Shafts, Bearings, and Bushings

The hex shaft standard#

FRC has largely standardized on 1/2" hex shaft (with 3/8" hex for lighter loads). A hex profile transmits torque to gears, sprockets, and wheels without needing keys or set screws to grip a round shaft. Most COTS gearboxes, wheels, and sprockets are bored for 1/2" hex.

• ThunderHex / rounded hex is a hex shaft with rounded corners. It still indexes gears like a hex but the rounded corners let it ride in round bearings, which simplifies assembly.
• Round shaft is still used in some places (then torque is transmitted with keys, pins, or set screws).

Shaft materials#

• 6061-T6 aluminum for light-duty shafts and tube axles.
• 7075-T6 aluminum for high-strength hex shafts handling torsion.
• Steel (e.g., 4140) where maximum strength and wear resistance are needed, at the cost of weight.

Bearings#

A ball bearing lets a shaft spin with very little friction. The FRC workhorse is the R8 bearing (1/2" ID, 1-1/8" OD, 5/16" wide), often in a flanged version so the flange stops it from pushing through a hole. There are hex-bore flanged bearings (e.g., AndyMark's FR8ZZ-HexHD) that accept a 1/2" hex shaft directly. A chrome-steel R8 is rated for roughly 1,150 lbf dynamic / 540 lbf static radial load — far beyond typical FRC needs, so they rarely fail from load (they fail from contamination, side-loading, or a bent shaft).

Bushings#

A bushing is a plain sleeve (often bronze or plastic) with no rolling elements. It is cheaper, lighter, and more tolerant of dirt, but has more friction. Bushings suit slow or lightly loaded pivots (e.g., some arm joints).

Mounting and retention#

• Press or slip bearings into bearing blocks or directly into bearing holes in your tube/plate.
• Retain shafts axially with shaft collars, e-clips/retaining rings, or machined shoulders.
• Keep things square: a misaligned bearing binds the shaft and wastes motor power, and over time it can overheat and seize.