It is after all a simple ballandsocketjoint; it has certain loads imposed on it; it needs reliability over a defined life; it must contain materials suitable for the working environment.
Ballandsocketjoint at shoulder allows movement in 3 planes.
球窝关节—在肩膊容许三平面的运动。
3
They made joints in stone keeping the lintels in place horizontally using a tongue and groove jointand subsequently fixing the upright and lintel with a ballandsocketjoint or mortice and tenon.