Upgrade Servo Horn Fastener To Address Coupler Weakness

Sawppy the Rover’s steel shaft upgrade for bigger flatter detents, along with applying thread locking compound to set screws, was the final task completed before it was time to go visit JPL. Since Sawppy will be running throughout the day, I expected that the next weakest point in the system will surface. And indeed one did! The winner of “next weakest link in the chain” is the shaft coupler holding the servo horn to the steel shaft.

The coupler was 3D-printed and designed around the LewanSoul LX-16A servo and the servo horn mounting screws that came in its package. LewanSoul products using these servos are made of thin sheet metal, so these screws are very short. In fact they are exactly as long as the servo horn is thick, possibly to ensure that it’s impossible to accidentally fasten the servo horn into the servo body.

Unfortunately, what this also means is that when used to fasten a 3D-printed plastic part, there is not enough thread engagement for a good grip. So when one of Sawppy’s corner steering is over-stressed, the screws tore out of the servo horn.

This particular coupler design only used three screws. I tried to perform a field repair by fastening into alternate holes on the horn but that quickly tore out as well. By the end of the day we have this mess: a servo horn where every hole is damaged.

Chewed up servo horn

It’s clear that to accomodate a 3D-printed shaft coupler, we can’t rely on the short screws that came in the servo package. We’ll have to upgrade to something longer and these screws (McMaster-Carr catalogue item# 98685A220) look like good candidates.

Longer screw for servo horn

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