Hollow-Bore Rotary and Gimbal Motors: A Project Selection Checklist

This guide organizes the information needed to assess a hollow-bore direct-drive rotary axis. It does not confirm that a motor, encoder, or drive combination is available.

Describe the axis

Start with the payload and the motion it must perform. Include the load inertia, orientation, acceleration profile, travel, dwell, repeatability requirement, and any disturbance torque. If the axis moves a cable bundle, tube, or optical path through its center, include that in the bore and clearance drawing.

Separate static holding requirements from motion requirements. A motor that can hold a load at rest may still be unsuitable for the required acceleration, speed, thermal duty, or emergency stop.

Distinguish cogging from total torque ripple

Cogging torque is a no-current effect associated with the magnetic and slot geometry. Torque ripple during operation can also come from current ripple, commutation error, encoder feedback, drive tuning, mechanical runout, and load variation.

When low-speed smoothness matters, state the test method and acceptance limit you need. A measured trace should identify the motor, fixture, angular resolution, direction, speed, temperature, drive state, and signal processing. A topology name by itself does not prove a ripple level.

Define mechanical and thermal interfaces

Provide the stator and rotor dimensions, through-bore, mounting pattern, bearing arrangement, air-gap reference, axial clearance, and installation sequence. For a frameless assembly, identify who is responsible for alignment, retention, and bearing support.

Describe the housing or cooling path, ambient conditions, nearby heat sources, and the continuous duty. Ask for model-specific ratings with their mounting and test conditions. Do not transfer thermal or runout values from a different fixture.

Plan feedback and commissioning

Specify the encoder type, resolution, electrical interface, reference or index behavior, cable routing, and servo drive model. Ask the supplier and drive vendor to confirm the supported configuration and required commissioning steps.

Do not use generic PID values or assume protocol compatibility from a product-family name. Current limits, electrical angle, pole-pair information, feedback scaling, and safety functions must match the actual motor and drive.

What to include in an RFQ

• Payload, axis orientation, inertia, motion profile, and holding duty.
• Required bore, envelope, mounting, bearing, and installation details.
• Smoothness or ripple requirement and the acceptance measurement method.
• Thermal boundary, operating environment, and duty cycle.
• Encoder, drive, controls, cabling, and safety requirements.
• Quantity, destination, required drawings, and test records.

For a possible motor alternative, send the reference model and its official interface drawing. A dimensional or electrical review is not a fit guarantee or an endorsement by the referenced manufacturer.