The 41 second clock failure syndrome

[anorlunda]

I'm curious about this failure mode.

I had a cheap ($3.82) wall clock from 2017. On Tuesday, it stopped working, getting stuck at the 41 seconds position. The second hand twitches every 1 second, but it does not advance. Since it was so cheap, I threw it away and bought a new one (also $3.82). The new one claims to be a different brand, but it failed in less than 1 hour use.

The new clock with a new battery, also fails the same way and also at the 41 second position. Actually, it stuck once at 49 seconds, but a dozen or more times at the 41 second mark. If I lay it down horizontally on a table, it keeps good time and doesn't get stuck.

I could spend $382.00 for a clock, but the money could go to cosmetics. Worse, it might have the identical quartz movement with a single AA battery. That movement seems ubiquitous in recent years. I've seen it in many brands of clocks.

Do those movements use stepper motors? Aside from the fact that 31-59 seconds is lifts the second hand and thus uses more torque, is there any good reason for random failures to result in such exact symptoms?

If the clock movement has injection molded plastic gears, there may be a remnant of the parting-line/sprue/gate- marks/ejector-pin-marks in the same place on both the old and new gears. That would make it a common mode failure.

Any more speculations?

 

[berkeman]

the fact that 31-59 seconds is lifts the second hand and thus uses more torque

That seems to be the reason for the failing position window.

Have you checked the batteries? As the battery wears out, failing at 41 seconds seems pretty common, in my experience.

 

[anorlunda]

That seems to be the reason for the failing position window.

Have you checked the batteries? As the battery wears out, failing at 41 seconds seems pretty common, in my experience.

Brand new battery in the 2nd clock. But I'll try a different brand battery.

 

[berkeman]

Measure them with a DVM to be sure they are good. I have two different storage boxes of batteries in my shop, and apparently one of them was little used for many years (I wasn't paying attention). I pulled some "brand-new" AA Energizer brand batteries out of one of the plastic packages and they failed right away! I checked the date on the package and saw that it had been on my shelf for over 10 years. Sheesh.

 

[hutchphd]

If the clock movement has injection molded plastic gears, there may be a remnant of the parting-line/sprue/gate- marks/ejector-pin-marks in the same place on both the old and new gears.

But the hands are just a friction fit and not keyed so it would not be a common mode. I think the issue is most likely the gravity torque on the high speed hand. So either a bad batch of motors or most likely battery (clean the contacts!) issues.
Those are very impressive little mechanisms in my experience.

 

[Baluncore]

Maybe designed for a single 1.5V cell, but being run on a single 1.2V rechargable cell ?
Maybe the time of day for failure is due to temperature of the cell and electronics.

The clocks are driven by a small 1 Hz, 4 step motor with a PM rotor.
You can turn over the field winding so they run backwards.

If it is a balance problem, failure should occur first just before 45 seconds.
The second hand is only geared down by 15:1, so torque is required from the motor.
Remove and balance the second hand by adding a small counter weight.

 

[DaveC426913]

Have you checked the batteries? As the battery wears out, failing at 41 seconds seems pretty common, in my experience.

+1. Common to see clocks fail in 31-59 range. And it's often a bad battery.

 

[anorlunda]

OK. I've tried Amazon, IKEA, and Duracell so far. Somewhere, I have some 1.8V lithium AA batteries. That should do it.

 

[berkeman]

Maybe gravity is a lot stronger where you are now? Have you checked that?

 

[Vanadium 50]

I could spend $382.00 for a clock, but the money could go to cosmetics.

Indeed. Somehow you never struck me as the kind of guy to wear lipstick and eye shadow.

 

[hmmm27]

So, what happens when the clock is tilted ? ie: 3,6,9 up top.

 

[Baluncore]

I have a similar clock, but I use 1.2V batteries that need to be recharged every 6 months or so. It keeps very good time, but it creeps ahead by about a minute per week when it needs a battery recharge. When the national News seems to start a minute of more late, I recharge the clock battery and correct it's time. I did not at first recognise the speedup prior to stopping on the 40 second hill climb, but now that I am watching for it, it gets recharged and so does not get to stop at 40 seconds anymore.

It does still stop dead after several years, but not at 40 seconds, and recharging the battery does not fix it. I used to think that was end-of-life for the clock, but now I know it is due to the end-of-life for very small midges.

Parachute spiders wait for a warm day with a gentle breeze, usually the day after rain, then they climb trees and leap off, to be delicately spread across the face of the Earth. When a parachute spider escapes my family of tame huntsman spiders, it may take up unsafe residence inside a poorly sealed space such as an electrical fitting, or my clock.

