Let’s not beat around the bush, almost all modern mechas have been designed to be toys or at least toys in mind. Robot toys are nothing new in itself, but the modern level of robot toy design can be attributed to Takara and Bandai, mostly because they wanted their toys to sell. While you may not like Transformers or Gundams all that much, both of them are good examples how even a simple joint can be effective in a toy when done right.
If you hit google or library and look for mechanical joints, you’ll get a large amount of referential materials how joints are designed and for what purpose. Pivots are another essential piece when it comes to joints you to use, so add it to your list of things to look up. At its core, how your mecha can move is all about joints and pivots. Outside the reality how much stress they can take, what’s powering them and all the stuff you’d need to take into account in real life, in fictional robots it’s enough to just have them seemingly work in a plausible manner.
We’ll go over two basic types of joints this time and their limits and possible uses, there’s no real use to get into more elaborate designs now. Joints’ themselves are not just limited to their own directions, they are naturally affected whatever design your mecha has, and it’s not uncommon to see e.g. a Gundam’s movement limited by its armour design. Gundam Unicorn is a good example of a design that can’t make the best of its knee-joint due to large portions in the back of the leg. Similarly, TSFs’ design in itself limits their motion at times.
The first is a basic joint, a hinge joint, henceforth called a single joint.
The good sides on this joint is that it’s simple and easy to do in many ways. It doesn’t require anything fancy to work just fine, but it’s limited by the surrounding design to a fault. At its simplest, it allows 180-degree motion if there is nothing to stand in its way. If you have elbow guard in place that doesn’t slide down or is not positioned at the side, you may lose half of the possible movement from the joint. Be it elbow or knee, the same principle applies nonetheless. Playing with the shapes around the joint can net you a large field of motion, but usually this means sacrificing something in the design, but maybe you want to go for that more unique look with your piece.
Maybe the most used joint in modern Gunpla is the double hinge joint, or just double joint, essentially a piece that uses two points of connection instead of just one. While the single joint design is robust and can usually withstand a lot of abuse from the user, the double joint is more about freedom of movement without sacrificing the design.
The application of these joints in robots are most often found in shoulders, elbows, pelvis and knees. The legs and arms are often combined form of having a single joint combined with a pivot to the torso. In principle, this should give an arm 360-degree freedom of pivoting in a full half-sphere motion.
Neither joint is really better of the other, despite some arguing some. Both have their applications and core uses. For example, the pelvis joint that connects to the leg very rarely is a double joint for the simple fact that a single joint with a pivot works far better than a double joint.
As mentioned, the armour design you apply limits how well the joints move. However, it should also be kept in mind how large, robust and strong the joints should be in what part. An elbow joint does not have to take the same amount of weight and stress than the joints in the legs, which need to carry the whole robot’s weight. Of course, you can get rid of the legs and replace them with tank treads or something else.