Basic Machines 2: Joints and Linkages
There are endless ways to join structural elements together to create movement, and each unique system of connections will give you a different type of motion. There are three types of joints that are most common, however: rotating joints, sliding joints, and bending joints. Each type of joint will have its own advantages and limitations, and its own weak points and points of greatest friction.
Bending joints, like a hinge, will give you limited motion in one axis. A hinge will be able to move at most 180 degrees, most likely less depending on where your two structural pieces are hinged together. The rigidity of a hinge joint is useful when you want to limit motion between two pieces to one plane.
Rotating joints are joints where circular structural elements rotate about a common axis. Think of a small pipe inside a slightly larger collar and you've got the picture. In a rotating joint, there is friction between the roatating shaft and the collar, or bushing, in which it's rotating. This can be managed by using some form of oil or grease lubricant, by using a high-density plastic bushing that has very little friction, by using an oil-impregnated bushing, or by using a bearing or some sort. Bearings consist of some sort of metal chassis containing balls or cylinders that are free to roll within the chassis, so they provide friction relief by turning. With a rotating joint, you want to make sure the shaft and the bushing fit together snugly, with just enough room to turn. Too tight, and it won't turn. Too loose, and the shaft will shift off the ideal axis of rotation and bind up.
When dealing with any rotational motion, it's important to know where the system will bear the load, or weight. An axial load is a load parallel to the shaft, and a radial load is a load perpendicular to the shaft.
Rotating joints are perhaps the most common joints due to the flexibility they offer; for example, many bending joints, such as hinges, are made of a rotating joint with two radial loads. In addition, to get more than one axis of motion, a spherical rotating joint called a ball-and-socket joint (like your hip joint) is necessary. In a ball and socket joint, a rod with a spherical end (ball) is fitted into a cup (socket) at the end of a second rod. Lubricant is placed between the ball and socket to reduce friction.
Sliding joints are joints where one surface slides against another. Like rotating joints, care must be taken to reduce friction and ensure a snug fit to avoid finding because of a misfit. Ideally, the sliding section of s sliding joint should be at least twice as long as the width of the slot. This will minimize the chance of the piece binding up.
Stiff and flexible support members can be linked using various joints to create a variety of movement and support systems. Below are a few examples:
Scissor linkages combine several stiff supports in an X fashion, with rotating joints at the corners. A scissor can retract and extend a great distance with reasonable support.
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