FRC Hex Shaft, Bearings, and Bushings: Sizes, Fits, and Why Shafts Wallow Out

FRC robots run on two hex shaft sizes: 3/8" and 1/2", measured flat to flat. Reach for 1/2" hex as your default: it's the size most drivetrains, gearboxes, and high-load mechanisms are built around. Drop to 3/8" hex when you want the same key-free torque transmission but need to save weight or space and the load doesn't demand the bigger shaft, like an idler stage, a lightly loaded roller, or a compact intake. Both sizes skip the machine key entirely: torque transfers through the six flat faces of the shaft pressing against the six flat faces of the bore, so nothing can spin relative to the shaft once it's on.

This guide covers the parts around that shaft: flanged bearing vs. bearing block vs. bushing, where press fit belongs versus clearance fit, how to actually retain a shaft, and why hex bores round off over a season.

Flanged bearing, bearing block, or bushing

These three options let a shaft spin relative to a structure, at three different cost, weight, and complexity points.

Flanged bearings are the default. A flanged bearing has a lip around its outer diameter that sits flush against the face of a plate, so it drops into a hole and can't punch through under axial load. The standard FRC parts are the 1/2" hex ID / 1.125" OD bearing (sold as FR8ZZ-HexHD at AndyMark, with equivalents at WCP and REV) and the 3/8" hex ID bearings (FR6ZZL-Hex shielded, 1.125" OD, and FR62RS-Hex sealed, 0.875" OD). If a shaft only needs support at a single plate or tube wall, this is almost always the right part.

Bearing blocks solve a different problem: supporting a shaft where there's no plate to drop a bearing into, most often at the end of a drivetrain rail. A bearing block is a machined aluminum block, usually two pieces clamped around a tube, that captures a flanged bearing and bolts to the rail. WCP sells a lightweight two-piece Side Bearing Block and a one-piece Gearbox Bearing Block for WCD-style gearboxes. AndyMark's West Coast Drive Style Bearing Block Kit uses the same FR8ZZ-HexHD bearing and is rated to withstand upwards of 1500 lb of axial force along the tube when installed correctly, which is why most FRC drivetrains skip a separate belt or chain tensioner: you tension by hand, then clamp the block down. If your plate material is too thin or soft for a press fit, AndyMark also sells a bearing plate that spreads the load across four bolted holes instead.

Bushings trade rolling elements for a plain sleeve of bronze, Oilite, Delrin, or acetal running directly against the shaft. They cost roughly a quarter of an equivalent bearing, weigh less, and are toleranced for a clean slip fit on standard FRC shaft stock, but they run hotter and wear faster at speed. Use them for low-load, low-RPM rotary joints: arm pivots, pneumatic cylinder clevises, linkage joints. Skip them anywhere that spins fast or carries real radial load, like a flywheel, an intake roller, or a drivetrain. See the intake design guide and the elevator and arm design guide for where these actually show up on a mechanism.

Press fit vs. clearance fit: two different jobs

One shaft assembly has two different fits happening at once, and mixing them up is where most teams get into trouble.

• Bearing outer race into the plate or block bore: usually a light press fit, just enough interference that it seats and stays put without a hammer. Press on the outer race only, never through the balls, with a socket or driver disc sized to touch just the race. Waterjet and laser-cut bores rarely hold a tight enough tolerance to guarantee a consistent press every time, so some teams draw the hole at nominal bearing diameter and press once burrs are cleaned up; others cut a deliberate slip fit and retain the bearing with screws or rivets instead, which is easier to hit consistently than a precise press fit.
• Shaft through the bearing's hex bore: a clearance or slip fit, not a press. The shaft should insert and remove by hand while the hex flats still carry torque; the outer race staying fixed in the plate is what actually holds things in place. A shaft that needs pounding into a bearing is wrong-sized on one part or the other.

Watch shaft stock tolerance here. AndyMark's own listing for 3/8" aluminum hex shaft stock warns it "may be a tight fit in some of our 3/8 in. hex products," and buyer reviews on that page describe batches that wouldn't fit their bearings or gears without extra machining. That's exactly the kind of variation that gets a shaft pounded into a bearing, wrecking the race, or a bore reamed oversized, creating slop. If a shaft doesn't slide freely by hand, don't force it.

Retaining the shaft

A bearing lets a shaft spin; it does nothing to stop it walking axially out of the assembly. You need a retaining feature on each side of anything that has to stay put, like a gear, sprocket, or pulley.

• Shaft collars (collar clamps): a split or slotted ring that clamps around the shaft with a screw. WCP's light-duty clamping collars are cut thinner than a generic McMaster collar to save axial space and come in bores matching FRC shaft sizes down to 3/8" hex. AndyMark's hex collar clamps come in a 10-32 heavy-duty version and a 4-40 thin version, machined from 6061-T6 with small bosses on each face so they sit directly against a bearing without interference. Use collars when you're still adjusting shaft position or expect to service the mechanism later, since they unbolt.
• Retaining rings (snap rings): a stamped ring that drops into a groove cut into the shaft. Thinner and lighter than a collar, useful when axial space is tight, but they need that groove machined first and are effectively a one-way install, better for a position you've already finalized.
• Spacers: plain tube or nylon washers that take up slack between a bearing and a moving part so nothing shifts sideways. They don't retain anything by themselves, but a wrong-length spacer is one of the most common causes of a shaft that binds or has too much end play.

Why hex bores wallow out

"Wallowing out" is what happens when the hex profile in a bore rounds off into something closer to a circle. It's usually the softer of the two mating parts that goes first, most often an aluminum gear, pulley, or sprocket hub riding on a shaft, though a soft aluminum shaft can round off just as easily against a harder steel bore.

The mechanism is straightforward: torque only transfers through the six flat contact patches, and under a cyclic or reversing load, like a drivetrain constantly accelerating and braking or an indexer that oscillates, those flats take repeated peak stress at the corners with every direction change. If the material yields even a little each cycle, the corners round, the contact patch shrinks, and the same torque now concentrates on an even smaller area. It's a self-accelerating failure: a little play produces a lot more play, fast.

An oversized clearance fit that lets a part rock, an undersized shaft not fully engaging all six flats, and repeated shock loads like driving into a defense bot all make it worse. Steel hex shaft holds up to that cycling better than aluminum, which is why high-cycle mechanisms are often built with steel shaft instead, and why aluminum hex-bore gears are typically hard anodized (Type III) rather than left bare. Once a mechanism has developed slop, teams commonly shim the gap with shim stock or use a retaining compound like Loctite 638 to fill the play and bond the hub to the shaft, though bonding it means giving up the ability to service that joint without cutting or heating it apart. If the flats are visibly rounded, replace the part; shimming a badly worn bore only delays the failure.

Quick reference

Vendor part numbers shift between seasons, so check the current WCP, AndyMark, and REV catalogs before you spec a build. For the full walkthrough with diagrams, LearnFRC's mechanical build track covers shafts, bearings, and bushings in its power transmission module.