The speed changes or drifts during the test.

Speed is a closed loop servo system. The speed and deflection information comes from the optical encoder underneath the tester. This encoder is open and can collect dust. Remove the front cover on your dual ball screw tester. For the single screw testers you must remove the bottom cabinet and lean the tester on its side. You will see a flexible plastic disk with clear and dark lines on it. Clean the disk with water and mild detergent. The disk passes through a small slotted sensor. Blow this sensor clear with compressed air.