Editor's Note: For all motors utilizing non-contact commutation technology (such as Brushless DC motors and Permanent Magnet Synchronous Motors), their service life is primarily constrained by the reliability of the bearing system that supports rotation (e.g., oil-impregnated bearings, ball bearings, fluid dynamic bearings).
What is URD? Background and Origin
In DLP projector applications, motors are required to operate reliably long-term under the demanding conditions of confined space + high-temperature environment (85°C or higher) + sustained high-speed rotation (7200~14400 rpm). Traditional Ball Bearings face three critical shortcomings:
- High-Temperature Longevity Issue: The rolling contact between balls and raceways is metal-on-metal. Grease deteriorates rapidly at high temperatures, and NRRO (Non-Repeatable Run-Out) worsens with wear, leading to unreliable lifespan.
- High-Speed Quiet Operation Issue: Ball dimensions cannot be perfectly uniform, causing periodic impact noise and vibration during rotation, which becomes increasingly intolerable as image quality demands rise.
- Engineering and Maintenance: Ball bearings are susceptible to damage from dropping and impact, making handling management difficult on production lines and increasing the cost of motor traceability management.
URD (Ultra Reliable Dynamic Bearing) = Nidec's self-developed oil-film/fluid dynamic sliding bearing solution. It replaces ball bearings with an oil film, fundamentally solving the above problems. In essence, it belongs to the FDB (Fluid Dynamic Bearing) family. It represents Nidec's transplantation and specialized optimization of the mature FDB technology from HDD spindle motors for the DLP color wheel motor application.
Core Operating Principle
When the shaft rotates, the oil in the microscopic gap between the shaft and the bearing sleeve generates "dynamic pressure" due to shear flow, lifting the shaft to form an oil film with a thickness on the order of a human hair's diameter. The shaft and metal do not make direct contact; instead, the system relies on the oil film for load support + oil film damping for vibration absorption.
This enables three things ball bearings cannot achieve: no metal-on-metal contact wear, inherent damping for noise reduction, and greater tolerance for minor eccentricity.
Four Key Technical Measures in URD's Structural Design
The four targeted designs implemented in URD to achieve "long life under high-temperature conditions":
- Sintered Oil-Impregnated Material + Low-Evaporation Specialized Lubricant: Ordinary bearing oil evaporates rapidly at high temperatures (85°C+) → oil film thins → dry friction → seizure. URD selects a low-evaporation-rate oil formulation to slow oil depletion at the source.
- Increased Oil Volume + Setting an Oil Tank (Reservoir/Baffle Structure): Increases the initial oil reserve and incorporates an oil tank / baffle in the structure to prevent oil leakage to the outside. The life criterion is defined as: when the oil content rate falls below 10%, the friction coefficient rises sharply and lubrication fails (refer to the oil consumption curve model on Document Page 20).
- Oil Groove + Taper Structure: Oil grooves and tapered surfaces are machined into the rotating body/sleeve. This guides oil circulation and distribution while suppressing oil scattering and external outflow—serving as a second line of defense beyond sealing.
- High-Sealing Encapsulation: Enhances the sealing at the junction between the sleeve and housing, the sleeve retention structure, and the bonding/seal areas to prevent slow oil seepage from assembly gaps.
The logical chain of these four measures: Good oil → More stored oil → Prevent oil from escaping → Tight sealing = Persistent oil film = Extended lifespan.
Advantages of URD Bearings
4.1 Noise: The Most Directly Perceivable Advantage
Test Conditions: 7200 rpm, no load, anechoic chamber measurement, microphone 4cm above the motor.
| Sample | Ball Bearing Noise [dB] | URD Noise [dB] |
|---|---|---|
| 1 | 36.7 | 33.6 |
| 2 | 38.6 | 29.4 |
| 3 | 37.4 | 34.0 |
| … | … | … |
| MAX | 40.3 | 36.5 |
| AVE | 38.11 | 34.03 |
- URD is approximately 4 dB quieter on average, and the peak noise is also reduced from 40.3 to 36.5.
- FFT waveform analysis shows: Ball Bearing noise has discrete peaks at the ball pass frequency (audibly perceived as a "gritty" sound), while the URD spectrum is smoother—perceptibly quieter.
