1. Pull the plastic Speed Knob off the rotary encoder shaft

2. Spray some cleaning alcohol onto the slit shown below

3. Try and rotate the shaft slightly and also push the shaft in a few times to clear out all the dust. 

4. Repeat steps 2 if there is more dust in the shaft. 

The shaft sometimes becomes stiff or jams when fine dust makes it into this slit. 

Sometimes cleaning alcohol is able to clean out the dust, but if it does not you can try a little bit of WD-40 in the same spot. You can rotate the speed dial and push it in and out a few times to help clear the dust out.