Add Idler (2nd Axis Mounting Point) on Micro Servos for Robotic Projects

In humanoid robotic projects, servos are used at joints to move different segments of the robot, most of the time it is best to mount each segment at 2 or more points on the rotating axis of the servo for stability and proper transfer of torque..

Smart servos designed specifically for robotics have idlers on the opposite side of servo output shaft, so moving segments the robot are connected to the servo at 2 points on the rotating axis, one at the servo output shaft (or servo horn) and one at the idler at the opposite end.

I use a variety of standard servos in my robotic projects, one issue I've encountered is that almost all of these servos were designed for the RC industry, only the output shaft need to be connected (such as steering a RC car or control the rudder of RC planes), and don't come with options to add an idler, so I wanted to explore different ideas on how to add an idler to these standard servos.

Step 1: Add Idler Mounting With Riveting Nut

My initial idea is to add a screw mounting hole on the back fo the servos directly opposite of the servo output shaft, then use the appropriate screw to mount a flanged bearing to the newly added hole, and the flanged bearing will function as the idler.

I found some small M2 through hole riveting nuts that seem suitable for my purpose. I drilled a hole on the back of the servo, pushed a riveting nut through, then put a drop of Super Glue between the nut and the plastic servo cover.

I tested this setup with some 3D printed parts and it worked pretty well, but the process seemed like a hack job so I though about redesigning the servo back cover that includes the idler mounting hole.

Step 2: Redesigning Servo Cover With Idler Mounting Hole

I designed a few variation of the back cover then 3D printed them to test, after picking the final design, I begged my machinist friend to make 30 prototype pieces for me.