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When two rotating shafts can't be perfectly aligned — whether in an RC boat drivetrain, a small CNC motor mount, or a DIY automation rig — a rigid connector transmits vibration, accelerates wear, and eventually fails. These universal flexible shaft couplings are designed for hobbyists, makers, and light-industrial builders who need a compact, metal U-joint connector that bridges mismatched or offset shaft ends without sacrificing torque transmission.
Each coupling is a metal universal joint (U-joint) style connector machined to accept two shaft diameters simultaneously, covering input-to-output combinations from 2 mm to 10 mm (~0.08 in to ~0.39 in). The all-metal construction provides rigidity under load while the universal joint geometry allows angular misalignment between the two connected shafts. Available in 23 size combinations and pack quantities of 1, 2, 5, 10, or 20 pieces per order.
Typical applications include RC boat propeller shaft-to-motor connections where the motor and prop shaft are offset, stepper motor to lead screw adapters in 3D printers and CNC routers, small servo linkages in robotics and automation, and replacement U-joint connectors in hobby-grade power transmission systems. The wide size matrix — including same-diameter pairs like 4-4mm and 8-8mm, and cross-diameter pairs like 6-8mm and 8-10mm — makes these couplings a practical stock item for any workshop.
Universal joint geometry allows angular misalignment between input and output shafts, absorbing minor offset without binding — essential for RC boat drivetrains and compact motor mounts.
Covers shaft diameters from 2 mm to 10 mm in both same-diameter and cross-diameter pairings, so you can match mismatched motor and driven shaft sizes in a single connector.
Order 1, 2, 5, 10, or 20 pieces per SKU — ideal for single repairs, spare-parts kits, or stocking a workshop with matched couplings across multiple builds.
Compact form factor and metal construction make these couplings a direct fit for RC boat propeller shaft connections, where space is tight and the motor-to-prop alignment is rarely perfect.
Pairs like 3-5mm, 6-8mm, and 8-10mm let you connect a motor shaft and driven shaft of different diameters without a separate adapter — reducing part count and assembly complexity.
The U-joint design transmits rotational torque continuously while accommodating angular misalignment, reducing vibration transfer to bearings and frames in sensitive builds.
| Construction | All-metal universal joint (U-joint) |
|---|---|
| Shaft Diameter Range | 2 mm – 10 mm (0.08 in – 0.39 in) per end |
| Size Combinations Available | 23 input–output diameter pairings |
| Same-Diameter Options | 2-2mm, 2.3-2.3mm, 3-3mm, 3.17-3.17mm, 4-4mm, 5-5mm, 6-6mm, 8-8mm, 10-10mm |
| Cross-Diameter Options | 2-2.3mm, 2.3-3mm, 2.3-4mm, 3-2mm, 3-3.17mm, 3-4mm, 3-5mm, 3.17-4mm, 4-5mm, 5-8mm, 6-4mm, 6-5mm, 6-8mm, 8-10mm |
| Pack Quantity Options | 1 / 2 / 5 / 10 / 20 pcs |
| Overall Length | See variant options |
| Torque Capacity | See variant options |
| Max RPM | See variant options |
| Weight (per piece) | See variant options |
The size label shows the input shaft diameter first and the output shaft diameter second, separated by a dash. So "6-8mm" means the coupling accepts a 6 mm shaft on one end and an 8 mm shaft on the other. If both your shafts are the same diameter, choose a same-diameter option like "6-6mm" or "8-8mm".
A U-joint (universal joint) coupling uses a cross-shaped yoke mechanism to transmit torque through an angle, making it well-suited for applications where the two shafts are not collinear. A flexible beam coupling uses a helical-cut metal body that flexes to absorb misalignment. U-joints handle larger angular offsets and are commonly used in RC boat drivetrains and automotive-style power transmission, while beam couplings are more common in precision CNC and encoder applications.
Yes. These all-metal U-joint couplings are designed for hobby-grade RC boat propeller shaft connections, where the motor and prop shaft are often offset and the coupling must transmit torque reliably at moderate RPM. For high-performance racing RC boats with very high RPM or shock loads, verify the specific torque and RPM rating of the size you select against your motor's output specifications.
The couplings are all-metal construction. If used in a wet or marine environment (such as an RC boat), apply a light coat of waterproof grease or corrosion-inhibiting oil to the joint mechanism after installation and periodically during use. This will extend service life and maintain smooth articulation.
Yes, these U-joint couplings can connect a stepper motor shaft to a lead screw when there is angular or parallel misalignment between the two. Select the size that matches your motor shaft diameter on one end and your lead screw diameter on the other — for example, "5-8mm" for a 5 mm stepper shaft to an 8 mm lead screw. Note that for precision CNC applications requiring zero backlash, a rigid or zero-backlash coupling may be more appropriate.
U-joint couplings of this type typically use set screws (grub screws) to clamp onto each shaft end. Tighten the set screw against a flat on the shaft (if present) or directly onto the round shaft for a secure grip. If your shaft does not have a flat, consider filing a small flat to prevent the coupling from rotating under load. Refer to the coupling's bore and set screw size when selecting a compatible hex key.
Yes. When you select a pack of 2, 5, 10, or 20 pcs, all pieces in the pack are identical — same input and output shaft diameter combination. Multi-packs are priced at a lower per-unit cost than single-piece orders, making them economical for workshop stock or multi-machine builds.
