Belts & Pulleys
STAGE 1B · POWER — Routing power with synchronous belts in Fusion 360
Why Belts, Not Chain
Belts are quiet, light, and need no lubrication
- Toothed belts grip pulleys — no slipping
- Great for shooters, intakes, swerve, elevators
- Trade-off: less abuse-tolerant than #25 chain
- Fixed center distance — no easy tensioner take-up
Frame belts as the go-to for clean, high-RPM, low-load runs. Chain (#25, #35) wins for high-torque dirty jobs like drivetrains under impact. A common rookie mistake is using belt where chain belongs, then shredding teeth. Mention that swerve modules (MAXSwerve, SDS) use belts internally.
Gt2, Htd & The Profile
Synchronous belts have molded teeth, not friction
- HTD = curved tooth, very common in FRC
- GT2 = refined HTD, less backlash, tighter fit
- Teeth must match pulley groove profile exactly
- Never mix a GT2 belt on an HTD pulley
Drive home that the belt PROFILE and the pulley PROFILE are a matched pair. GT2 and HTD look similar but GT2 has flatter, fuller teeth. WCP and REV sell matched belts and pulleys — buy them as a system. Students who eyeball it end up with belts that ride up and skip.
Pitch Is The Language
Pitch = distance between adjacent teeth
- 5mm HTD: workhorse for FRC power runs
- 3mm HTD (GT2 3mm): compact, lower-load jobs
- Pulley size is named by tooth COUNT
- Belt length is named in tooth count too
Pitch is the unit everything is measured in. A '5mm HTD 36T' pulley has 36 teeth at 5mm pitch. Belts are sold like '5mm HTD 100T' or by mm length like '500-5M'. Tell students: pick ONE pitch for a subsystem and stick to it. Mixing 3mm and 5mm parts is a classic ordering disaster.
Belts only mesh when pitch matches
Same pitch, same profile, same width — the belt, both pulleys, and your math must all agree.
Set Up Your Parameters
Modify > Change Parameters before modeling
- Add 'pitch' = 5 mm, 'pulleyT_driver', 'pulleyT_driven'
- Add 'beltTeeth' for the belt length
- Drive sketches off parameters, not raw numbers
- Change one value, the whole layout updates
In Fusion, User Parameters live under Modify > Change Parameters (the fx icon). This replaces Onshape Variables. Demo adding one and referencing it in a sketch dimension by typing the name. The payoff: when you tweak gear ratio later, the model rebuilds. Students forget to set units — make pitch '5 mm' not '5'.
Pitch Diameter Math
PD = (pitch × teeth) ÷ π
- 5mm × 36T ÷ π ≈ 57.3 mm pitch diameter
- The belt rides on the pitch diameter, not the OD
- Center distance math uses pitch diameters
- Let Fusion parameters compute it for you
Pitch diameter is where the belt's tension line sits — slightly inside the tooth tips. You can type expressions like (pitch*36)/PI directly into a Fusion dimension field; PI is built in. Students confuse outside diameter with pitch diameter; for center distance, always use pitch diameter.
Spacing The Two Shafts
Center distance = gap between the two shaft axes
- Belt length must reach around both pulleys
- Belts come in fixed standard lengths only
- So center distance is constrained, not free
- Solve for a length that lands on a real belt
Unlike chain (where you add/remove links), belts come in discrete tooth counts. You can't just pick any center distance. Workflow: pick pulleys, pick a stock belt length, then SOLVE for the exact center distance that belt needs. Emphasize you usually design the frame around an available belt, not the other way around.
Two Ways To Solve
- Frame geometry fixes center distance
- Compute required belt length
- Round to nearest stock belt
- Add an idler or tensioner for slack
- Pick a stock belt tooth count
- Solve center distance to match it exactly
- Move a shaft to that spacing
- Cleanest — no tensioner needed
Most FRC teams use the right-hand method: choose a belt from REV/WCP/AndyMark stock, then place shafts at the exact center distance. Use an online calculator (or VEXpro/WCP belt calculator) to get the spacing. The left method needs a tensioner because your geometry rarely matches a stock belt perfectly.
Keeping The Belt Tight
Too loose skips teeth, too tight kills bearings
- Exact center distance = no tensioner needed
- Idler pulley or roller presses on the belt back
- Slotted mounting holes let a shaft slide to tension
- Tension the slack side, never the taut side
Three tensioning strategies: exact spacing (best), slotted holes to slide a shaft, or an idler/tensioner pulley. Idlers can press on the smooth back (any size) or the toothed side (must match pitch). Over-tensioning is the silent killer — it overloads shaft bearings and motor output bearings. Snug, not banjo-string tight.
Pull In Real Pulleys
Don't model pulleys from scratch — insert them
- Insert > Insert McMaster-Carr or Insert Derive
- Download STEP/F3D from REV, WCP, AndyMark
- Match bore: 1/2in hex, 3/8in hex, or round
- Joint pulley to shaft as Rigid
Real teeth are tedious to model and not worth it. Vendors publish CAD: REV, WCP, AndyMark, ThriftyBot. Insert the STEP/F3D, then use Assemble > Joint (Rigid) to lock the pulley to its hex shaft. Check the bore matches your shaft — 1/2in hex is the FRC default. The SpurGear add-in is for gears, not belts.
Your Task
- New design, set User Parameters (pitch=5mm)
- Insert an 18T and a 36T 5mm HTD pulley
- Place on two 1/2in hex shafts, 2:1 ratio
- Set center distance for a real stock belt
- Rigid-joint each pulley to its shaft
- Confirm belt length lands on a stock size
- Capture a top view showing center distance
- File > Share > Public Link in Fusion
- Paste the link on AltHub
- Note your chosen belt tooth count
Give them ~25 minutes. Watch for: parameters with no units, OD used instead of pitch diameter, and center distances that don't match any real belt. Push them to look up an actual WCP/REV belt length and back-solve the spacing. Walk the room and check that joints are Rigid, not loose.
🧰 Add-ins for this step
Use the installed AltSkripts / FRC-COTS tools here — don't do it the slow way.
- C-C Distance — HTD 5mm / GT2 3mm pulley spacing.
- Belt Generator — build the belt solid around the two pulleys.
Belts Run On Pitch Match Or It Skips
- Pitch + profile + width must all agree
- Center distance is set by stock belt length
- Insert vendor pulleys; Rigid-joint to hex shafts
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.