Build the speed-torque envelope
The first calculation is a continuous and transient speed-torque envelope at the driven shaft. Include breakaway torque, normal process torque, upset conditions, acceleration time, minimum stable speed, overspeed scenarios and any reversing or inching operation. For conveyors, starting with material on the belt can differ sharply from empty running. For mills, process charge and starting sequence can dominate. For pumps and fans, the process curve and control method matter.
Do not convert a gearbox ratio into a motor speed and stop there. Check mechanical losses, service factors, inertia reflected through the old transmission and the torque that the new motor must supply during each event. If measured data are unavailable, identify the calculation method and uncertainty. A transparent envelope is more useful than one oversized torque number because it guides electromagnetic, thermal, drive and structural design together.
Low speed changes cooling and control
Producing high torque at low speed creates a thermal and control problem as well as an electromagnetic one. A shaft-mounted fan may provide little airflow at low rotational speed, so cooling architecture must be reviewed against continuous torque. Drive current, switching, sensor strategy and tuning must support stable operation through the required speed range. Applications near zero speed may need different feedback or commissioning practices from applications that run steadily above a minimum speed.
Share the lowest continuous speed, dwell time, overload duration and permissible temperature rise. If the equipment cycles or stalls by design, state that clearly. The cooling method, sensor placement and protection thresholds should correspond to this duty. A direct-drive concept that meets peak torque briefly may still be unsuitable if it cannot reject heat during the real cycle.
Evaluate reliability without slogans
Fewer mechanical transmission elements can remove lubrication points, wear interfaces and alignment tasks. That can simplify maintenance, as official ABB and Siemens direct-drive materials explain for their own systems. The remaining components, however, become more critical. Motor bearings, cooling, sensors, drive electronics, cable routing and structural connections still require a maintenance and spares plan.
Create a before-and-after failure-mode review. List each component removed, retained or added, its inspection interval, likely failure signal and replacement method. Confirm whether site technicians can access the components and whether the plant has the required diagnostic tools. Reliability improvement is credible when it is tied to a specific architecture and maintenance plan, not when it is presented as an automatic result of the words direct drive.
Direct-drive RFQ checklist
Send the driven-equipment description, existing drivetrain drawing, motor and gearbox nameplates, shaft speed, continuous and peak torque, starting method, load profile, inertia, coupling details, bearing arrangement, voltage, proposed drive, environment, cooling limits and available installation envelope. Photographs with scale references are useful, but dimensioned drawings remain necessary.
For conveyors include belt speed, loaded start condition, incline and take-up arrangement. For mills include charge condition, starting sequence and any inching requirement. For pumps and compressors include the process curve, minimum speed and control objective. ENNENG can use this package to decide whether a TYDP or other low-speed configuration deserves detailed review.