[handzbagz]

Just how likely is this to happen though, it would have to be the only light source and be flickering with exactly the same frequency as the machinery is traveling at (which i assume for most devices varies a lot).

Not saying it can't happen but it does seem stupidly hard to achieve.

[nwallin]

It's not true that it has to be the only light source. All of the light sources have to be the same type and on the same circuit, which is fairly common.

Many AC motors are synchronized to the frequency of the power line, by the same principal that turbines in the power grid are. So 60hz AC often means 30hz = 1800 rpm motor, and 59.8hz AC means 29.9hz motor. So if you have LEDs with a half bridge rectifier and your machinery is 180 degree rotationally symmetric, it will appear to not be moving, even if the line frequency fluctuates.

[NeedMoreTea]

Pretty common, about as common as the wagon wheel effect in 24fps film movies. In just the same way, the item can appear to stop, run slow, or run backwards, just as with film. No it does not have to be the only light source, just the nearest one to the machinery.

Fluorescent tubes are most noticeable, LEDs a little less. Tungsten or Halogen are the only sensible option for workpiece illumination.

[mcv]

Or, of course, just use a LED that properly smooths out the DC. It's not hard to do, and a quality LED manufacturer should get this right. The only excuse for flickering is a cheap LED that cuts cost for circuitry.

[NeedMoreTea]

Oh I agree, but a surprising number of LED bulbs have some degree of flicker, even from those makes you might expect to do better. A purpose made work lamp should get it right though. Hopefully. Maybe.

For my hobby stuff, I just bought a couple of spare halogens that should see me out.

[ouid]

I believe halogen is also tungsten.

[NeedMoreTea]

It is. Just running at higher temperature. They tend to be marketed as something separate and distinct though.

[m463]

I would imagine there's a higher-than-average chance of this because most AC motors are definitely syncronized with the 60hz (or 50hz) AC source. There's a chance the lights might also derive from this.

[Dylan16807]

> There's a chance the lights might also derive from this.

If the light is flickering it's absolutely guaranteed that it's derived from this.

But it's hard to imagine a lightbulb that emits a short enough pulse of light to make something look stopped. Even an absolute garbage one-way rectifier will be emitting light more than a quarter of the time. That can make a tool look odd, but it won't make it look still.

[nitrogen]

If the light is flickering it's absolutely guaranteed that it's derived from this.

Not necessarily. If it's a 50/60 or 100/120 Hz flicker then yes, but LED lights with a cheap switching power supply might still flicker at the switching frequency, which could be say 400Hz or something.

[Dylan16807]

You have to be intentionally wasting money to put in a transformer big enough to handle 400Hz, and once you get into the lots of KHz where a supply like that is happy I think your "please don't explode" capacitor on the transformer is enough to prevent flicker.

[simcop2387]

It doesn't have to be going the exact same speed, a multiple would do the trick too. It can also be dangerous if it's close to the same rate since it can then look like it's going slower even if not stopped.

[TheSpiceIsLife]

Or the rotating / reciprocating object may appear to be rotating / reciprocating slowly in the opposite direction.

Even after 20 years in the metal fabrication industry that scenario still spooks me.

[ajuc]

> which i assume for most devices varies a lot

Most will flicker at a multiply of the power line frequency (60 or 50 Hz depending where you live).

It's so common, that most digital cameras have a setting to set the power line frequency, so they can reduce the flickering in the footage.

Incidentally - many alternating current engines are also working with multiples of the powerline frequency, because it's easier that way.

[icelancer]

This is uncommon but not rare. Have you ever seen wheel blur or other artifacting of such? It's very easy to make happen artificially under controlled conditions which means it can't be that hard to happen accidentally.

Edited to add: Have you ever heard of a timing light for an engine? This works exactly the way you are describing.

[calvinmorrison]

Timing lights for engines work this way for clarity: The distributor on an engine has a spinning rotor and a number of points (one for each sparkplug) on the outside the circle the rotor forms while spinning are a series of contacts across which electricity jumps to send voltage to the spark plug.

The timing light is then pointed at the flywheel on an engine, which has numbers or marks stamped into it. Each time the spark plug fires the timing light (which is hooked into that same current via induction) lights up for a brief amount of time to show at what timing offset the engine is currently at. (this all happens at hundreds of rpms a minute).

so I'm not sure it's the same

[dsfyu404ed]

The effect might not be rare but getting hurt by it is incredibly rare. Machines produce noise and vibration. Nobody is going to be fooled and try to pull a moving part out of a lathe because of some LED lighting except in the most exceptional or bizarre circumstances.

[cc439]

Machine shops are very loud (plus you'd be wearing ear protection) and a lathe/mill that vibrates noticeably is either broken or incapable of performing the very task its designed for.

[roenxi]

Nobody? That is a high bar. Mechanics tend to be pretty cluey, but for the effort of getting proper lighting why nobody should be taking that sort of risk. Sight is one of those fundamental lets-feed-in-useful-information sensors for keeping situations safe.

[dsfyu404ed]

Most people spec'ing out machine shops are not experts in lighting other than more = better. This is a really, really low risk we're talking about. Like basically the stars have to align for someone to get hurt.

[TheSpiceIsLife]

I started to write a lengthy rebuttal, but decided to go with something more concise.

Never underestimate the ability of machinery to rapidly render you dead.

Never underestimate the ability of otherwise intelligent people to intentionally disable safeguards, or guninely make mistakes.

[icelancer]

No, it simply isn't that rare. You haven't been around wood shops and machinery much if you think that.

[mcv]

Are there any statistics on this? That seems like the obvious way to settle this issue.

So far, the discussion sounds like it's a theoretical possibility. If it's a real possibility, then it's something that should have happened a number of times.

[bootlooped]

You would also have to ignore or miss any other signs of very fast rotation, such as noise or air movement or the power switch being on.

[m4x]

It's easy to miss those signs too. It's common to have earmuffs and sometimes a dust mask on when using power tools, and power switches often don't clearly indicate state. Many start/stop control boxes don't have any visual indication of state at all.

[ric2b]

Someone might hear the noise and feel the vibration but think something is broken inside because the drill isn't rotating. I'm not sure exactly how that would lead to an accident but it might.

[lmilcin]

Multiple light source of the same general characteristic (don't even have to be the same type of bulb) may produce this result.

The rotation frequency would have to be close to a relatively simple multiple (or fraction) of AC frequency but the way electric motors are contstructed if the piece is directly connected to a motor it is very likely it is true.

[erikig]

I can also imagine the sound and moving air being masked by safety gear making it even more likely to note the movement.