It is amazing what you can see under a binocular operating microscope. The small spiders use the clock as a hunting lodge. Each time they foray, to bring back a midge, they grow in size and the midge's remnant skeletons build up in the clock, until the spider is too big to exit, when it sheds a last exoskeleton in the clock, before softly departing to seek it's destiny outside time. The accumulated hard exoskeletons of the predator and prey will in the end block the rotation of the seconds reduction gear train. That is when I remove the mechanism and carefully separate the two halves. Then I can blow out the fragments of exoskeletons left there by the young spider to chock the gears that kept it awake at night.

 

[Twigg]

If the clock movement has injection molded plastic gears, there may be a remnant of the parting-line/sprue/gate- marks/ejector-pin-marks in the same place on both the old and new gears. That would make it a common mode failure.

Since you asked for speculations, I'm going out on a limb. It's possible that since the clock is so cheap, the manufacturer might be using the same dies for the injection molding since 2017, and the fit of the gears could have seriously degraded compared to the initial batch. After all, who would return a $3.82 clock? Makes more sense for them to get every possible use out of the dies to keep costs down, even if some clocks are dead on arrival. This doesn't explain the 41 second phenomenon (at least, you'd think one clock might fail nearby on 43 seconds or 39 seconds, or something).

 

[Rive]

Do those movements use stepper motors?

Surprisingly, yes. I did not even know about this kind of stepper.

I too had some problems with some cheap clocks. After some time I've found a solution: they did not like some batteries. Amongst the classic zinc-carbon batteries, they did not like the fresh ones (they stuck often) while with the already half-depleted ones they worked fine.

Maybe you can try yours with something rechargeable. They give some lower voltage...

 

[hutchphd]

This doesn't explain the 41 second phenomenon (at least, you'd think one clock might fail nearby on 43 seconds or 39 seconds, or something).

Allow me to reiterate #5. The hands are in no way keyed to the gears. They are a friction fit and so a particular mold problem in a gear will not show up at any particular time. You can buy the movements for a few dollars at Banggood. They are occasionally very useful.

 

[anorlunda]

Allow me to reiterate #5. The hands are in no way keyed to the gears. They are a friction fit and so a particular mold problem in a gear will not show up at any particular time. You can buy the movements for a few dollars at Banggood. They are occasionally very useful.

I'm not convinced. Despite the friction fit, automated manufacturing machines are likely to press fit the second hand at the same angle relative to the gear for all movements. So, it might be better to say "potentially the problem will not show up at any particular time."

By the way, I found these movements for sale on alibaba.com for at little at $0.60 each.

The picture appears to show a solenoid plus a ratchet rather than a stepper motor.

 

[hutchphd]

So, it might be better to say "potentially the problem will not show up at any particular time."

Having torn these apart I am certain the pick and place for the gear train does not worry about each gear and so the relative orientation of hands to the mold will involve very very small probabilities. Six gears with 20 teeth gives more than a million.

 

[Rive]

The picture appears to show a solenoid plus a ratchet rather than a stepper motor.

But it really that type of stepper. Weird design, never would have thought that it's even possible.

I think the failure is due some poor manufacturing of the gears (loose gears can lock under force).

Could you check please whether the issue would become some 'eleven second syndrome' if you hang the clock upside down?

 

[anorlunda]

Could you check please whether the issue would become some 'eleven second syndrome' if you hang the clock upside down?

Excellent suggestion. The experiment is underway. I'll report when conclusive.

 

[anorlunda]

OK experiment complete.

1. I replaced the old battery used during the initial failures.
2. I verified that the clock still sticks at 41s. Confirmed.
3. I hung the clock upside down.
4. After 4 hours, it kept perfect time. @Rive's experiment confirmed.
5. I re-hung the clock right side up. Sure enough, it promptly stuck again.

So I guess the lesson is: Weak batteries may contribute, but the sticking is likely caused by manufacturing imperfections in the plastic gears. The same imperfections may be present in different brands of clocks that use the same movement manufacturer.

p.s. I thought it was a fun mental exercise to re-train my brain to tell time with the clock upside down. It wasn't difficult. But my wife was firm. She said, "NO."

 

[Twigg]

Allow me to reiterate #5.

Whoops, I done goofed. Thanks for reiterating.

 

[Baluncore]

p.s. I thought it was a fun mental exercise to re-train my brain to tell time with the upside down clock upside down. It wasn't difficult. But my wife was firm. She said, "NO."

I have modified clocks to run backwards which gets interesting when strangers first see them.
The best I find is a mathematical clock; running backwards with 0 on the right and Pi on the left, with i at the top, x marks 10 o'clock.

 

[anorlunda]

I have modified clocks to run backwards which gets interesting when strangers first see them.

 

[berkeman]

1. I hung the clock upside down.
2. After 4 hours, it kept perfect time. @Rive's experiment confirmed.

Wow, I would not have guessed that! Interesting experiment and result!

 

[anorlunda]

Wow, I would not have guessed that! Interesting experiment and result!

As I think about it, the solenoid design in #16 is not symmetrical. The plunger works with gravity one way and against gravity the other way.

