How do you design a music box to last longer per wind? - Page 1
Forum section: electronics / general

author: engineheat
author_rank: Frequent Contributor
timestamp: December 14, 2018, 12:35:06 am
content: For one my applications I need a slow and quiet rotating mechanism. Rather than deal with electronics which adds complexity and cost, I want to try a mechanical solution. The music box spring motors is of the perfect torque/speed. However, most only last 2-3 min every wind. I need it to last at least 10 min per wind.What factors affect the running time and is what I want feasible? I think I need to alter the gear train and use longer/stiff springs...

author: Ampera
author_rank: Super Contributor
timestamp: December 14, 2018, 01:42:51 am
content: You sort of answered your own question there.Spring motors store mechanical energy that gets put into them, and then release it into a system, usually with some sort of governor that exchanges the release of a small amount of energy (usually to spin something up) to get a more controlled release.So, a longer/stiffer spring that takes more energy to wind, will release more energy when unwound, There's always loss in any mechanical system due to things like friction and wear, but a spring motor is just like a battery, the more energy you can stick into it without it exploding/degrading, the more energy you can get out, as energy can't just disappear.

author: beanflying
author_rank: Super Contributor
timestamp: December 14, 2018, 01:50:46 am
content: Spring mechanics are part science/engineering and to make them part art/experience.Your current spring fully wound has potential energy of X that gets released at a rate of Q through the mechanism (whatever it is) your music box needs presumably most of Q to make it work for the current 3 minutes.Thinking about this logically unless you change X or your potential energy you will not get to 10 minutes no matter how you change the mechanism. So you need to store 3-4 times more energy capable of releasing it at or near Q.My texts with anything on spring design are long gone but you will find what you really need is a spring of about the same cross section but 3-4 times the length (minus a few additional losses) if memory serves leaving the rest of your mechanism the same.There are companies around that wind custom springs for industrial applications. I used one a long long while ago for a custom job for a part we couldn't get for an oddball mechanical timer.Personally in this day and age smooth rubber belts, pulleys and brushless motors would be how I would look at doing it.EDIT - Was typing this while AMPERA made a reply

author: BrianHG
author_rank: Super Contributor
timestamp: December 14, 2018, 02:12:07 am
content: The technical info and terminology you require is here:https://youtu.be/SOFx50Jsx-g?t=128To slow things down, you need to adjust either the gearing to the 'governor', or increase the 'governor's' blade size.

author: beanflying
author_rank: Super Contributor
timestamp: December 14, 2018, 02:16:25 am
content: Quote from: BrianHG on December 14, 2018, 02:12:07 amGood video but if you 'slow things down' it is no longer the music it was. Little thing called conservation of energy kicks in so you need more stored energy.

author: BrianHG
author_rank: Super Contributor
timestamp: December 14, 2018, 04:17:13 am
content: Come on, wouldn't be fun to hear your music with a tempo of 100bpm slowed down to 50bpm?Anyways, I left the video to give the OP some ideas.I might try removing the wind-up spring, tie a super weak DC motor to the governor and give that a try. With one of those cheap super-low power 1.5v dc fan motors, you know, the ones which can be powered by a small solar cell, the music box should run for months non-stop on a single D cell battery.

author: Stray Electron
author_rank: Super Contributor
timestamp: December 14, 2018, 06:36:04 pm
content: Find a larger spring motor out of a toy or an old wind-up clock. Some of the better clocks were supposed to run up to eight days on one winding. I used to have a clock from a B-24 and it would run for days and it had four or five dials on it. I don't recall exactly how long it would run but I know that it ran for several days at least. Unfortunately, I mounted it in a hot rod that I built and sold it with the car when I left CaliforniaSome of the old cars also used spring wound clocks too. I still have one around here somewhere out of my old MB 190SL. Check Ebay.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 14, 2018, 06:48:05 pm
content: There is some complexity and cost going to be involved in whatever solution you choose.Another option would be to steal a trick from at least some car clock radios. They had a spring driven clock mechanism with a spring that only wound a half turn or so. The clock would run for several minutes on that half turn. When the spring reached the end of its half turn a contact closed and a solenoid wound it back a half turn.You could implement something similar with your music box situation.Personally, I would look into DC gearmotors. Small ones with torque capabilities comparable to music boxes are cheap (same price range as music box motors) in surplus sites, and commonly have an output shaft running from a few to a few dozen rpm at 12 v. Pick a voltage that gives the rotation rate you want and the electronics becomes a switch to turn it on and off.

author: Rick Law
author_rank: Super Contributor
timestamp: December 15, 2018, 09:50:25 pm
content: Quote from: CatalinaWOW on December 14, 2018, 06:48:05 pmAnother good target to try may be timer motors that old appliances use.For example, this old Whirpool/Maytag Washer Timer Motor 418-080-20, it runs at (according to the ebay page) 1/9.26 rpm. You would still need to gear it to the time you like.https://www.ebay.com/itm/Whirlpool-Kenmore-Maytag-Washer-Timer-Motor-418-080-20/113398504256The one I linked to above is rather expensive. On my old (now replaced) washer, I was able to find the whole timer box (timer+gear+relay) at less than $15. Something like this new one:https://www.ebay.com/itm/Kenmore-3351119-Washer-Timer-OEM/153178573357If I recall correctly, both my dish-washer and cloth-washer ran on a 1rpm timer motor according to the number printed on the motor. I did my cloth washer twice not because of motor failure but because the shaft to the end-user knob that sticks out keep breaking. The motor was still running fine after 15+ years. Both time, I found the box at around $15 on ebay. So I think you may be able to find a lone motor (not in the gear/relay box) cheaper than the one listed in my first link.