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Recently I started to study about "Design of mechanical machinery". Since I am a person who is very sensitive about words I want to know the difference between these notions and their precise definitions:

  1. Linkage
  2. Kinematic chain
  3. Mechanism
  4. Mechanical element
  5. Mechanical machine

It will be very good if you introduce some source textbooks which treat these concepts very carefully and accurate as well.

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  • $\begingroup$ Welcome to Engineering! The latter part of your question looks like a resource-hunting question. Such questions are prone to becoming out-dated, and are therefore considered off-topic for this site. Simply asking for the difference between the terms, however, seems fine imo. $\endgroup$ – Wasabi Nov 26 '16 at 21:05
  • $\begingroup$ @Wasabi Thank you. Then can you tell me what's the difference between the terms? Also another question: Do you think I'm allowed to post such engineering questions on Physics Stack Exchange too? $\endgroup$ – Hamed Begloo Nov 27 '16 at 6:54
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    $\begingroup$ Never post the same question in two stack exchanges, it is very much frowned upon. Especially since this is certainly the right site for this question. $\endgroup$ – Wasabi Nov 27 '16 at 10:34
  • $\begingroup$ @HamedBegloo - Cross-posting the exact same question between different SE sites is strongly discouraged. That said, you can ask similar or related questions on different sites. So, you might ask a question about resolving a combination of forces on a beam at the physics site (even though it's still on-topic here) and then ask how a force relates to deflection on this site. But, like I mentioned, bulk copy/pasting to separate sites is discouraged and likely to result in the questions getting deleted if anyone notices. $\endgroup$ – Chuck Nov 28 '16 at 19:52
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Here are some definitions for you, based on my experience with these terms:

  1. A linkage is a group of bodies (or links) connected together by joints.

  2. A kinematic chain is a subset of linkages, specifically referring to linkages with rigid, ideal joints, and rigid links. Since the joints and links are rigid, this allows you to use geometry to relate the kinematics (position and velocity) of each link to every other link in the chain.

  3. A mechanism is synonymous with mechanical machine and refers to any collection of parts that can perform a mechanical action or transformation. For example, a gearbox is a mechanism that converts (transforms) an input torque and speed to a different output torque and speed without the addition of energy. The relationship between the input and output mechanical variables is known and quantifiable.

  4. A mechanical element is the smallest subdivision of a machine that is still useful for analysis (similar usage as in chemistry, if you consider what happens beyond the scale of atoms to be the domain of physics). For example, if you are considering the gearbox from the previous example, and are interested in the motion of each shaft and gear within the box, then the individual gears and shafts would be considered as mechanical elements. However, if you were analyzing the entire drive train of a car, it would be more convenient to consider the entire gearbox as a single mechanical element, ignoring the internal motion and focusing on its input-output characteristics as it interacts with the rest of the system.

Terminology like this can vary from place to place, so it is important to consider your audience. If you are writing an internal report or presentation for a group of engineers or in a course where the terminology is known to you, just use their terminology. Otherwise, it is important to include definitions for any terminology you might use.

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  • $\begingroup$ Thank you for your answer. I don't remember precisely but I think I read somewhere that a Mechanism is a Linkage/Kinematic chain with at least one grounded joint and a Mechanical machine is any group of connected mechanisms. Do you agree with these definitions too? $\endgroup$ – Hamed Begloo Nov 28 '16 at 18:22
  • $\begingroup$ @Hamed I suppose since a mechanism would need to be grounded with respect to some reference in order to do anything useful that definition also works. As for the mechanical machine, if you connect a mechanism to another mechanism, are they still separate mechanisms or do they form a single, larger mechanism? Can a machine be constructed from a single mechanism? In these cases it all depends on the terminology you want to use, and how much you need to divide your machine to describe it in a useful way $\endgroup$ – BarbalatsDilemma Nov 28 '16 at 18:29
  • $\begingroup$ Thank you for your helps. I voted your answer up but since I want to see others' opinions as well I prefer to make question to be remained open. Thank you again. $\endgroup$ – Hamed Begloo Nov 28 '16 at 18:33
  • $\begingroup$ @Hamed sure thing, it's a good idea to see if anyone else can give a better cited answer, since mine is based more on experience $\endgroup$ – BarbalatsDilemma Nov 28 '16 at 18:35
  • $\begingroup$ according to the definition i have seen a mechanical machine is one that has no degrees of freedom once attached to one engine. So for example a robot is not a machine in this terms. But many machines. Wereas a steam locomotive is a machine (adjustable controlls are still permitted) $\endgroup$ – joojaa Nov 28 '16 at 20:27
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Kinematic chain is an assembly of link in which the relative motion of the links is possible and the motion of each relative to the other is definite. A linkage is obtained if one of the link of kinematic chain is fixed to the ground. If motion of any of the movable links results in definite motions of the others,the linkage is known as mechanism.

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