A practical selection guide for engineers comparing frameless torque motors by torque-speed duty, OD/ID geometry, winding, cooling path, and RFQ readiness.
Choosing a frameless torque motor is not the same as choosing a catalog servo motor. You are selecting a rotor and stator kit that must become part of your mechanical structure, thermal path, encoder stack, and control loop.
For OEM buyers, the best early decision is to evaluate the motor as part of the whole axis. A motor that looks strong on a torque table can still fail if the housing cannot remove heat, the rotor cannot be clamped safely, or the drive voltage does not match the winding.
Here is the sequence I use when reviewing a buyer's first frameless motor inquiry. It keeps the discussion close to the real axis instead of drifting into catalog numbers that cannot survive prototype testing.
A practical order for turning machine requirements into a useful RFQ.
| Takeaway | Practical meaning |
|---|---|
| Select the axis, not only the motor | The housing, bearing, encoder, drive, and cooling path can change whether the motor data is realistic. |
| Continuous torque needs a thermal basis | Ask what housing, cooling condition, ambient temperature, and winding limit support the quoted value. |
| OD/ID changes are not minor | A larger bore can reduce active electromagnetic area and may require more OD, stack length, cooling, or a lower torque target. |
| Winding must match the drive | Bus voltage and current limit decide whether the motor can hit both torque and speed. |
| Sample goals should be staged | Fit, electrical, thermal, and full-axis validation are different sample objectives. Mixing them too early slows the program. |
Before you compare frameless torque motor suppliers, define these six items:
| Decision | Minimum input worth sending | What it prevents |
|---|---|---|
| Torque-speed duty | Continuous torque, peak torque, speed, and duty cycle | Oversizing from peak torque only |
| Geometry | OD, ID, stack length, axial envelope | Late redesign when rotor/stator do not fit |
| Electrical window | Bus voltage, current limit, drive model | Winding mismatch and speed shortfall |
| Thermal path | Housing material, cooling method, ambient temperature | Unrealistic continuous torque claims |
| Integration method | Stator retention, rotor clamping, lead exit, sensor | Unsafe or unstable sample assembly |
| Validation target | Fit check, torque test, thermal test, or production pilot | Samples that cannot be accepted internally |
If your team does not have all six, send the unknown values as "TBD" and explain what is fixed. That is still better than hiding uncertainty.
Score each row 0-2. A total below 8 usually means the supplier will need clarification before a reliable quote.
| Area | Evidence needed | 0 | 1 | 2 |
|---|---|---|---|---|
| Geometry | OD, ID, stack length, mounting envelope | Unknown | Partial | Ready |
| Duty profile | Continuous torque, peak torque, speed, duty cycle | Unknown | Partial | Ready |
| Electrical limits | Bus voltage, current limit, drive model | Unknown | Partial | Ready |
| Thermal path | Cooling method, housing material, ambient temperature | Unknown | Partial | Ready |
| Integration | Air gap, rotor clamping, sensor, lead wire exit | Unknown | Partial | Ready |
| Validation | Fit sample, electrical test, thermal test, pilot plan | Unknown | Partial | Ready |
Do not start from peak torque alone. The first sizing question is the load profile:
| Input | Why it matters |
|---|---|
| Continuous torque | Defines sustained thermal load. |
| Peak torque | Defines acceleration and transient margin. |
| Target speed | Affects back EMF, voltage headroom, and drive choice. |
| Duty cycle | Separates short bursts from continuous operation. |
| Cooling method | Determines whether the quoted torque is realistic in your housing. |
If the axis has multiple operating modes, separate them. For example: idle hold, normal motion, fast indexing, emergency stop, and calibration.
A better way to describe duty is:
| Mode | Torque | Speed | Duration | Repeat rate | Notes |
|---|---|---|---|---|---|
| Hold | Continuous holding torque | 0 rpm | Long duration | Constant | Usually thermal-limited |
| Move | Running torque | Target rpm | Seconds | Per cycle | Include acceleration and load inertia |
| Peak event | Peak torque | Low or changing speed | Milliseconds to seconds | Occasional | Defines overload margin |
| Index | Torque during acceleration and deceleration | Variable | Per move | Cycles per hour | Important for rotary tables |
The supplier does not need a perfect model at the first contact, but they do need enough information to avoid quoting a motor that only works in a short burst.
Some projects can start from an existing frameless torque motor family. Others need custom winding, geometry, wiring, or mechanical interface changes.
Use this decision table before RFQ:
| Situation | Better path |
|---|---|
| OD, ID, and stack length are flexible | Start with an existing product family |
| OD and ID are fixed by the machine housing | Ask for custom geometry review |
| Drive voltage/current is already fixed | Ask for winding selection support |
| Continuous torque is high in a sealed housing | Ask for thermal model or test basis |
| Cable, optics, or bearing must pass through the center | Evaluate hollow-shaft motor options |
| Existing supplier is too expensive or too slow | Ask for equivalent architecture review, not just copied part number |
For a first pass, review frameless torque motors. If the geometry is already constrained, compare custom OEM motor assemblies.
Frameless motors are attractive because they remove the external motor housing. That also means the customer housing becomes part of the motor system.
The most important geometry fields are:
If the ID is driven by a bearing, cable bundle, slip ring, reducer, or optical path, include that information in the first RFQ. A hollow-shaft requirement can change the electromagnetic design direction.