 

[hutchphd]

You may be able to rotate the gearbox by pi relative to the clock housing. The hands pull off easily. Might be a center nut or clips in back. So you will have the worlds most expensive 5 dollar clock. My refrigerator is now worth many thousands of dollars I reckon.
I used several of these motors to drive small (10inch) dollar store Mylar foil trees at 1RPM on my Christmas table last year. Each one had its own color changing spotlight LED. It was the lockdown.

 

[Baluncore]

The picture appears to show a solenoid plus a ratchet rather than a stepper motor.

As I think about it, the solenoid design in #16 is not symmetrical. The plunger works with gravity one way and against gravity the other way.

There are probably many ways of doing it.
In one, a single coil is wound on a core similar to the one shown. At the gap in the core, there is a hole that surrounds the PM rotor. You can see a white round gear at the bottom that hides the gap and rotor. The end of the rotor shaft can be seen in the transparent housing.

The crystal oscillator and divider produces a square wave output that drives alternate positive and negative pulses of current through the solenoid. Each pulse advances the PM rotor to the next position. The pulses to the solenoid were once produced by coupling the square wave through an AC electrolytic capacitor to the solenoid. I expect they are now generated by an internal digital pulse generator.

It appears the solenoid core is now asymmetrical, so the clock cannot be reversed by simply turning the core over. Reversing the solenoid terminals makes no difference, so modifying the rotor is the only way to reverse the clock.

When an electrolytic capacitor was used it could dry out and sometimes needed replacing. It did have the advantage that you could change the capacitance value to adjust for a different battery voltage.

I don't believe gear manufacturing imperfections are the problem. The special lubricant used on the nylon gears may redistribute over time. That may explain why it works upside down, but not the right way up.

Another test would be to push the second hand to point the opposite direction, so the gear does work on the opposite stroke.

 

[Baluncore]

This doesn't explain the 41 second phenomenon (at least, you'd think one clock might fail nearby on 43 seconds or 39 seconds, or something).

I am sure it does happen, but it will keep trying before advancing again. That advance is dependent on environmental noise and any building vibration that might nudge it up another step. When you get home, open then close the front door, stomp across the room, the clock may take another step forward. In a noise free environment, the clock should first stall at 45 sec.

We only notice a stopped clock when it stagnates at one point for a while. There are cases where it appears to lose time before it manages to advance again. Any noise or building vibration may give it the nudge it needs. The amplitude of the available nudge needed to get to the next step falls rapidly from 39 to 45 sec.

The time relationship to peak torque, Tp, is;
45 seconds is 45 - 0 sec = 0° before Tp; Cos( 0 °) =100% of Tp. 0.0% to step to next.
44 seconds is 45 - 1 sec = 6° before Tp; Cos( 6 °) = 99.5% of Tp. 0.5% to step to next.
43 seconds is 45 - 2 sec = 12° before Tp; Cos(12°) = 97.8% of Tp. 1.7% to step to next.
42 seconds is 45 - 3 sec = 18° before Tp; Cos(18°) = 95.1% of Tp. 2.7% to step to next.
41 seconds is 45 - 4 sec = 24° before Tp; Cos(24°) = 91.4% of Tp. 3.7% to step to next.
40 seconds is 45 - 5 sec = 30° before Tp; Cos(30°) = 86.6% of Tp. 4.8% to step to next.
39 seconds is 45 - 6 sec = 36° before Tp; Cos(36°) = 80.9% of Tp. 5.7% to step to next.

If it can get to 43 seconds it will take 2.2% noise to get to 44 sec, then 0.5% noise to pass 45 sec.
So it looks like the time it stops is at about 8.6% short of peak torque when it will need 3.7% noise to advance.

 

[Tom.G]

Since the problem is position sensitive (rotating the clock body by pi), it sounds like a bad bearing surface is contributing to the problem.

 

[sophiecentaur]

Remove and balance the second hand by adding a small counter weight.

Or just remove it for ever. It's rare to need second accuracy on a wall clock when we all (?) have wrist watches and cell phones for stopwatch / timer facility.

 

[Baluncore]

Or just remove it for ever.

Then how could you know that the clock had stopped ?

 

[sophiecentaur]

Then how could you know that the clock had stopped ?

First time it let's you down and you get to the train after it's left.

 

[neilparker62]

p.s. I thought it was a fun mental exercise to re-train my brain to tell time with the clock upside down. It wasn't difficult. But my wife was firm. She said, "NO."

For -1 second I thought it might be fun to work out the mathematical transform from t' to t where t' is the time on the upside down clock and t is the 'actual' time. But then said NO. And I don't even have a wife

 

[sophiecentaur]

The high gearing to the second hand may explain why cheaper watches had a small seconds dial and not a sweep second (full diameter). I never considered that before.

 

[JT Smith]

Then how could you know that the clock had stopped ?

Instead of removing the second hand maybe just cutting it down to a very short nub will fix the problem and yet still allow you to see if it's moving or not.