Sprag and roller clutches are both types of one-way (overrunning) clutches used in automotive and industrial applications to allow rotation in one direction while locking in the other. In torque converters, they enable the stator to lock for torque multiplication during high-slip conditions and freewheel during low-slip cruising.
Sprag clutches, using wedge-shaped elements, have largely replaced rollers in modern designs due to superior performance, though rollers remain in cost-sensitive or simpler setups. Below is a detailed comparison based on key attributes.
| Aspect | Sprag Clutch | Roller Clutch (or Trapped Roller) |
| Design | Uses multiple barrel- or figure-8-shaped sprags (wedges) loaded by springs between smooth inner/outer races; no ramps needed. More complex machining. | Employs cylindrical rollers in a cage between races with asymmetric ramps/holes; springs push rollers up ramps for wedging. Simpler but requires ramped surfaces. |
| Operation/Engagement | Sprags tilt and self-wedge with minimal rotation (slight twist) for instant lock; freewheels by retracting under forward torque. Low drag in overrun. | Rollers roll freely in overrun; wedge by climbing ramps under reverse torque. Requires more rotation to engage fully, potentially causing slight delay. |
| Torque Capacity | Higher in given dimensions (e.g., up to 759,300 Nm in heavy-duty models); supports more elements per size for better load distribution. | Lower relative to size (e.g., up to 70,000 Nm in larger models); limited by fewer rollers and ramp design. |
| Size/Compactness | More compact radially; fits more elements in less space, ideal for tight automotive housings like torque converters. | Bulkier due to ramps; requires larger overall envelope. |
| Cost | Higher due to precision machining of sprags and springs; justified for high-performance needs. | Lower and more economical for high-volume or basic applications. |
| Durability/Wear Resistance | Superior; even load distribution reduces point loading and heat; resists vibration/transients with anti-rotation options (e.g., ARO sprags). | Moderate; higher contact pressures can cause wear, galling, or slippage at extremes; sensitive to contamination. |
| Speed Suitability | Excellent for high speeds (e.g., electric motors, highway cruising); minimal centrifugal issues and smooth engagement. | Good for medium speeds but may slip or overheat at very high RPMs due to roller dynamics. |
| Applications (Automotive Focus) | Common in modern torque converters (e.g., stator one-ways), automotive starters, differentials, high-HP transmissions (e.g., TH400 with 34-element sprags); also industrial backstops, conveyors. | Used in older torque converters, automotive starters, heavy-duty gearboxes, differentials; suited for oil-lubricated environments like barring drives. |
| Pros | Compact, high torque/space ratio, quick/precise engagement, vibration-resistant, efficient at high speeds. | Simple, cost-effective, easy to maintain, reliable in low-to-medium duty with good lubrication. |
| Cons | More expensive, complex (spring fatigue possible under extreme heat), harder to service. | Lower torque density, bulkier, potential for uneven wear or slippage under overload/high speeds. |
In automotive contexts like torque converters, sprag clutches are preferred for their robustness in high-torque launches, though roller designs persist in budget rebuilds. Selection depends on factors like space constraints and operating speeds—consult OEM specs for compatibility.
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