Flexible couplings and universal joints explained for mechanical power transmission, covering misalignment types, angular velocity variation, and practical engineering limits.
Flexible Couplings.—Shafts that are out of alignment (misalignment) either laterally or angularly can be connected using various designs of flexible couplings. These couplings also allow a limited amount of axial movement in one or both shafts.
Some flexible couplings transmit torque through disks or diaphragms. A simpler design consists of two flanges connected by links or endless belts made from leather or other strong, pliable materials. In other designs, the flanges contain projections that engage molded rubber or elastomeric elements to accommodate uneven motion between shafts.
More advanced flexible couplings use toothed flanges that mesh with correspondingly toothed elements, allowing relative movement while maintaining torque transmission. Such couplings generally require lubrication unless one or more components are made from self-lubricating materials.
Other coupling types use metallic diaphragms or bellows that flex elastically to accommodate angular, axial, and parallel misalignment without lubrication.
The Universal Joint.—Originally known as the Cardan or Hooke’s joint, this coupling connects two shafts whose axes intersect at an angle. Numerous designs exist, all based on the same fundamental operating principle.
As a general rule, a universal joint does not perform satisfactorily if the shaft angle α exceeds about 45°. In most power transmission applications, the angle should preferably be limited to approximately 20°–25°, except in cases of low rotational speed and low transmitted power.
The angular velocity of the driven shaft is not constant throughout a single revolution when a universal joint is used. Even if the driving shaft rotates at a uniform speed, the driven shaft undergoes periodic acceleration and deceleration. Therefore, universal joints should not be used in applications where uniform motion of the driven shaft is essential.
Determining Maximum and Minimum Velocities
If shaft A rotates at a constant speed, shaft B reaches its maximum speed when shaft A occupies the aligned position shown in the illustration. The minimum speed occurs when the fork of shaft A has rotated 90° from that position.
The maximum angular velocity of the driven shaft is obtained by multiplying the driving shaft speed by the secant of angle α. The minimum angular velocity equals the driving shaft speed multiplied by the cosine of angle α.
Example: If the driving shaft rotates at 100 RPM and the shaft angle is 25°:
- Maximum speed = 100 × sec 25° ≈ 110.34 RPM
- Minimum speed = 100 × cos 25° ≈ 90.63 RPM
- Total speed variation = 19.71 RPM
Some technical content adapted from Wikipedia.org
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