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I have some questions in the basics of machinery terminology

  1. Can a spring ever be a link? Most books (eg. Ham et al. 58, Shigley pg. 5) consider links to be rigid. However, I have come across articles using springs as links.

  2. Are joints always the same thing as kinematic pairs, or do they ever differ in any particular circumstances?

  3. Is mechanism a subset of kinematic chain or is chain a subset of mechanism? Some books mention if there is no fixed link, then the combination cannot be called a mechanism but can be a kinematic chain.

Chris Mueller
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katipra
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2 Answers2

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Answering your first question: In the control field of mechanical engineering, a spring can indeed be a link. In the book Modern Control Engineering by Ogata, Katsuhiko the spring is frequently used as a link!

Answering your second question: "Reuleaux called the ideal connections between components that form a machine, "kinematic pairs". He distinguished between higher pairs which were said to have line contact between the two links and lower pairs that have area contact between the links. J. Phillips shows that there are many ways to construct pairs that do not fit this simple classification." Kinematics

Therefore, I would say in a broad sense they're called kinematic pairs but I think there might be a lot more different types of joints that might have different names. It really depends in which circumstance the subject is being discussed, but in Kinematics they are called indeed both names.

Answering your third question: "Rigid bodies ("links") connected by kinematic pairs ("joints") are known as kinematic chains. Mechanisms and robots are examples of kinematic chains. The degree of freedom of a kinematic chain is computed from the number of links and the number and type of joints using the mobility formula. This formula can also be used to enumerate the topologies of kinematic chains that have a given degree of freedom, which is known as type synthesis in machine design." Kinematics

With that being said, if there is no fixed link connected by kinematic pairs, then there are neither kinematic chains nor mechanisms. Mechanism is simply an example of kinematic chains, which makes mechanism a subset of kinematic chains.

Edwardo9
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@katipra I am adding this as another answer since it was too long for the comment section. But it's a reply to your aforementioned comment.

Good news. Based on the actual definitions of Merriam-Webster of machine vs. machinery:

"Machine: 1) an assemblage of parts that transmit forces, motion, and energy one to another in a predetermined manner. 2) an instrument (as a lever) designed to transmit or modify the application of power, force, or motion. 3) a mechanically, electrically, or electronically operated device for performing a task."

"Mechanism: 1) a process, technique, or system for achieving a result 2) mechanical operation or action."

Therefore, the best conclusion would be that machine would be the most suitable word for an IC engine.

Also, I have found a reliable link in NASA's database that mentions "When discussing engines, we must consider both the mechanical operation of the machine and the thermodynamic processes that enable the machine to produce useful work."

[https://www.grc.nasa.gov/WWW/K-12/airplane/icengine.html]

Edwardo9
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