A buyer-side guide to using hollow-shaft frameless torque motors for optical gimbals, cable pass-through axes, indexing tables, and low-backlash direct-drive systems.
Hollow-shaft torque motors are used when the center of the axis must remain open. The opening may carry optics, cables, slip rings, vacuum lines, tooling, bearings, or encoder hardware.
For gimbals, rotary tables, inspection stages, and precision automation axes, a hollow-shaft frameless torque motor can remove gearbox backlash and keep the axis compact. The tradeoff is that mechanical integration must be planned carefully.
These are the items I check before treating a hollow-shaft torque motor quote as realistic, especially when the center aperture, smooth motion, and mechanical runout are as important as the torque number.
Large aperture designs need early review of bore size, air gap, bearing reference, and rotor clamping.
| Takeaway | Practical meaning |
|---|---|
| Define the aperture before torque discussion | The required clear ID and what passes through it can change the motor family, OD, stack length, and winding direction. |
| Smoothness is application-specific | Gimbals care about pointing stability and vibration; rotary tables care about repeatability, indexing duty, and thermal repeatability. |
| Large ID trades against active motor area | A bigger opening may require more OD, more stack length, better cooling, or a lower torque target. |
| Mechanics control final accuracy | Bearing stiffness, housing concentricity, encoder layout, and air gap control determine whether the motor can perform as expected. |
| Installation planning is part of supplier qualification | Rotor retention, magnet handling, lead routing, and post-assembly checks should be reviewed before samples ship. |
Common applications include:
The shared requirement is usually smooth rotary motion with a large aperture and low backlash.
Not every hollow-shaft axis has the same buying priority:
| Application | Primary value | Main RFQ risk |
|---|---|---|
| Optical gimbal | Smooth pointing and cable/optical pass-through | Torque ripple, vibration, environmental assumptions |
| Antenna pointing | Large aperture and holding torque | Thermal load during hold |
| Rotary indexing table | Low backlash and repeatable indexing | Temperature rise over repeated cycles |
| Semiconductor inspection axis | Smooth motion and stable geometry | Runout, encoder compatibility, particle/process constraints |
| Cable-through automation module | Packaging and wiring simplicity | ID requirement reducing available motor torque |
| Slip-ring rotary axis | Open center and continuous rotation | Mechanical interface and lead routing |
Use this table to decide what the supplier should optimize first. A gimbal motor conversation should not sound identical to a rotary table conversation.
A large inside diameter is valuable, but it reduces the space available for copper, magnets, and iron. The supplier may need to adjust:
If the aperture is fixed by optics, a slip ring, or a bearing, send that exact value in the first RFQ. Do not describe it only as "large hollow shaft."
The ID/OD ratio is one of the first things to review. A large ID with a small OD leaves limited radial space for the active motor structure. In that case, the supplier may recommend:
If OD cannot increase and stack length cannot increase, the realistic torque may be lower than expected. Discovering that before the mechanical design is frozen saves a painful redesign.
For optical and pointing systems, smoothness can be more important than raw torque.
Useful RFQ details include:
| Requirement | Why it matters |
|---|---|
| Pointing speed | Determines voltage and back EMF requirements. |
| Holding torque | Defines continuous thermal load. |
| Disturbance torque | Defines peak margin. |
| Torque ripple target | Affects optical stability and control tuning. |
| Cable or optical pass-through | Drives ID and rotor geometry. |
| Environmental condition | Affects insulation, coating, and thermal assumptions. |
If vibration or pointing stability matters, include the measurement target your team uses internally.
For gimbals, also clarify:
Avoid vague terms like "high precision" without a measurable target. If you cannot share a numeric target, describe the test method your team uses.
For indexing tables and automation axes, the motor is only one part of repeatable motion. Bearing stiffness, encoder feedback, fixture inertia, and housing thermal behavior matter.
Ask the supplier to review:
If the table runs many cycles per hour, continuous thermal behavior is more important than the highest peak torque number.
