Servo - Wormgearunit - Family HT - High - Torque Gearbox Series 98 150 % output torque backlash < 1 arcmin highest stiffness HP - High - Performance Gearbox Series 58 100 % output torque backlash < 2 arcmin high stiffness E - Economy Gearbox Series 59 100 % output torque backlash < 6 arcmin high stiffness B - Basic Gearbox Series 57 90 % output torque backlash < 12 arcmin normal stiffness