Combined Gearbox

Remind students of Stage 1A: a single gear pair gives torque but also fixes the center distance. A NEO spins ~5676 RPM free; an arm or wheel needs far less. Today we stack a gear stage and a belt stage so we get the ratio we want and route the output where it physically needs to be. Tell them: gears are stiff and compact, belts let you span distance and absorb shock.

Walk through the arithmetic live. Each stage multiplies. A 4:1 gear then a 2:1 belt is 8:1 total. Stress that you read ratio as driven-teeth over driving-teeth. Common mistake: students add ratios instead of multiplying. Also note efficiency: each gear mesh ~96-98%, each belt ~98%, so an 8:1 combined is still ~94% efficient end to end.

This mirrors Onshape Variables. In Fusion it is Modify > Change Parameters. Define everything up front so the gearbox is editable. Demo adding a parameter, then show typing 'pinionT' into a dimension field. Tell students: never hardcode tooth counts into geometry. The whole point of a combined gearbox is iterating the ratio fast.

Install the SpurGear add-in from the Scripts and Add-Ins menu if not already loaded (it ships with Fusion under Utilities). FRC standard is 20DP, 14.5 degree pressure angle. Generate the pinion and gear separately, each as its own component so we can joint them. Remind them to bore the final gear for 1/2in hex (0.500 across flats) since most FRC shafts are hex, not round. Common mistake: mismatched DP between pinion and gear will not mesh.

This is the number that makes gears actually mesh. Center distance for a spur pair equals the sum of teeth over twice the DP. For 12T+48T at 20DP that is exactly 1.5 inches. Have them compute it from their parameters so it auto-updates. If the spacing is off even 0.02in the gears bind or backlash badly. This is the most common reason a student gearbox does not turn.

The belt stage takes the gear-stage output and reduces again. Pull pulley STEP files from WCP or REV, or model them. Key constraint students miss: belt length is discrete. You cannot use any center distance you want; you pick a stock belt (e.g. 100T, 105T HTD) and solve the center distance for it. There are belt-length calculators online. For class, give them the target center distance. Sketch the belt path so they can see it does not hit the frame or other components.

In Fusion, Mates become Joints. Each shaft gets a Revolute joint to the grounded plate. Then Assemble > Motion Link ties two joints together at a ratio so when you spin the pinion the gear turns the right amount. Do the same for the belt pulleys (18:36). Demo dragging the input and watching the whole train animate at 8:1. This is the payoff moment; it visually confirms the math. Common mistake: forgetting to ground the housing, so everything spins together.

Ground this in real FRC hardware. Insert a MAXSwerve or MK4 module STEP and show them: the motor reduces through gears at the top, then a belt carries torque down to the wheel at the bottom. Same pattern on arms: a planetary gearbox (like a MAXPlanetary) does the big reduction, then a belt or chain reaches the pivot so the motor can sit on the frame, not out on the arm where its weight hurts. This is exactly why we teach combined gearboxes.