# Thread: Tip: Directional Terms For Polar/Cylindrical Coordinates

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## 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.

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 point on a 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 (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 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'd noticed the fusion, changeing the linear equations to angular with the old omega used to mess folk up too,
Mark

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Minor quibble.....

"Tangential" is at right angles to both the axis and a radial line, through a point which is on the OD of a cylinder or circle, or curve. Also may be said as a line which touches a circle or curve etc at one point only and does not pass through it.

It is not possible to be "parallel to a point on a circle or some other curve" as you wrote.

I mention it only because you are trying to correct a matter of bad descriptions..... and so you do not want to give another one.

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OK, perhaps I should have said, "parallel to a curve at a point on that curve" to be technically correct. It sounds like you have not studied calculus because calculus does make it possible to find a line parallel to a curve at a point on that curve. I was trying to be as simple as possible for greater understanding. There would be other ways of describing it like, locally touching a continuous curve at only one point on that curve (while holding open the possibility of touching that curve at other points that are not local to the chosen one.

And your double perpendicular thing also works.

Originally Posted by J Tiers
Minor quibble.....

"Tangential" is at right angles to both the axis and a radial line, through a point which is on the OD of a cylinder or circle, or curve. Also may be said as a line which touches a circle or curve etc at one point only and does not pass through it.

It is not possible to be "parallel to a point on a circle or some other curve" as you wrote.

I mention it only because you are trying to correct a matter of bad descriptions..... and so you do not want to give another one.

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Originally Posted by Paul Alciatore
.... It sounds like you have not studied calculus because calculus does make it possible to find a line parallel to a curve at a point on that curve. ....
LOL.

Yes "to a curve at a point".... but never "parallel to a point", as a point has nothing to be parallel to....

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Useful writeup, Paul.

Heh. I suspect you youngsters haven't seen carbon resistors with radial leads. I haven't either for 50 or 60 years...

-js

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Originally Posted by J Tiers
LOL.

Yes "to a curve at a point".... but never "parallel to a point", as a point has nothing to be parallel to....

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Originally Posted by Mark Rand
See post #3

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Oh come on Jim, it hasn't been that long ago that chip resistors came into use. Perhaps 25 or 35 years ago. And there are still a lot of resistors with leads around, even today.

Originally Posted by Jim Stewart
Useful writeup, Paul.

Heh. I suspect you youngsters haven't seen carbon resistors with radial leads. I haven't either for 50 or 60 years...

-js

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Originally Posted by Paul Alciatore
Oh come on Jim, it hasn't been that long ago that chip resistors came into use. Perhaps 25 or 35 years ago. And there are still a lot of resistors with leads around, even today.
But not so many with RADIAL leads, that look like an old "Globar" resistor.

I happen to have a number of them that I just found when cleaning up. There were also some of the very similar tubular radial lead capacitors.

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