Rate of decent is important when using a vibrator on the screen. I never measured mine, but I hazard a guess to say 15 degrees +, depends on how dry the material is. The wetter it is - the steeper the decent.
I have screened several yards with this one over the yrs. It's a 12v 380 motor, the output shaft is about 1/16th, the counter weight is 4mm rod coupler from Mecanno with a 8mm length bolt screwed into the collar to secure it to the shaft. I think the motor's rpm is about 11 000. The motor disintegrates eventually, but each one does last a long time.
http://youtu.be/15xwKUVjyBw