A shaft coupling is a mechanical connector that joins two rotating shafts so torque can pass from a motor to a driven part. In robotics, couplings are used between motors, gearboxes, wheels, lead screws, encoders, and joints. They matter because even small alignment errors can create extra friction, vibration, bearing wear, and position error.
Choosing the right coupling helps a robot move smoothly, accurately, and reliably.
Key Facts
- Torque transmitted by a shaft is τ = F r, where F is tangential force and r is shaft radius.
- Mechanical power in rotation is P = τω, where τ is torque and ω is angular speed in rad/s.
- Rigid couplings work best when shafts are precisely collinear and have very little misalignment.
- Flexible couplings can tolerate small angular, parallel, or axial misalignment while transmitting torque.
- Oldham couplings use a sliding center disk to handle parallel offset between shafts with nearly constant angular speed.
- Shaft speed conversion is ω = 2πN/60, where N is speed in revolutions per minute.
Vocabulary
- Shaft coupling
- A device that connects two rotating shafts so torque and motion can be transferred between them.
- Torque
- A twisting effect that causes rotation, equal to force times the perpendicular distance from the rotation axis.
- Misalignment
- A condition where two shafts are not perfectly lined up in position, angle, or axial spacing.
- Rigid coupling
- A coupling that holds two shafts in a fixed relationship and allows almost no relative motion between them.
- Oldham coupling
- A three-part coupling with a sliding center disk that transmits torque while allowing parallel shaft offset.
Common Mistakes to Avoid
- Using a rigid coupling on misaligned shafts is wrong because it can force the shafts and bearings to bend, increasing friction, heat, and wear.
- Ignoring the coupling torque rating is wrong because a coupling that is too small can slip, deform, crack, or fail during acceleration or shock loads.
- Confusing angular misalignment with parallel misalignment is wrong because different coupling designs handle these errors in different ways.
- Overtightening set screws without proper shaft flats or keys is wrong because the coupling may still slip under load and the shaft can be damaged.
Practice Questions
- 1 A robot motor delivers 0.80 N m of torque through a coupling to a wheel shaft spinning at 120 rad/s. What mechanical power is transmitted?
- 2 A coupling is rated for 2.5 N m. If a robot joint requires 1.6 N m during steady motion and experiences shock loads 1.8 times larger, is the coupling rating sufficient?
- 3 A motor shaft and a lead screw are parallel but their centerlines are offset by a small distance. Explain why an Oldham coupling may be a better choice than a rigid coupling.