Shafts & Bearings
Shafts carry rotating components (wheels, pulleys, sprockets, rollers). Bearings let those shafts spin smoothly inside the frame. Every mechanism on the robot has at least one shaft and at least two bearings, so understanding how these work together is foundational.
Shaft types
1/2" hex
0.500" flat to flat
The primary FRC standard. Used for drivetrains, main mechanism shafts, and anything under significant load. Almost all COTS pulleys, sprockets, wheels, and hubs are designed for 1/2" hex bore.
3/8" hex
0.375" flat to flat
Lighter and smaller. Good for lower-load applications like intake rollers, conveyor shafts, and light pivots where 1/2" hex is heavier or bulkier than necessary.
Round
Various (commonly 1/2")
Used for dead axle applications where the shaft is fixed and components spin on it. Also used when interfacing with non-hex components.
ThunderHex (rounded hex)
ThunderHex is hex shaft with rounded corners. It was developed by WCP/VEX and is now the most common hex shaft in FRC. The rounded corners serve an important purpose: they allow the shaft to work with standard round bore bearings in addition to hex bore bearings.
Why this matters: A regular hex shaft requires a hex bore bearing, and the tolerances between the two can be tight enough to cause fitment issues (especially after tapping the end of the shaft, which can slightly deform the hex profile). ThunderHex in a round bearing gives you a shaft that transmits torque through hex-bore components (pulleys, sprockets, hubs) but pilots smoothly through round bearings in the frame, which are easier to source and more forgiving with tolerances.
A common and reliable setup: ThunderHex shaft with round flanged bearings in the plates, and hex-bore components on the shaft. The hex bore components grip the hex portion of the shaft for torque transfer, and the rounded corners ride in the round bearings for smooth rotation. This gives you the best of both worlds.
Dead axle vs. live axle
These are two fundamentally different ways of designing a rotating mechanism, and understanding the difference matters for every subsystem you design.
The shaft spins. Components (wheels, rollers, pulleys) are fixed to the shaft, and everything rotates together. Bearings sit in the side plates, and the shaft rotates inside those bearings.
Advantages:
Simple to build. Slide hex-bore components onto a hex shaft and they're locked together.
Flexible plate placement since the plates don't have to be on the outside of the shaft.
Disadvantages:
The shaft transmits all the torque, which means it's under more stress.
To disassemble, you typically need to pull the shaft out through the bearings, which means removing shaft retention and anything else on the shaft.
Common on: intake rollers, conveyor rollers, shooter flywheels, and any mechanism where multiple components on the same shaft all need to spin together.
The shaft is fixed (bolted to the frame and doesn't rotate). Components spin on bearings that are mounted on the shaft. The shaft itself is just a stationary pin.
Advantages:
Stronger, because the shaft doesn't need to transmit torque. It's only resisting bending loads.
Easier to service since you can remove the spinning component without pulling the shaft.
The shaft can be bolted at both ends for very rigid support.
Disadvantages:
You need to drive the spinning component directly (through a belt to an integrated pulley, for example) rather than just driving the shaft.
Slightly more complex to design since the bearing is on the spinning component, not in the plate.
Common on: drivetrain wheels (many swerve modules use dead axle), arm pivots, and mechanisms where strength and serviceability matter more than simplicity.
If you're not sure which to use, live axle is simpler for most mechanisms. Dead axle is worth the extra design effort when the mechanism is high-load, needs to be serviced frequently, or when the shaft is long and you want maximum rigidity.
Bearings
Flanged ball bearings are what you'll use for almost everything. The flange is the flat lip around the outside of the bearing that sits against the outside face of the plate, which prevents the bearing from pushing through the hole.
Bore
The hole in the middle. This matches your shaft. For 1/2" hex shaft you need a 1/2" hex bore bearing. For ThunderHex with round bearings, you need a 13.75mm (or 0.5413") round bore bearing.
Outer diameter
Determines the hole size in your plate. Check the vendor's datasheet for the recommended hole diameter.
Bearing hole in plate
Needs to be the right size: tight enough that the bearing doesn't wobble, loose enough that you can press it in by hand or with light tapping. Check the vendor's recommended hole size and use that exact dimension in CAD.
Flanged vs. non-flanged
Use flanged for almost everything. The flange prevents the bearing from being pushed through the plate. Non-flanged bearings are only used when the bearing sits inside a bore (like inside a hub) rather than in a plate.
Where to buy: REV and WCP both sell hex bore and round bore flanged bearings in the standard FRC sizes. Keep spares in your hardware bins because bearings wear out over a season, especially in high-RPM applications like shooters.
Bushings
Bushings are a simpler and cheaper alternative to bearings. They're just a sleeve (usually bronze or Delrin) that the shaft rotates in. They have slightly higher friction than ball bearings but take up less space and cost almost nothing.
In FRC, bushings are useful for low-speed pivots and linkage joints where a full ball bearing is overkill. For example, a shoulder bolt acting as a pivot pin for a pneumatic cylinder mount can ride in a bronze bushing rather than a bearing. They're also useful when space is very tight and a standard flanged bearing won't fit.
For anything spinning at meaningful RPM (rollers, wheels, flywheels), use a real bearing.
The golden rule
Never cantilever a shaft. Always support it at two points (two bearings, or a bearing and a bolt through the end). A shaft supported at only one end will wobble, bend, and eventually fail. If you look at your design in CAD and see a shaft sticking out with only one bearing, fix it before manufacturing.
This also applies to components on shafts. A wheel at the very end of a long shaft with one bearing close to the frame and nothing on the other side is cantilevered. Add a plate with a bearing on the outside to support it.
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