how to find the rpm of a motor shaft using the change in the geometry of the shaft?

[amir_aj67]

Hello everyone, I am new here so sorry if the question has been asked before, I searched it here and could not find anything.
I am looking to find the motor's RPM using the changes in the geometry of the motor's shaft. I can not install anything on the shaft, all I can do is to maybe cut or sand the surface of the shaft so it will be flat in one area and curvy on the rest, and some sort of sensors that will find out when it is flat and when it is not and send it as electric pulses to my controller.
does such a sensor exist? it should be fairly small (4*4*4 cm) so it can be screwed some were close to the shaft.
I have been using hall-sensors and tachometers before but for this application we can not use any of them.
will appreciate your help

 

[Rleo6965]

Try using small Limit Switch with Roller.

 

[duke37]

It depends on the shaft size.
You can use a reflective detector with the shaft painted white/black.

 

[Electrobrains]

You will have to debounce a mechanical contact, it will soon wear out and what about possible speed?

As duke37 suggests, why not using a reflective Photointerrupter? Paint the shaft black (or white) and draw a white (or black) line on it, or attach a bit of aluminum foil on the shaft.

See my links and example in the thread below: transistor circuit for counting wheel rotations