The most commonly used joint in animation armatures is the sandwich-plate ball and socket joint. Sandwich-plate style ball and socket joints are the easiest to machine due the simplicity of their configuration. The sandwich-plate ball and socket joint consists of one or two bearings clamped between two plates. The plates are drilled to 'hug' the balls. The plates are held together by a tightening screw. The tighter the screw, the harder the joint will be to move. The balls rotate within the plates and create a rotating joint. Ball and socket joints are commonly used for hips, shoulders and necks.
Collet joints are another kind of ball joint. They are much more difficult to machine and are therefore used far less frequently. The ball, or bearing, rides inside the collet and swivels.
Collet joints are often used to replicate spinal columns as several collet joints can be assembled together to form a line of rotating ball joints. The flexibility of this strand of joints resembles and functions in a similar fashion as the human spine. Collet joints may also be used to fabricate tails or long necks. They also work as shoulder joints for four-legged creatures or as the primary structure in the torso of humanoids.
The step-block joint uses a rotating ball at one end and often features a swivel connection at the other. The joint is comprised of a main piece, called the step-block. The bearing rests in a hole in the step-block and a capture plate in placed on the other side of the ball. A tightening screw goes through the capture plate and into the step-block, thus holding the ball in place. This part works similar to a sandwich-plate ball and socket joint.
At the other end of the step-block is a swivel joint. This consists of a smaller block that connects to the step-block with a pin, allowing the two blocks to rotate separately.
This joint works well for hips, shoulders, elbows, spines or feet.
The hinge joint is the oldest joint used in animation. The hinge joint moves linearly in one direction, either vertically or horizontally. The main advantage of the hinge joint is that, because it only moves in one direction, it moves exactly were you want it to with very little difficulty. There is no "play" in the motion and the joint moves perfectly along its axis. The disadvantage is that this limited mobility has few practical applications when building the armature of an organic creature. There are very few joints in any living creature that move strictly along a single axis.
Hinge joints are good, however, for elbows and knees. They are also used for animating robots, whose movements are intended to appear more mechanical in nature.
The universal joint is a modified version of a hinge joint. This joint is built by assembling two rods that intersect at a 90 degree angle. By using two bars that work off of swivel joints, all axis of motion can be achieved within a single joint. All motion in a universal joint occurs from one point, thus offering a unique and versatile range of motion.
Universal joints are frequently used where the puppet has a large bulky mass and requires additional support.