Tip: Directional Terms For Polar/Cylindrical Coordinates
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  1. #1
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    Default Tip: Directional Terms For Polar/Cylindrical Coordinates

    I have seen several instances where different people have used improper terms for the direction of forces or stresses in the past week or two. There is apparently some confusion over terms like "radial", "axial", and "tangential". These are three terms that can be applied to cylindrical objects, like bearings, shafts, gears, pulleys, and many other parts that are typically produced on a lathe by turning. Therefore I have produced this drawing to help show how these three terms are properly applied.



    As the drawing shows, the word "axial" is the proper way to describe the direction that is parallel to an axis (of a cylinder). Therefore a thrust bearing is a way of controlling axial movement or force.

    The word "radial" refers to the direction along a radius of a circle or sometimes, by extension, in a direction that is at a right angle to some central axis. Therefore a standard ball bearing is primarily designed to oppose radial movement.

    The word "tangential" is used to describe a direction parallel to a circle at a point on that circle or some other curve. Therefore, when we are turning something in our lathes that part is experiencing a tangential force at the point where the tool is in contact with it. Or a pulley experiences tangential force where the belt contacts it.

    When we are describing forces or directions in machines, it is important to use the correct terminology to prevent confusion. I hope this is of some help.

    Additional note: The terms "radial" and "axial" are also used in describing some electronic components. The standard resistors with wire leads that are cylindrical in shape with the wires attached to the centers of the two ends are called an axial style component. This applies to capacitors, diodes, fuses, and other components that are made in this general form factor. It can also apply to components of other shapes, such as rectangular, if the wires come out from the centers of two opposite faces.

    Some electrolytic capacitors are cylindrical in shape but have two wires attached on ONE end and neither of them is at the center. So they are on a radius from the center of that round end and this is referred to as a radial style component. Electrical engineers are not as particular about geometry as mechanical engineers so other capacitors that have a pair of parallel leads on one side of the package are also referred to as radial style. I guess one big reason for this is because they would be handled in a very similar manner by automatic insertion machinery: the radial style wires do not need to be bent prior to inserting them in a PCB while axial leads would. So capacitors that are flat or rectangular can also be called radial style if the two leads are on one side and come out parallel to each other. Again, other types of components (resistors, diodes, fuses, etc.) can also be referred to as radial style.

    I do not think that the mechanical world shares this more general use of the terms that the electronic world uses.

    I will be posting this Tip on other boards so don't be surprised if you see it somewhere else.

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    I get where you're coming from but "axial" only describes parallel to AN axis. It doesn't necessarily have to be the diametral one - there are three, don't forget. Your diagram does show widely used terms for use when referring to work in a lathe though.

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    Quote Originally Posted by eKretz View Post
    I get where you're coming from but "axial" only describes parallel to AN axis. It doesn't necessarily have to be the diametral one - there are three, don't forget. Your diagram does show widely used terms for use when referring to work in a lathe though.
    Referring to polar/cylindrical coordinates in topic is also bit confusing as then you get polar axis, azimuth and whatnot.

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    Well technically you are correct. In fact you can define an axis in any direction you wish and one object can have any number of axis: 3, 5, 10, dozens, hundreds, thousands, whatever. But if you are going to start referring to a direction with the descriptive word "axial" it would be really, really nice to have defined that axis before doing so. And if it is not an obvious axis, then you should say which axis that direction is axial to. If I or most people see a cylindrical object and someone talks about the axis, I and they will assume the obvious axis is the one being discussed. If you talk about axial forces on a rotating shaft, almost everyone will envision the obvious axis through the centers of the circular cross sections of that shaft. Although it may be technically correct, I doubt that anyone would imagine you are speaking about an axis that is perpendicular to that obvious one. That direction would be referred to as radial or along a radius.

    I was attempting to give a common sense differentiation of these three terms that are often confused. I was not trying to go into competition with the people who write dictionaries.



    Quote Originally Posted by eKretz View Post
    I get where you're coming from but "axial" only describes parallel to AN axis. It doesn't necessarily have to be the diametral one - there are three, don't forget. Your diagram does show widely used terms for use when referring to work in a lathe though.

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