Re: pic: 1557 crab module
 

2 points define a line. In the same way, 2 bearings constrain a shaft. Adding a third point (bearing) over-constrains the line (shaft). It is inevitable that there will be some misalignment between these three bearings. If your tolerances are very good, and the misalignment is very small, the consequences are increased shaft stresses and friction. If your tolerances are not as good, the mechanism may bind and not function at all.

Another example of this problem is a wobbly pedestal style table or chair. 3 points define a plane. When you add a fourth point, such as a four legged table or chair, you are over-constraining the design. If the tolerances are good (and the surface the object is placed on is a smooth plane) the issue is unnoticeable. If the tolerances are not tight enough, or if the table is placed on a surface that is not a smooth plane, the table or chair will wobble between two different three-point planes. A rectangular table with legs on the corners is less likely to suffer from wobbling because a virtual degree of freedom is provided by the flexing of the table surface, allowing for four constraints to all be met.