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Minimum Constraint Design

February 4, 2010

by Lawrence Kamm

MinCD, as opposed to RedCD (Redundant Constraint Design), or, meeting in the middle, semi-MinCD. This is a pretty entertaining piece with plenty of food for thought.

“The essence of minimum constraint design is not in limiting the number of constraints to the minimum number required for a rigid body but rather in eliminating overconstraints.” (p. 34)

“Any position or uniform motion of any rigid body can be resolved into exactly six component axes, three linear and three rotary.” (p. 8): Linear in the X, Y and Z axes, and then rotary about those axes, best described as “pitch” (front-to-back rotation), “roll” (side-to-side) and “yaw” (around the vertical axis, or ability to hold a straight course). The six components are “degrees of freedom”, and anything that limits a freedom is a “constraint”.

His tripod example for attaching two parts in 2.1 is ingenious, and it is a solution he returns to in many designs: one leg, resting in a conical hole, constrains the object to a fixed point in space about which it may rotate, the second leg, resting in a v-groove, halts this rotation, and the third leg, resting on the top of the lower part prevents vertical rotation. Without the seating force of the spring it could be easily overturned in the opposite direction. “Among the examples of seating forces are gravity, centrifugal force, fastener force, air or hydraulic pressure, and magnetic or electro-magnetic force.”

He goes onto some real-world examples, and my favorite has to be the comparison between the Danish and English three-legged chairs, where the Danish example is much more stable with the pair of legs at the back balanced by the sitter’s legs at the front. The English example takes no advantage of the “human servomechanism”, and can easily “tip over sideways when leaning back” (p. 20). Props to the Danes, inventors of Lego.

His concluding considerations for MinCD design with flexible bodies carry these principles into larger more complex systems, and his discussion of the “whiffletree” is very entertaining. It takes the distribution of a point load to the next logical level: linked point loads are like a spread footing in compression, but have the advantage that they can work in tension as well. In inverted compression they make an elegant diagram of the forces contained within a classical architectural capital.


From → Mechanisms

One Comment
  1. It sounds like you really digested this topic well. Kamm can be a little hard to get through at times but you hit on all the major points. Nice work.

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