Choosing between a bare frameless motor kit and an all-in-one integrated joint module is one of the most consequential early decisions in robot design. The wrong choice can add months to your development timeline and tens of thousands of dollars in hidden engineering costs.
Key Takeaways (TL;DR)
- A frameless motor is a stator+rotor kit only —you must source and integrate the gearbox, encoders, housing, and driver separately.
- An integrated joint module (like ZHR-H or ZHR-P) bundles all components into a single bolt-on actuator with up to 36 Nm/kg torque density.
- DIY frameless assemblies typically require 200—00+ engineering hours per joint design. For most teams, this eliminates any cost savings.
- Integrated modules are the correct choice for prototypes, runs under 5,000 units/year, and any project prioritizing time-to-market.
- Frameless motors only make economic sense for very high-volume production (>5,000 units/year) with highly specialized form-factor requirements.
What is a Frameless Motor—/h2>
A frameless motor —sometimes called a "torque motor kit" or "motor cartridge" —is a brushless DC motor supplied as a bare stator and rotor without any housing, bearings, encoders, or shaft. It is designed to be embedded directly into a host mechanical structure.
Crucially, purchasing a frameless motor is not an end in itself. To create a functional robot joint, the engineer must also procure and integrate:
- A reduction gearbox (harmonic or planetary) —sourced separately
- An absolute output encoder —sourced and calibrated separately
- An input (motor-side) encoder —for current-loop commutation
- A custom CNC-machined aluminum housing or structural frame
- Output bearings and precision shims
- A motor driver / controller (e.g. ESCON, ODrive, or custom FOC board)
- Thermal management strategy (thermal paste, potting compound, or heat path design)
Empirical data shows: Engineering teams at university and startup robotics labs routinely underestimate frameless motor integration time by 3—×. A joint that appears simple on paper (motor + gearbox + encoder) often requires 6—2 months of iteration before reaching reliable performance.
What is an Integrated Joint Module—/h2>
An integrated joint module —also called an all-in-one actuator or smart actuator —is a complete, pre-assembled robotic joint unit. It integrates the frameless motor, gearbox, dual absolute encoders, housing, bearings, and often a motor driver into a single device with standard bolt-circle mounting flanges.
The ZHR-H Series (Harmonic) and ZHR-P Series (Planetary) are examples of integrated joint modules designed specifically for collaborative robots, humanoid robots, and exoskeletons.
Side-by-Side Comparison: Frameless Motor vs Integrated Module
| Criterion | Frameless Motor Kit | Integrated Joint Module (ZHR) |
|---|---|---|
| Unit Component Cost | Lower (motor only) | Higher (all-in-one) |
| Total Engineering Hours | 200—00+ hrs/joint | ~8—0 hrs integration |
| Time to First Prototype | 6—8 months | 2— weeks |
| Achievable Torque Density | Depends on designer skill | Up to 36 Nm/kg (ZHR-H) |
| Backlash Performance | Variable (assembly-dependent) | <20 arcsec (ZHR-H Harmonic) |
| Firmware / Driver | Develop from scratch | CAN/EtherCAT/RS485 ready |
| Warranty & Support | None (DIY assembly) | Full manufacturer warranty |
| Best For Volume | >5,000 units/year | 1 unit to 5,000+ units |
The Hidden Costs of Frameless Motor Integration
The bill-of-materials cost of a frameless motor looks attractive at first glance. However, engineering cost is the dominant expense in any robotic actuator project at volumes under 5,000 units per year.
1. Mechanical Design: Custom Housing Machining
A frameless motor has no housing. The robot's structural frame must be precision-machined to serve as the motor housing, with tight tolerances (<0.02mm) to ensure concentricity and axial alignment with the gearbox input shaft. A single CNC housing iteration typically costs $800—3,000 and 3— weeks of lead time.
2. Encoder Integration: Dual Absolute Encoders Required
High-performance force-controlled joints require two absolute encoders: one on the motor input shaft (for commutation) and one on the output (for closed-loop position control and compliance estimation). Sourcing, mounting, and calibrating magnetic encoders in a compact space typically requires 40—0 hours of embedded firmware development.
3. Motor Driver Firmware: FOC from Scratch
A bare frameless motor requires a Field-Oriented Control (FOC) motor driver. Implementing a production-quality current-loop FOC algorithm from scratch —with proper gate driver protection, thermal cutoffs, and CAN/EtherCAT communication —requires 3—2 months of embedded firmware engineering by a specialist, or using an off-the-shelf driver that adds weight and cost.
When Integrated Modules Win: The ZHR Advantage
For the vast majority of robotics projects and companies, integrated joint modules deliver superior total value. The ZHR-H and ZHR-P series eliminate all the integration challenges described above:
- Pre-aligned optical/magnetic dual encoders calibrated at the factory —zero encoder integration work required.
- Zero-backlash harmonic reducers tested to <20 arcsec in ZHR-H, or shock-rated planetary gearboxes tested to 300% peak overload in ZHR-P.
- Standard communication interfaces: CAN, RS485, and EtherCAT supported natively —plug-and-play with ROS 2 and most robot control frameworks.
- Thermal path optimized: The anodized aluminum housing doubles as a heat sink, delivering continuous operating margins that a bare DIY assembly cannot replicate without a thermal engineer.
Frequently Asked Questions
Q: What is a frameless motor in robotics—
A frameless motor is a brushless DC motor supplied as a stator and rotor kit without housing, bearings, or shaft. It must be integrated into a custom robotic structure and combined with separate gearbox, encoders, driver, and firmware.
Q: Which is cheaper: frameless motor or integrated module—
The component cost of a frameless motor is lower, but total cost of ownership (TCO) typically favors integrated modules for volumes under 5,000 units/year. Engineering a joint from a frameless motor typically requires 200—00+ hours of mechanical, firmware, and electrical engineering —costs that dwarf any component savings.
Q: When should I choose a frameless motor over an integrated module—
Choose a frameless motor when: (1) your form factor is so unique no standard module fits, (2) volume exceeds 5,000+ units/year with custom tooling amortized, or (3) your operating environment (e.g., underwater, radiation) requires fully custom sealing beyond any off-the-shelf module.
Q: What torque density can integrated joint modules achieve—
The ZHR-H Series integrated joint module achieves up to 36 Nm/kg peak torque density. This rivals the performance of custom-engineered frameless motor assemblies, but with zero integration engineering time required.
Not Sure Which Actuator is Right—
Use our interactive Product Selector or schedule a free 30-minute consultation with our engineers. We'll analyze your joint requirements and recommend the optimal integrated solution.