For a rotary table RFQ, include the cycle profile:
| Cycle item | Example input |
|---|---|
| Index angle | 30 degrees, 90 degrees, 180 degrees, or continuous |
| Move time | Acceleration, cruise, deceleration |
| Payload inertia | Fixture and workpiece inertia if known |
| Dwell time | Time between moves |
| Cycles per hour | Production duty |
| Positioning target | Repeatability and accuracy expectation |
This lets the supplier discuss torque-speed and thermal behavior more realistically.
Hollow-shaft frameless motors usually require careful assembly because the rotor and stator are separate parts.
Control these risks before sample build:
The installation guide covers these issues in more detail.
The air gap is the controlled space between rotor and stator. If the tolerance stack is poor, the motor can lose performance or suffer physical contact.
Ask these questions before sample build:
For precision axes, the motor supplier and mechanical team should review the stack together. A motor cannot compensate for an unstable bearing or housing reference.
CAD helps after the motor family and aperture direction are known. For early discussion, share your envelope and ask for a preliminary drawing path. For later review, request:
Tie every CAD request to a project name and drawing revision. This reduces the risk of using the wrong model during mechanical design.
Ask these during the first supplier discussion:
| Question | Why it matters |
|---|---|
| What is the largest practical ID for this OD and torque target? | Checks whether geometry is realistic |
| What thermal condition supports continuous torque? | Prevents overpromising in sealed axes |
| Can you provide torque-speed and temperature-rise data? | Helps compare motor options |
| What CAD formats are available? | Supports mechanical packaging |
| What assembly method do you recommend? | Reduces air gap and magnet handling risk |
| What outgoing tests are available? | Supports supplier qualification |
For B2B programs, the quality of clarification questions often tells you more than the first quoted price.
Use these questions to separate a useful engineering quote from a shallow price reply.
| Topic | Ask | Useful answer |
|---|---|---|
| Thermal basis | What cooling condition supports the continuous torque? | Housing/cooling assumption and temperature basis |
| Winding fit | How does the winding match our bus voltage and current limit? | Kt, back EMF, current, speed, and drive comments |
| Mechanical integration | What mounting and air-gap control method do you recommend? | Stator retention, rotor clamping, and inspection notes |
| Validation | What tests can be provided before production release? | Back EMF, resistance, insulation, torque, thermal, outgoing record |
| CAD control | When can CAD be released and how is revision controlled? | Model family, drawing revision, STEP/IGES scope |
Subject: Hollow-shaft torque motor RFQ for [gimbal / rotary table / automation axis]
Hello framelesstorquemotor.com engineering team,
We are evaluating a hollow-shaft frameless torque motor for [application].
[optics / cables / slip ring / bearing / tooling][values][values][hold / scan / index cycles per hour / continuous rotation][values][values][values or TBD][STEP / IGES / 2D drawing][values]Please advise suitable model family, geometry risks, winding direction, and what data is still missing before the quote can be trusted.
Send these details for a first review:
For product options, review large hollow-shaft torque motors. For optical pointing applications, start with aerospace and optical gimbals. For RFQ support, contact [email protected].
Send the required clear inside diameter and explain what must pass through it, such as optics, cables, slip rings, bearings, tooling, or encoder hardware. The ID requirement changes the electromagnetic and mechanical design direction.
A larger ID leaves less radial space for copper, magnets, and iron. To recover torque, the design may need more OD, more stack length, better cooling, a different winding, or a lower torque target.
Gimbal RFQs should define pointing speed, holding torque, disturbance torque, torque ripple or vibration target, cable or optical pass-through, environmental assumptions, and whether the motion is hold, scan, step-and-hold, or continuous.
The rotor and stator are separate magnetic parts. Air gap tolerance, rotor axial retention, magnet handling, lead wire strain relief, housing concentricity, and post-assembly electrical checks should be planned before sample build.
It is usually better when the application needs low backlash, smooth direct-drive motion, a clear center aperture, and compact integration. A geared axis may still be better when cost, holding load, or torque multiplication matters more than backlash and aperture.
Frameless torque motor sourcing and application engineering. 10+ years in industrial motion control supply chain between China and global OEM markets.
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