The tradeoff is simple: a larger ID can reduce electromagnetic area. To recover torque, the design may need more OD, more stack length, stronger magnets, different winding, better cooling, or a different mechanical layout.
This is why "similar to this motor, but with a bigger hole" is not a small request. The aperture is part of the motor design.
A winding choice is not just a line item. It connects torque constant, back EMF, resistance, inductance, speed, and drive current.
Useful RFQ fields include:
If your drive current is limited, a higher torque constant winding may help. If your speed is high, back EMF can become the limit. The motor supplier should review both sides together.
Common winding mistakes include:
| Mistake | What usually happens |
|---|---|
| Choosing high Kt without checking top speed | Motor cannot reach speed at available bus voltage |
| Choosing high speed winding without checking current | Drive overheats or cannot deliver required torque |
| Ignoring inductance | Current control becomes harder at the target switching frequency |
| Treating peak current as continuous current | Thermal estimate becomes unrealistic |
| Changing drive model after sample approval | Winding selection may no longer be optimal |
If your drive model is fixed, send the datasheet or the current/voltage limits. If your drive is not fixed, ask the supplier what electrical range would make the motor easier to quote.
A frameless motor usually transfers heat through the stator into the customer housing. Continuous torque is only meaningful when the thermal path is defined.
Ask the supplier to clarify:
For high duty applications, do not accept a torque number without asking how heat leaves the motor.
Ask for the thermal basis in plain language:
If the answer is unclear, treat the continuous torque value as a preliminary number. For production programs, sample validation should include temperature rise under your actual duty cycle.
The motor is not ready for sample approval until the integration method is clear.
Check these items before releasing a sample order:
For detailed installation topics, see the frameless motor installation guide.
The biggest assembly risks are not always visible on a datasheet. Exposed permanent magnets can pull tools or parts unexpectedly. Air gap error can reduce performance or cause contact. Rotor axial movement can damage the motor. Lead wires can fail if strain relief is ignored.
Before a paid sample, align on one of these sample goals:
| Sample goal | What to verify |
|---|---|
| Mechanical fit sample | OD/ID, stack, lead exit, mounting method, air gap access |
| Electrical validation sample | Winding, resistance, inductance, back EMF, drive compatibility |
| Thermal sample | Continuous duty in customer housing |
| Full axis prototype | Motion control, encoder, load, thermal, and assembly process |
Trying to validate all four with an unstable drawing set usually causes delays.
Many buyers ask for CAD first. In practice, CAD is only useful when the intended configuration is clear.
A productive first request is:
If the motor is custom, use the CAD file as a controlled engineering artifact tied to a model, revision, and project name.
Use this request format:
Please share the drawing package for project [project name], motor family [candidate family], revision [drawing revision], with OD [value], ID [value], stack length [value], lead exit [position], and CAD format [STEP/IGES].
That is much better than "send CAD," because it keeps the mechanical package tied to the quoted configuration.
Good supplier communication is not only about price. Ask questions that reveal whether the supplier understands frameless motor integration:
| Question | What the answer should include |
|---|---|
| What thermal condition supports the continuous torque? | Housing/cooling assumption and temperature basis |
| What can be customized? | Winding, stack, OD/ID, lead wire, sensor, potting, interface |
| How should the rotor be installed? | Clamping, fixture, magnet safety, air gap notes |
| What tests can be provided? | Back EMF, resistance, insulation, hi-pot, torque, thermal, outgoing record |
| When can CAD be released? | After family/configuration scope is clear |
| What data is missing from our RFQ? | A specific clarification list |
If the reply is only a price and a lead time, treat it as a rough commercial signal, not an engineering quote.
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: Frameless torque motor RFQ for [application/module]
Hello framelesstorquemotor.com engineering team,
We are evaluating a frameless torque motor for [machine/module].
[robot joint / gimbal / rotary axis / medical device / automation axis][values or TBD][values][values][values][values][values][values][values]Please advise suitable model families, customization options, lead time, and what data is still missing before a quote can be trusted.
Send these details to reduce quote loops:
You can also use the custom frameless motor RFQ checklist.
A catalog-like frameless motor is a good starting point when the geometry, speed, and torque target are close to an existing family. Custom engineering becomes valuable when you need a specific OD/ID ratio, winding, stack length, lead wire exit, sensor interface, or mounting geometry.
For product-family review, start with frameless torque motors. For project-specific geometry, review custom OEM motor assemblies or contact the engineering team at [email protected].
Send the application, OD, ID, stack length, continuous torque, peak torque, target speed, duty cycle, bus voltage, current limit, cooling method, sensor plan, sample quantity, and annual forecast. If some values are unknown, mark them as TBD and explain which dimensions are fixed.
Continuous torque depends on winding temperature, cooling condition, housing contact, duty cycle, and mounting method. A motor tested in a good aluminum heat path can show a different continuous torque than the same motor in a sealed plastic or poorly conducting housing.
Custom review is worth it when OD, ID, stack length, lead exit, winding, sensor interface, air gap, or mounting geometry is constrained by the machine. If geometry and electrical limits are flexible, an existing product family is usually a faster starting point.
No. Peak torque only describes short overload capability. Selection also needs continuous torque, speed, duty cycle, thermal path, drive current, voltage headroom, and mechanical integration method.
Preliminary drawings can be shared after the candidate family and envelope are clear. For custom programs, CAD should be tied to a project name, revision, OD, ID, stack length, lead exit, and selected winding direction.
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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