This is just a curiosity question but how are press fit pinions able to adhere to the shaft and perform so effectively. Like is there something that occurs at the molecular level or something between the shaft and pinion when you press on?
no, press fit pinions are a sort of interference fit, as the hole in the pinion to accept the shaft will either be the exact size of the shaft or usually slightly undersized, when the two are pressed together, super high fiction that is caused by the tension created on the pinion and compression on the shaft when the two are made intimate which allows force to be transferred through the assembly with essentially no loss or slippage as the friction force is greater than the rotary force being generated by the motor
It’s just friction. With a very large normal force.
This varies from device to device. Some use the interference method above, some will press onto a spline shaft and some will actually heat one part to expand it while cooing the the second part to shrink. When the parts return to the same temperature, they will be locked in place.