Two-Stage Gearbox

Anchor this: last lesson they built a SINGLE reduction. A NEO free-speeds at 5676 RPM, a Kraken x60 at ~6000. A 4in wheel at that speed is ~110 ft/s — absurd. We need roughly 8:1 to 12:1 for a drivetrain. Ask: why not just one 10:1 gear pair? Because the driven gear would be enormous (a 12-tooth pinion needs a 120-tooth gear). Two stages split the work so both gears stay small. Common mistake: students think more teeth always means slower; remind them ratio is driven/driving teeth.

Do this live on the whiteboard. Stage 1: 12T pinion drives 48T = 4:1. Stage 2: 16T drives 48T = 3:1. Multiply: 12:1. Stress that you compound by MULTIPLYING, not adding. Then connect to real gears: in FRC we use 20DP gears with 1/2in hex bores. Tooth counts must be whole numbers, so not every ratio is buildable — that is why we choose teeth first. Output torque is input torque × 12 (minus efficiency, ~95% per stage).

This is the conceptual heart of the lesson. The layshaft (also called countershaft or jackshaft) is what makes it a compound gearbox. Two gears live on it and are keyed to the same hex shaft, so they rotate as one unit. The Stage 1 driven gear receives power; the Stage 2 pinion (smaller, same shaft) sends it on. Common confusion: students draw the second pinion on the wrong shaft. Both Stage 1 gear and Stage 2 pinion ride the layshaft.

In Onshape this would be Variables; in Fusion it is User Parameters under Modify > Change Parameters. Pitch diameter = teeth / diametral pitch. A 48T 20DP gear is 48/20 = 2.4in PD. Center distance between two gears = (PD1 + PD2) / 2. Set these up NOW so when a student wants to retune the ratio, the whole sketch flexes. Common mistake: hardcoding center distances, then nothing updates when they change tooth counts.

Fusion ships a SpurGear sample add-in (Utilities tab > Add-Ins > Scripts and Add-Ins). It builds true involute gears. Set pressure angle to 20 degrees to match FRC AndyMark/WCP/REV gears. After generating, sketch a 1/2in hex and extrude-cut the bore so it mounts on hex shaft. Make each gear a separate Component (right-click > Create Components from Bodies) so you can joint them. Tip: gear thickness around 0.375in for a typical FRC gear; thicker for high torque.

Now packaging. Sketch on the plate. Fix the input bore. Dimension the layshaft bore at Stage 1 center distance from input — but it can swing on an arc, so the angle is a design choice (this is the 'triangle' students get to lay out). Then the output bore is Stage 2 CD from the layshaft. Use 1.125in bores for standard 1/2in hex thunderhex bearings (or 0.875in for smaller). Encourage them to angle the layshaft so the gearbox fits their available space.

Onshape Mates become Fusion Joints. Each shaft gets a Revolute joint (one rotational DOF) to the plate. Gears lock to shafts with Rigid joints. The two layshaft gears are rigid to each other and the shaft, so they MUST spin together — that is the compounding. To animate properly, add a Motion Link between input and output revolute joints at the computed ratio. Common mistake: forgetting a joint and the gear floats, or making everything rigid so nothing spins.

A real gearbox is two plates with the geartrain sandwiched between. Shafts ride in bearings (REV/WCP hex bearings press into the plate bores). Standoffs or a spacer set the plate gap so gears have axial clearance. Motors bolt to one plate over the input shaft — show the NEO (25-tooth-ish bolt circle) or Kraken pattern. Remind them to leave ~0.020in clearance so a slightly-thick gear doesn't rub the plate. This is where packaging gets real.