Minor mystery about my tumbler

[Pat Cannon]

OK this is of no real importance, but it's been a minor mystery to me for years. And then in thinking about it as I was formulating the question to post here, I think I figured it out. So I thought I'd go ahead and post about it, just in case anybody else ever wondered the same thing (or maybe this was obvious to everybody else in the world):

I have your basic vibrating brass tumbler (a hand-me down from Strad, BTW, thank you sir), the kind with the threaded rod sticking up the middle to hold the lid on. When the machine is running, and I loosen the nut, rather than staying in same place or wandering randomly up and down with the vibration, the nut always climbs upward, and at a pretty good rate, maybe a half inch per second. Why is this?

It reminded me of the old electric football games. The player pieces had little spines on their bases, angled backward to provide the 'bias' to move forward as the playing field vibrated (no need to take my word for this, young people):


But what I didn't get was, what provides the upward bias for my nut? Shouldn't gravity provide a downward bias if anything? Is it the way the threads are cut? Does vibration at that frequency reverse gravity?

Then it hit me: the rod isn't just shaking. More precisely, it's swinging, since it's fixed at the bottom. The nut's being thrown, not perpendicular to the rod, but at a tangent to the arc of its path as the rod swings back and forth; a vector, often referred to as "centrifugal force", with a component away from the center of the rotation, just like when you windmill your arm to swing the ketchup to the mouth of the bottle and the cap comes off and you have to clean ketchup off the kitchen ceiling, except in just a couple degrees of arc rather than 360.

OK. I can sleep tonight. After I copy this into an email to the Nobel committee.

[BigDog58]

While this doesn't answer your question I have a suggestion to help the tumbling rate of your tumbler. I have one of the cheap Frankford Arsenal Tumblers that has the threaded rod all the way from the bottom of the bowl through the lid. I worried about the exposed threaded rod hitting amd scoring my brass. So, I took a small plastoc funnel and inverted it (upside down) and placed it over the rod. It definitely increased the tumbling effect and it also protects the brass and the tnreads on the rod. Just a simlple idea that might help someone else.

[OldmanFCSA]

I will try to explain.

Turn unit over.
Look at direction of angled plates that hold bowl for the vibration.
Turn back upright.
With wingnut loose on threaded rod,
when vibration occurs,
the bowl raises slightly carrying the wingnut with,
then on reverse the bowl drops slightly and wingnut doesn't catch up with that movement until the next up vibration,
and the next up raises it more.

A left hand thread would try to tighten.

Similar to gun barrels , a normal twist on barrel will cause barrel to tighten in receiver. A FCSA Member friend had a rifle with left hand thread and barrel would loosen while shooting.

[xd ED]

Precession (mechanical) - Wikipedia

Precession, also called epicyclic fretting precession,[1] is the process of a round part in a round hole rotating with respect to that hole because of clearance between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force.[2] Fretting between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other."[2]

Automobile lug nuts
Automobiles have also used left-threaded lug nuts on left-side wheels, but now commonly use tapered lug nuts, which do not precess.

Bicycle pedals
Bicycle pedals are left-threaded on the left-hand crank so that precession tightens the pedal rather than loosening it. This may seem counter-intuitive, but the torque exerted due to the precession is several orders of magnitude greater than that caused by a jammed pedal bearing.

Bicycle bottom brackets
English threaded bicycle bottom brackets are left-threaded on the right-hand (usually drive) side into the bottom bracket shell. This is the opposite of pedals into cranks because the sense of the relative motion between the parts is opposite. (Italian and French threaded bottom brackets have right-hand threading on both sides.)

Bearings in manual transmissions
A bearing supported gear in a manual transmission will rotate synchronously with its shaft due to the dog-gear engagement. In this case, the small diametrical clearance in the bearing will induce precession of the roller group relative to the gear mitigating any fretting that occurs if always the same bearing rollers push against the same spot on the gear. Typically the 4th and 5th gears will have precession inducing features, while 1st through 3rd gears might not since cars don't spend long period of time on those gears (like on highway driving). Transmission failure due to lack of precession is possible in gear boxes when low gears are engaged for long periods of time.

Where I first learned of the phenomena>

On bicycles, left hand threads are used mainly in three places, on left pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction.

For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.

Precession of right side BB cups is less obvious because the rotating load is only partial. The largest load being chain tension, that together with the moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has little tendency to turn.

Freewheel cones are more obvious candidates for precession, their load being mainly radial, and rotating continuously in the direction that would unscrew a right hand thread. There are other such but less common threads on bicycles.

Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.

Left hand threads would not be required on left pedals if a design common on cars were used. Before the advent of conical lug nuts, many cars used left hand threads on left side wheels. Today, stories of wheels rolling away from cars no longer make news, the conical seat having solved this problem on car wheels as it could on bicycle pedals.

However, unscrewing is not the main problem for pedals, but rather crank failure caused by fretting erosion of the pedal eye. Fretting initiates cracks that can cause sudden and unsuspected pedal separation when the eye of a crank breaks. Because this occurs equally with right and left cranks it is the more important reason for a conical spindle face and crank eye. This has been tested.

Jobst Brandt

Sheldon Brown.com

[xd ED]

I will try to explain.

..

Similar to gun barrels , a normal twist on barrel will cause barrel to tighten in receiver. A FCSA Member friend had a rifle with left hand thread and barrel would loosen while shooting.

When one considers the relationship between rifling and threading, the potemtial conflict seems obvious, but I' would never have thought of it had I not read that.

[Scratch]

I'm pretty sure the nut rises on the threads due to black magic. If I were you I'd tie it to a stake and burn it.

[TH3180]

Aren't pain meds great?

[Synergy]

All you gotta do is take the threaded rod out and flip it end for end to get your left handed thread so the nut will tighten.

[OldmanFCSA]

All you gotta do is take the threaded rod out and flip it end for end to get your left handed thread so the nut will tighten.

I sincerely hope you are NOT that stupid.

[brad3579]

All you gotta do is take the threaded rod out and flip it end for end to get your left handed thread so the nut will tighten.

I am confused

[Synergy]

I was worried about someone thinking I was a compete idiot with that statement LOL, take special note of the


I work with idiots all week long, to overcome the frustration I get away with BSing them like that all the time, its fun hearing them say "REALLY"

[xd ED]

All you gotta do is take the threaded rod out and flip it end for end to get your left handed thread so the nut will tighten.

That only works south of the equator

[Synergy]

BTW its refreshing to see my BS doesn't work on you guys, it gives me hope for mankind.

[brad3579]

All you gotta do is take the threaded rod out and flip it end for end to get your left handed thread so the nut will tighten.

I am confused

That only works south of the equator

Alright, now I understand

[Pat Cannon]

Oh jeez you guys, I procrastinated a couple hours of freelance work yesterday thinking about this and was gonna make it up today.

Tim, I'm not even taking Tylenol anymore. In fact I'm even out of beer.