- Time-based comparison: In high-temperature life tests, Ball Bearing noise continuously worsens over time (ball wear → increased clearance → rising noise), whereas URD noise changes very little over time, only spiking near end-of-life.
4.2 Comprehensive Performance Matrix Comparison
| Evaluation Dimension | Ball Bearing | URD | ADB (Pure Hydrodynamic/Full Oil Film) |
|---|---|---|---|
| Rotational Speed Capability | ★ (~×2 Poor) | ★★ (×2~×4) | ★★★ (×3~) |
| Noise | ★ | ★★ | ★★ |
| No-Load Current | ★★ | ★★ | ★★★ |
| Load Capacity (CW weight/unbalance) | ★★★ | ★★ | ★ |
| Cost & Producibility | ★ | ★★ | ★ |
| Continuous Life (High Temp.) | ★ | ★★ | ★★★ |
| Start/Stop Life | ★★ | ★★ | ★ |
| Temp./Humidity/Shock Tolerance | Generally Weak | Overall ★★~★★★ | Humidity ★ |
Interpretation:
- Ball Bearing's sole absolute advantage is "ultimate load capacity" (due to the metal-on-metal contact area), but this is not the bottleneck in DLP color wheel applications characterized by light load, high speed, and continuous operation.
- ADB offers the longest ultimate life but is weak against Start/Stop shock, sensitive to humidity, high-cost, and has poor production flexibility.
- URD's positioning = The optimal cost-performance sweet spot: Life is far superior to Ball, cost/producibility is far better than ADB, load capacity is sufficient, and it performs stably at high temperatures.
4.3 Regarding Service Life
Life Definition: Oil content rate drops below 10% → Lubrication film breaks down → Friction coefficient increases sharply → Life ends.
By tracking oil consumption/evaporation to fit a curve, life can be predicted at the design stage:
| Model | Test Conditions | Predicted Life |
|---|---|---|
| 17S-URD | 85°C / 10800 rpm / Horizontal / With dummy disc | ≥ 28,600 h (MTTF nominal: 20,000 h) |
| 17S-URD-DP (Dual-Plane Balance Version) | 85°C / 14400 rpm / 4X | MTTF 30,000 h, with current reduced to 200mA |
| 28S-URD (Large) | 85°C / 10800 rpm / 32,000 h continuous operation | MTTF ≈ 54,000 h |
Temperature Rule of Thumb: Life is shortened by approximately 1.5 times per 10°C increase (not the classic 2x, based on actual measurements). Therefore, reducing motor self-heating + controlling the ambient temperature on the customer's device side are the most effective levers for extending life.
URD vs. Ball Bearing Comparison
| Ball Bearing | URD | |
|---|---|---|
| Friction Mechanism | Rolling + sliding of balls, metal contact is inevitable | Oil film completely separates shaft and sleeve, no metal contact |
| Wear Source | Ball/raceway fatigue spalling, grease drying out | Only oil evaporation/leakage (no solid part wear) |
| Vibration/Noise | NRRO + ball pass frequency → Discrete peaks | Oil film damping → Smooth spectrum, lower noise floor |
| High-Temp. Performance | Grease degradation accelerates → Life collapses rapidly | Low-evaporation oil + sealing → Predictable slow oil consumption |
| Cost | Requires precision balls + sealing components | Sintered sleeve + oil + structural sealing → More suitable for mass production |
Why is it called "Ultra Reliable"?
- Quantifiable Reliability: Lifespan is not guessed but based on a physical model of oil consumption. Life can be predicted at the design stage using oil content rate curves.
- Operational Robustness: 85°C continuous operation + color wheel imbalance load + start/stop shock. URD performs reliably without lagging behind in all dimensions—temperature, humidity, vibration, and shock.
- Production Track Record: The URD series helped Nidec capture the No.1 world market share in DLP motors.
Nidec Motor Series Using URD Bearings
- 10S Series Motors ：10S220L270、10S22Q100、10S220H280
- 17S Series Motors ：17S220H270、17S220E120、17S2203070、17S220F060
- 28S Series Motors：28S2203080