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.

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    $\begingroup$ This seems like several questions. Why not break each out into its own question? It would be challenging for people to find just one of the four specific answers in the future, by searching alone. $\endgroup$ – wwarriner Apr 18 '16 at 17:21
  • $\begingroup$ @starrise I understand your concerns.I am new to this and will keep it in mind onwards. $\endgroup$ – katipra Apr 18 '16 at 17:26
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    $\begingroup$ @katipra - StackExchange encourages editing your questions to make them better. If other community members provide constructive feedback, you are expected to edit your question to add those requested details or changes. I have fixed some of the formatting issues, and I removed the 4th question as it is unrelated to the first three. Normally, SE Q&A is one question per post, but your first three questions are related to each other and should still be answerable in a reasonably sized answer. $\endgroup$ – user16 Apr 18 '16 at 17:45

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.

  • $\begingroup$ All real joints are more or less flexible. So each real link is a spring of sorts. But this is the realm of flexible multibodies which is different from rigid multibodies where a link is indeed rigid. In Kinematics links are assumed to be rigid since there are no dynamics in kinematics it would be pointless unless you are modelling a very slowly moving system. $\endgroup$ – joojaa Feb 20 '17 at 16:12
  • $\begingroup$ .@Edwardo9 The book "amazon.com/Fundamentals-Kinematics-Dynamics-Machines-Mechanisms/…" mentions in a peculiar statement that IC engines are neither mechanisms nor machines but does not give any explanation for it.Could it really be true that IC engines are neither? $\endgroup$ – katipra Feb 21 '17 at 18:50
  • $\begingroup$ @katipra Interesting. Would you be able to quote what exactly it is mentioned in that book? Also, the link unfortunately says page cannot be found :/ $\endgroup$ – Edwardo9 Feb 22 '17 at 12:42
  • $\begingroup$ @katipra I answered to your comment as an answer since it did not fit in the comment section! $\endgroup$ – Edwardo9 Feb 22 '17 at 17:39
  • $\begingroup$ @Edwardo9 See section 1.3 books.google.co.in/… $\endgroup$ – katipra Feb 24 '17 at 17:33

@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."


  • $\begingroup$ Dictionary definitions aren't necessarily the same as technical definitions. It's good to remember that we now live in a world where some dictionaries are adding another definition of literally that means "figuratively". The common use of language isn't always proper technical use. $\endgroup$ – JMac Feb 22 '17 at 13:38
  • $\begingroup$ @JMac Agreed indeed. Although, there was a lot of different definitions for those terms and I only picked the ones according to the subject discussed. Therefore, I don't see why the dictionary would not be able to provide technical definitions, since you can filter the definitions needed to disregard the "figurative" ones. $\endgroup$ – Edwardo9 Feb 22 '17 at 14:04
  • $\begingroup$ It's still hard to tell if a given definition is a technical definition. Often in physics terms are chosen for specific reasons. In English the words get adapted and the technical definition may not be the same as the dictionary definition, I. E. That of a layperson who isn't familiar with the technical nomenclature. $\endgroup$ – JMac Feb 22 '17 at 17:13

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