Definition
A mechanical coupling that connects two shafts which are not in line with each other, allowing rotational motion to be transmitted through a varying angle. It typically consists of two yokes joined by a cross-shaped center piece (a spider or cross), permitting the driving shaft to turn the driven shaft even when the two are not aligned.
Plain English
A flexible joint that lets one spinning shaft turn another spinning shaft, even when the two shafts meet at an angle instead of running in a straight line.
Context Anchor
Seen in aircraft maintenance when describing drive connections, control linkages, or tool drives that must carry rotation through an angle.
Derivation
Called 'universal' because it can transmit motion in any direction the connected shafts happen to point, rather than being limited to a single fixed alignment.
Why Pilots Care
Universal joints appear in flight control linkages, trim systems, and accessory drives. Wear or play in a universal joint can cause sloppy control feel or uneven operation, which is why they're inspected during maintenance.
Analogy
Like the joint in a socket wrench extension that lets you turn a bolt around a corner -- the handle and the socket aren't lined up, but turning one still turns the other.
Intuition Check
Universal does not mean the joint can be used for every possible job. Here it means the joint can carry turning motion between shafts that meet at an angle.
Example Sentence 1
The mechanic checked the universal joint in the control linkage for any free play before signing off the inspection.
Example Sentence 2
The rigger replaced the worn universal joint so the aileron pushrod could transmit full travel through its angled path.