Talk:Precession (mechanical)

gyroscopic precession

I have not seen in any paper/books/internet any discussion/analysis on the magnitude of the precession angle and how it varies with I or omega and the force or impulse tending to pull it off axis. Is there a simple formula for this. -Coli.white (talk) 21:40, 14 August 2008 (UTC)[reply]

I believe you are inquiring about Precession, not the mechanical precession discussed here. -AndrewDressel (talk) 23:43, 14 August 2008 (UTC)[reply]

mechanical process

"Fretting-induced precession is a purely mechanical process"...Inertia is also a mechanical process. "mechanical" should be replaced by "kinematic" which refers to mechanical processes that are NOT governed by inertia...the latter being called "dynamic". Diogenes —Preceding unsigned comment added by 67.168.194.102 (talk) 01:47, 7 October 2009 (UTC)[reply]

First, some definitions:

mechanical, adj. and n. Of the nature of a machine or machines; acting, worked, or produced by a machine or mechanism. - OED
inertia Physics. That property of matter by virtue of which it continues in its existing state, whether of rest or of uniform motion in a straight line, unless that state is altered by an external force. Originally used as L., and also called vis inertiæ (force of inertia). - OED

So, I don't see how inertia is also a mechanical process. A planet in orbit, which hardly fits the definition of mechanical, has plenty of inertia.

Second, some more definitions:

Kinematics is the science of motion without regard to the inertia of the objects involved not merely processes that are not governed by inertia.
Kinetics is the science of motion due to forces acting on masses.
Dynamics encompasses both kinematics and kinetics.^[1]^[2]^[3]

^ Hibbeler (2009). Engineering Mechanics: Dynamics. Prentice Hall. p. 3.
^ Beer and Johnston (1996). Vector Mechanics for Engineers: Dynamics. McGraw-Hill. p. 582.
^ Meriam and Kraige (2002). Engineering Mechanics: Dynamics. John Wiley & Sons. p. 3.

So, I don't see why "mechanical" should be replaced by "kinematic". -AndrewDressel (talk) 03:29, 7 October 2009 (UTC)[reply]

About the mechanical vs. kinematic talk: How about "Precession (engineering)" Laelele (talk) 01:02, 12 January 2011 (UTC)[reply]

Per Andrew, I think it ought to stay at "mechanical". Wizard191 (talk) 01:48, 12 January 2011 (UTC)[reply]

I changed from "is a purely mechanical process wich" to "is a process purely due to contact forces and" since precession (with it's companion nutation) is also mechanical. "Contact forces" is more to the point.Laelele (talk) 13:58, 12 January 2011 (UTC)[reply]

Maybe that "fretting-induced" isn't a very well chosen term. It's not induced by fretting (fretting = wear), fretting is rather a concequense. It's as I see it induced by relative clearance (play) and contact forces. Play allows for movement (= precession) wich in turn causes wear (= fretting). (You know the hen and the egg.) I'm now even more convinced that "Precession (engineering)" is a better title for the article. Laelele (talk) 14:21, 12 January 2011 (UTC)[reply]

I'm not convinced that "engineering" is better, because precession occurs in naturally occurring situations (unfortunately I can't think of any off the top of my head). One does not need to engineer a precession for it to occur. Wizard191 (talk) 15:44, 12 January 2011 (UTC)[reply]

Who other than an engineer would take this phenomenon into consideration? Thats my point. (OK a bicyclist, but his business is a product of engineering.) Laelele (talk) 21:59, 12 January 2011 (UTC)[reply]

Example

This: "If you use a bar to make pilot holes (for a fence) and you move the upper end of the bar in a circular path (the bar still in the hole) the bar will tend to spin in your hands. The torque is quite substantial." was removed for the reason of being a bad example. I think there should be some example to help the reader grasp the consept. I see nothing wrong with the example as such,

it emulates the bicycle pedals-situation but with the play emphasized. Maybe someone could help me phrase it better. Laelele (talk) 01:02, 12 January 2011 (UTC)[reply]

I removed the example, because the way you worded it, it is very unclear. I could hold the bar tightly and move it in a circular path while causing it not to precess. And thus I removed it. Wizard191 (talk) 01:47, 12 January 2011 (UTC)[reply]

But you would still feel the torque. Modify the example slightly: the bar now has splines (20 of ‘em) and the hole is now lined with splines (200 of ‘em). If you move the top end of the bar in a circular path you will always have two points of contact between the bar and the liner (forget about contact with the bottom of the hole). Now we can talk about torque. This is a good image of what is happening in the bicycle pedals-situation: at the two point of contact there is effectively no slip (like spline in spline) resulting in an enormous torque. As a result the pedals will be screwed in till there is no play, or the surfaces break down: fretting.

