
- Belt at the final high-torque stage.
- Forgetting bearings on both ends of a shaft.
- Center distance typed once, never updated.
- Modeling COTS gears by hand.
Run through fast. The overdrive flip is #1 — always sanity check output is slower. Belt-at-final-stage = skipped teeth and a dead mechanism mid-match. Unsupported shaft ends = wobble and broken gears. Each of these has bitten a real FRC team; tell a quick war story if you have one.
Your Task
- Lay out a 2-stage reduction in Fusion.
- Hit a target output RPM (you choose).
- Use real vendor parts + 1/2in hex.
- Drive center distance from User Parameters.
- Add revolute joints so it spins.
- Note your ratio + output RPM in a sketch.
- Fusion Share > Public Link.
- Paste the link on AltHub.
Give them a target, e.g. 'get a NEO down to ~300 RPM for an arm joint.' Encourage one gear stage + one chain stage to practice mixing transmission types. Circulate and check that center distance is a parameter and joints actually animate. Submission is a Fusion Share public link pasted to the AltHub board.
Power, Solved Pick For The Load
- Ratio = driven ÷ driving; multiply stages.
- Gears = tight+torque, belt = clean, chain = abuse.
- Parameterize CC, use vendor parts, joint it up.
Your Task
- Model what this lesson covers in Fusion 360.
- Use the AltSkripts tools where they apply.
- Save it with a clear name.
- In Fusion: Share → Public Link → Copy.
- Paste the link below.
- A coach reviews it in AltHub.