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Shaft & Structure Load Check

Whether the torque you are putting through a shaft will twist it. One V5 motor on a red cartridge out-torques a standard VEX shaft with no gearing at all.

Inputs

Gearing between the motor and this shaft.

Impacts load a shaft far above its steady torque. 1 is a smooth mechanism, 2 covers ordinary knocks, 3 or more for a drive that gets rammed.

Results

Will it survive?

—

Torque on the shaft

—N·m

Motor stall, times gearing.

With shock allowance

—N·m

Shaft yields at

—N·m

Shaft shears at

—N·m

Safety factor

—

Yield torque ÷ applied, shock included.

If it fails

—

Torque on the shaft, against where it yields—

Yield is where the shaft takes a permanent twist, not where it snaps. A shaft that has yielded stays bent and never comes back.

Hit it harder

Hit it hardertwistssnaps
2×

The torque on your shaft, on a line from nothing up past the point where it breaks. The first mark is where the metal starts to take a permanent twist; the second is where it snaps in two. Your motors set the steady torque. A knock multiplies it, so slide the shock factor up and watch the needle cross the marks. The line ends at twice the snapping torque. The shaft underneath is a sketch of what each stage looks like, not a calculated twist.

How this is calculated

Twisted shafts are one of the most common VEX failures, and they are almost always predictable. A shaft carries whatever the motors can produce multiplied by whatever gearing sits between, and gearing for torque is exactly what makes shafts fail.

torque on shaft = motors × stall torque × gear ratio

Note it uses stall torque. The shaft does not care about your usual operating point; it cares about the worst moment, which is when the mechanism jams and every motor pushes as hard as it can.

The number worth remembering

A standard 1/8" shaft takes a permanent twist at roughly1.24 N·m. A single V5 motor on a 100 RPM cartridge stalls at 2.1 N·m. One motor, geared 1:1, already exceeds a standard shaft.

Gear that motor down 5:1 for a lift and the shaft sees over 10 N·m, which is eight times what it can take and beyond even a high strength shaft. This is why lift shafts twist, and why the fix is a bigger shaft or splitting the load across two, not a stronger motor.

Why the square shape matters

shear stress = 4.81 × torque ÷ side³

Strength scales with the cube of the side length. Going from 1/8" to 1/4" doubles the side and therefore multiplies capacity by eight, which is why the high strength shaft is not a marginal upgrade but a completely different part.

It also means the square corners carry the stress. A shaft that has been rounded off by a loose gear is meaningfully weaker than a fresh one, and the analysis assumes sharp corners.

Shock is not optional

A drivetrain being rammed, or a lift dropped onto its stop, applies far more than its steady torque for a moment. Designing exactly to the stall figure means designing to fail the first time something hits you, which is what the shock factor is for.

What this does not cover

Only torsion in one shaft. Bending from a cantilevered wheel, bearing loads, gear tooth strength and screw shear are all real and none are here. A shaft can also fail in bending while passing this check comfortably.

Sources & assumptions

Stall torque is published for the 100 RPM cartridge andderived for the others by scaling inversely with speed. Gear tooth counts and shaft sizes are VEX published.

Shaft strengths are derived. VEX publishes no strength figures for its shafts. These are calculated from square-shaft torsion theory assuming 54 ksi yield and 64 ksi ultimate tensile strength, which is a common assumption for the steel used but is not confirmed by VEX. Treat them as a guide to which shaft to reach for, not as a certified rating.

Assumes a single shaft in pure torsion with sharp corners, at stall, carrying the full motor output. A shaft shared through a differential or supported at both ends by driven gears sees less than this.

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                                Comes after

                                Torque & Stall Checkfills in your gear sizes hereRequired Lift Torquefills in your gear sizes hereLift Speed & Gear Ratiofills in your motor count hereFour-Bar Liftfills in your motor count hereIntake Designerfills in your gear sizes here

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                                Shaft & Structure Load Check

                                Where your answer goes

                                Shaft Deflectionlearn why a shaft that won't twist apart can still bend enough to skip gearsuses your shaft size againBearing Load & Support Placementsee how hard that same shaft leans on each bearing holding it upuses your shaft size again

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