In Swedish we have a word for precession (engineering): “valsvandring”, vals = roll (as in a paper machine), vandring = wandering. We also have a word for the torque so caused “valsvandringsmoment”, moment = torque. Maybe you now see why I see this as engineering rather than mechanics Laelele (talk) 15:16, 12 January 2011 (UTC)[reply]

Perhaps you are confusing the English words "mechanics" and "mechanical". Mechanics, besides being the plural of mechanic "is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment." Mechanical, on the other hand, is better described by the OED definition quoted above. -AndrewDressel (talk) 22:11, 12 January 2011 (UTC)[reply]

Your new example is even worse than the old one, because its not precessing at all. What you are now referring to is a geartrain, which has nothing to do with this article. There are many ways to produce torque, that have nothing to do with precessing, such as the wheel and axle. Wizard191 (talk) 15:46, 12 January 2011 (UTC)[reply]

Of course its a gear train. I think Sheldon Brown is to the point with this experiment:"You can demonstrate this to yourself by performing a simple experiment. Hold a pencil loosely in one fist, and move the end of it in a circle. You will see that the pencil, as it rubs against the inside of your fist, rotates in the opposite direction." (http://www.sheldonbrown.com/gloss_p.html, under pedals) The important things here are: a) two points of contact (as in my example with its emphasize on play to isolate just that) and b) rub, that is friction, (that's why I introduced the splines). With no friction and unobstructed slip you will have no precession. Can we agree on a) and b)?

And it's not "fretting-induced": you move a file to make it wear off dust from a piece of wood. You would probably not say that "as dust came off a piece of wood it caused a file to move back and forth".Laelele (talk) 21:24, 12 January 2011 (UTC)[reply]

Right, it should be just "mechanical precession", not "fretting-induced precession". -AndrewDressel (talk) 22:17, 12 January 2011 (UTC)[reply]

”Vals vandring” (roller wandering) and ”vals vandrings moment” (roller wandering torque): why is it that we have words for this in Swedish? My theory is that it has to do with national pride. See Sven Wingquist. (SKF would later give birth to VOLVO.) Of course old school Swedish engineers had to be able to describe the benefits of his innovation and just what sort of problems it solved. Hence the special words.

Think of a shaft or a roller. It must somewhere have some sort of bearing: the simplest form, a bushing in a housing. Now let the roller spin and suppose there is some angular misalignment. Now we get what I call a a)two points of contact-situation. The bearing (bushing) can’t perform as intended, the lubricant film breaks op at the two points leading to increased b)friction. The bushing will then start rotating (wander) in the housing. It’s not that the roller wanders but it causes the wandering of the bushing, or at least a tendency to do so (roller wandering torque). If you return to my image with the bar with splines (I know you hate it, but I like it), and instead of moving the upper end in a circle you pull the end towards you and hold it fixed there. Now start spinning the bar and you will see that the lining with splines will spin as well. This is the original roller wandering-phenomenon. Now shift roles between what is fixed, what is intended to move and what moves as an unfortunate consequence. Now you have my example or the bicycle pedals-situation.

Your precession and my roller wandering are, as I see it, two sides of the same coin if we can agree on a) and b). When you talk to a bicycle repair man about the threding of pedals you say precession, a Swedish engineer would say valsvandring for historical reasons.

I still think there should be an example. What about Sheldon Brown's pencil?Laelele (talk) 01:13, 14 January 2011 (UTC)[reply]

You are missing my point, I understand what precession is and how it works. And yet your translation from Swedish is correct, however the example you wrote is poor. To those who don't understand the principle your example is only going to confuse them more instead of instruct. This is something that is far better described with a picture or video than words. As such, I recommend that you create a video or image seeing how you are so fired up about this topic. Wizard191 (talk) 19:02, 15 January 2011 (UTC)[reply]

Bicycle Pedals

It is stated that the pedal threading is counterintuitive, "but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing". Is that really true? One would think that the pedal bearing not being jammed is the normal condition, and that's why we care about the precession. That is to say, under normal use, the precession torque is present, but pedal bearing torque is nearly absent; i.e. do we perhaps want the wording to say that precession torque is several orders of magnitude greater than that of normally operating pedal bearings that haven't seized? If a pedal bearing is seized, how do you even pedal; the pedal won't stay horizontal under the foot as the crank rotates. KazKylheku (talk) 21:46, 5 August 2016 (UTC)[reply]

I don't think so, although it could be worded better to explain exactly what it means. As an avid bike rider all my life, I've experienced this.

Pedals are threaded so the top of the shaft spins toward the front wheel when tightening. When you push down on a pedal, the minimal torque from the bearing tries to loosen the shaft. The precession occurs opposite to this torque, and can tighten the shaft onto the crank so much that when it's time to change pedals, a lot of force on a wrench and usually some heat from a blow torch is needed to break them loose.

If the bearings on the pedal seize, it would normally begin backing out the threads until the pedal falls off. However, due to the tightness cause by the precession, that doesn't happen, Instead, it throws your foot off and comes around to slam the back of your leg. That is what I believe the author was trying to convey. Zaereth (talk) 22:12, 5 August 2016 (UTC)[reply]

A simpler way to visualize it is that the precession is caused by the the lateral load (the foot on the pedal) where as the torque is caused by the radial load (the friction in the bearing). Zaereth (talk)

[1] Hibbeler (2009). Engineering Mechanics: Dynamics. Prentice Hall. p. 3.

[2] Beer and Johnston (1996). Vector Mechanics for Engineers: Dynamics. McGraw-Hill. p. 582.

[3] Meriam and Kraige (2002). Engineering Mechanics: Dynamics. John Wiley & Sons. p. 3.

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