Question Keyway vs Keyless Gearbox Input for Rotary

Im trying to buy some gearboxes and alot of what im seeing the input does not have a keyway. Does the input side not deliver enough torque to matter?

Im looking at 80/90ST-M02430 servos the specs are a peak torque of 7.1 n-m. Looking to plug into a 50:1 reducer.

Also if you know a good site to find steel couplers that would be fantastic

Keys are meant only for orientation, never for torque transmission

ChatGPT “Motor shaft keys (along with the matching keyways in the hub or gear) are *specifically designed to transmit torque* between the rotating shaft and the attached component, like a pulley, gear, or coupling. The key fits snugly into both the shaft and the mating part, so when the shaft turns, the key acts as the mechanical link that transfers rotational force to the load.”

I had problems with the Amazon keys mostly that they were too small leading to slack. I was very satisfied with McMaster Carr. They have an oversized key which I need to hammer to get in. No slack. This is on the gear output side so we are talking 30nM of transmitted torque no slack, I bought a slide hammer to take them out.

ChatGPT is always right, for sure....

So every engine manufacturer and industry designer should trust ChatGPT in the future, every knowledge from the past is obsolete.


You have to use the right coupler for your use. Transmit high torque only with an key, will result in ware out of your shaft and coupler. A clutch that is clamped on the shaft can transmit much higher torques.

The keyed shaft is common, but its main drawback is that there inevitably exists a slight gap (fit tolerance) between the key and the keyway. This results in minor backlash during startup, shutdown, or reversal, which is a disadvantage for those seeking absolute control feel.
Keyless shafts typically use an interference fit or shrink fit. Their greatest advantage is the achievement of "zero-clearance" transmission, providing highly direct and smooth motion response without any backlash.

The key+keyway solution is widespread for historical reasons. It makes sense for agricultural machines where you have high torque and possible shock load and backlash is no issue. One of its advantages is also that it's "fool proof". There is no need to tighten a set screw to a well-defined torque so it doesn't slip.

For servos, keyed shafts make no sense. A slotted coupling with set screw can transfer more torque than the servo can deliver. A keyless shaft has no imbalance and is easier to manufacture.

Keys are even dangerous for servos. If the fitting tolerance isn't right you need a hammer to insert and remove the key and this can damage the shaft and the encoder.

Yes, I couldn't agree more with your perspective. The keyway solution is indeed outdated and ill-suited for servo applications. In our experience, we’ve encountered multiple instances where hammering during key insertion or removal led to encoder damage. Replacing an encoder is not only time-consuming but also a thankless task—often requiring realignment, recalibration, and significant downtime.

I dont think hammering is the way . but a file