Shafts, bearings and holding things in place

Gears, sprockets and pulleys all end up on a shaft. The shaft has three jobs: carry torque without slipping, spin freely without wobbling, and stay where it is along its length. Most "mystery" mechanism failures are one of those three going wrong.

Job 1: carry torque

How big the forces actually are

Torque is force times radius:

torque = force × radius   →   force = torque / radius

That means the closer to the centre of the shaft you are, the bigger the force for the same torque.

A goBILDA 312 RPM Yellow Jacket motor has a listed stall torque of 24.3 kg·cm, which is about 2.38 N·m. Suppose a hub grips a 6 mm round shaft (3 mm radius) only by friction:

force at the shaft surface = 2.38 N·m / 0.003 m ≈ 790 N

That is roughly the weight of an 80 kg person, and the joint has to hold it without slipping every time the motor stalls. Put a 3:1 reduction after the motor and the shaft carries three times the torque — about 2,400 N at the same radius. Friction alone struggles with numbers like that.

Why FTC shafts are hex

The fix is to stop relying on friction. A hexagonal shaft in a hexagonal hole cannot turn inside it — the flat sides push on each other directly. The torque goes through the shape, not through how tight something is.

5 mm hex shaft

REV's standard shaft: a stainless steel hexagon, 5 mm across, that you can cut to length.

8 mm REX shaft

goBILDA's standard shaft and motor output: a 7 mm hexagon with its corners rounded off to an 8 mm circle. The hex drives things; the round parts sit in 8 mm bearings.

D-shaft

A round shaft with one flat ground into it. goBILDA sells a 6 mm D-shaft. Better than plain round, but the single flat carries much less than a full hex.

Job 2: spin freely

Bearings, not holes

Bearing

A part that lets a shaft spin with very little friction while holding it in position. FTC mostly uses flanged ball bearings and plastic (Delrin) bushings.

A shaft spinning directly in a hole drilled through aluminium rubs, wastes power, and wears the hole oval within a few matches. Support every rotating shaft in a bearing.

• REV: motion brackets have 9 mm seats for REV's plastic bearings. REV also sells flanged ball bearings with an insert for 5 mm hex, which REV says perform better under high load or high speed.
• goBILDA: the pattern includes 14 mm holes sized for goBILDA's flanged ball bearings, and many parts accept them directly.

Two points of support

A shaft held by one bearing can tilt like a see-saw. Support it at two points, spaced apart, wherever you can. A wheel mounted between two plates, with a bearing in each, is the gold standard.

A shaft that sticks out past its last bearing with something mounted on the end is cantilevered. That is fine — many drivetrains do it for easy wheel access — as long as the shaft is still held in two bearings with some distance between them.

Live axle and dead axle

Live axle

The shaft turns, and the mechanism is locked to it. Power goes in through the shaft. Needs bearings at each support point and something to stop the shaft sliding.

Dead axle

The shaft stays still; the mechanism spins on bearings around it. Power goes in through a sprocket or gear bolted to the mechanism itself. The fixed shaft can double as a structural spacer between two plates.

Use a live axle when one shaft drives several things (two intake rollers on one shaft). Use a dead axle for a single rotating part between two plates, like an arm pivot.

The spin test

After assembling anything with a shaft, disconnect the drive (take the chain or belt off if you can) and spin it by hand. It should turn smoothly and coast. If it is stiff, something is wrong:

• Bearings not lined up across the two plates, so the shaft is bent between them.
• A collar or spacer squashing a bearing. Collars should only touch the bearing's inner ring, never the outer ring or the plate.
• Over-tight chain or belt pulling the shaft sideways.

Friction you can feel by hand becomes a slow, hot motor on the field.

Job 3: stay put

A spinning shaft also wants to slide along its length — it "walks" out of its bearings. Shaft retention stops that. You need to stop it in both directions.

Set-screw collar

A ring with a small screw that presses into the shaft. Cheap, but the screw can loosen, and it raises a burr on the shaft that makes parts hard to slide off later.

Clamping collar

A split ring that a screw squeezes around the whole shaft. Grips far better than a set screw and does not damage the shaft. Game Manual 0 recommends these, especially for high loads.

Other common methods:

• E-clips (retaining rings) snap into a groove in the shaft. Compact and secure, but they only go where a groove already is. goBILDA sells REX shafts with them.
• A screw and washer in each end of a shaft with a threaded hole. Use threadlocker so it does not back out.
• REV collars use a set screw tightened with a 1.5 mm hex key; REV notes you can swap it for an M3 hex cap screw.

Why things slip: a checklist

When a motor spins but the mechanism does not, work outwards from the motor:

1. Motor shaft to first part — loose clamp or set screw on the hub?
2. Hub or gear bore — has a plastic hex hole rounded out?
3. Chain or belt — skipping teeth (see the previous lesson)?
4. Last part to the mechanism — loose screws between the hub and the arm, wheel or roller?
5. Shaft retention — has the shaft slid so a sprocket no longer lines up with its chain?

Next

That completes the first-principles unit: power budgets, transmissions, and shafts. Next, apply all three to the most important mechanism on the robot — the drivetrain.

Sources and further reading

• Game Manual 0 — Shaft Retention
• Game Manual 0 — Motion Mounting (live, dead and zombie axles)
• Game Manual 0 — Direct Drive
• Game Manual 0 — Hardware Tips and Tricks (avoid set screws)
• REV DUO — Shafts and Bearings
• goBILDA — 8mm REX shaft
• goBILDA — 5203 Yellow Jacket 312 RPM motor (stall torque)
• goBILDA — The goBILDA pattern