Vibration.
If you can feel it vibrating, you already have a sensor. Your hand.
Better still to get some measure of the amount of un-balance is to set up a dial indicator against the motor in the radial plane where you wish to measure and see how much the pointer wobbles ,as long as the motor is not really hard mounted.
Given that they are close together, 2 worm drive hose clips with the heavy parts 180 degrees opposed mounted on the shaft should give no correction.
Use felt tip pen to mark a line on the shaft at 90 degrees to the big parts of the clamps, and 2 more at 120 degrees apart in a different color.
Now move the clamps around the shaft towards the mark. You have 'added' weight near the the line. Put a mark across both the clips so you can duplicate their angular position EXACTLY.
Run the motor and see how much it vibrates. Note how much wobble.
Move both the weights 120 degrees around to next color. Run it again. How much?
Move both the weights around 120 degrees to next color. Run it again. How much?
Now you might have some idea in which 120 degrees the weight needs to be added.
On a circular graph you can draw a vector from the center for each reading
at 120 degrees.
Now join the ends of the triangles and find the center of the triangle.
This gives the radial direction from the center of the relative amount of un-balance, and should give some idea of the angular position required to add weight. Now mark the heavy/light spot and fiddle with the clips to correct for the vibration.
Now you have 'added' weigh to fix the problem.
On the opposite side to where you added the weight you can remove weight by drilling/grinding to remove weight.
Remove the clips, start drilling and measuring and you will balance the motor. This will only be correct for this end of the motor. The other end needs doing too, and this is dynamic balancing. The correction at the other end may be totally different and at a different angle.
The 3 weight method is good because you can't screw up a triangle and you must get a vector result. If you want to practice before fixing it for real put an old pulley or piece of round stock with a hole on the end of the motor and practice on the stock. Even a piece of hardwood will work.
If you get inconsistent angular results and keep chasing your tail do each of the 3 measurements both clockwise and CCW, then draw the triangles for each direction, and average the resultant vector. Sometimes phase errors like this can result from the mountings interfering with the wobble measurement. Make sure the dial indicator stays in the same place for each of the test runs.
When getting close to balanced you will probably detect that the amount all of a sudden moves 90 degrees around the shaft. This is because the position you have selected to correct is off by a few degrees. At small un-balance amounts the angular position becomes quite sensitive.
I have balance tail shafts on cars using a similar method, but just using a piece of chalk to mark the heavy spot. Make sure the car is mounted safely!
Any comments...?
Neil
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.