Control over an electric motor is especially critical in large drilling rigs and a means of accurately and quickly measuring the speed of the rotor of an electric motor and correlating that information with other real-time information has been hampered by the failure of the prior art to attach optical encoders to electric motors with precision. The present device attaches to arbor of a cap on the end of the rotor which provides a centralized and sealed shaft extending through a covering plate on the end of the motor and directly attaching to an optical encoder.