Science Buddies: "Ask an Expert"

I have been tasked with making a mechanical clock that utilizes rubber bands as its energy source. I am completely stumped and any help would be much appreciated!

Thanks

That sounds like fun. I wish I could build one of those instead of other stuff I have to do.

If I were you, I'd look up "flying pendulum escapement" mechanisms. If you've ever played tetherball you've sort of seen this. A weight at the end of a string is propelled so that the string wraps around and around a stick. When it runs out of string and ball hits the stick, it comes back and starts unwinding. It can then wrap around the other way. If the energy that is propelling the ball and string is coming from an elastic-powered stick of its own, the powered stick will simply advance tiny bits each time the string wraps and unwraps. Make the string the right length and you can get it so the tiny movements happen once per second. Here's a nice video showing a real clock: https://www.youtube.com/watch?v=AKNOnGq0hUQ

Once you now have a stick that can move just a bit once per second, call that the second hand. Put a pulley on that and make another stick that can turn - and put a pulley on that with a circumference that's 60 times the size. But wait - isn't going to be huge? Yep, so do that in two steps. Let's call the stick with the flying pendulum the 'second' stick. Put a small pulley on that and next to it, put in a stick (we'll call that divisor1) that can turn which has a pulley that's 7.75 times the size. Connect the two pulleys with a rubber band. Then add another setup like that. A small pulley on divisor1 and a new one, 'minute' which also has the 7.75x size pulley. What happens then is that for every little movement of the second stick, you get 1/7.75 of that movement on divisor1 and (1/7.75)/7.75 or 1/60 of the movement on 'minute'. And then repeat again for hours. Does that make any sense? I wish it were easy to post a drawing here.

The clock I'm picturing in my head will be 7 sticks stuck into something, 5 of which will be turning at different speeds. I think if you hunt around on the internet for a flying pendulum clock you might see some inspiration. Good luck with your project. It should be a lot of fun to build.

Howard

Hi Howard,

Thank you so much for your help. I really really appreciate it!

I understand the concept of the tetherball, I am just a bit confused about the sticks and the pulley.

Thank you once again,
Georgie

Here's how you use pulleys to reduce or increase shaft speed.



Imagine that each of these pulleys have sticks stuck through them. The bottom of the sticks are in some sort of holder with holes (and bearings, probably) and so are the tops. That means the sticks can simply turn in place. Pulley might be stuck on the stick with the escapement. For every turn of that stick, the edge of pulley A travels 4*pi or about 12.5" The string or rubber band or whatever connecting pulley A to pulley B also moves 12.5". Since the circumference of pulley B is 8*pi or 25 inches, it only turns halfway for each revolution of pulley A and it's stick. That means that the speed of the stick through pulley B (stick2) is exactly half of the speed of the stick (stick1) through A.

Stick2 is also attached to pulley C. The ration in size of pulley C to pulley D is 4/16 or 1/4. So for every revolution of stick2, stick3 (the one through pulley D) goes 1/4 of a revolution. Stick3 speed is 1/4 of stick2 which is 1/2 of stick1. So stivk3 is turning at 1/8 the speed of stick1.

I don't have a good computer sketch tool handy to show the sticks. Take some pencils and sharpen them at each end. Stick a bunch of them into a piece of foam so they're sticking up straight. Slide the pulleys down on the sticks, connecting them with rubber bands. Jam another piece of foam on top. You have to make some allowance for them to turn, of course, but that's sort of what it might look like. http://www.robives.com/blog/rotating_re ... _prototype has this picture of a single shaft supported at both ends. It's using it for a very different purpose but it may help you see this. I'll suggest that dowels from the hardware store would be the right thing, or if you had access to a child's Tinkertoy set.



Getting the ratios set up so the three shafts rotate the correct speed relative to each other may not be too hard. Getting one to make one revolution per minute is the hard part - I think.

Howard

Thank you so much for your reply!

I definitely have a much clearer picture. From my understanding, each pulley, starting at A, acts as a second, minute or hour hand, respectively. I think I will begin putting my thoughts and ideas into action, hopefully it should work after a bit of hard work!

Georgie

Yes, that's what I'd suggest you explore. If you have access to precision wood or metal equipment and can cut gears that's another way, but sticks, pulleys and rubber bands should work and don't require a high level of tools. The reason I suggested using an intermediate pulley to divide your seconds into minutes is that a 60:1 ratio means that you have one really big pulley. If you divide first by the square root of 60 and then again by the same, you get the division you need without a pulley over a foot or two in diameter.

You might come up with another way of converting the motion and displaying the time - the flying pendulum though is a great mechanism for timing. Please write back with questions and especially with progress and pictures!

Howard

Hi Howard,

I have successfully managed to build the flying pendulum mechanism and it works pretty well. I am having trouble with the calculations regarding the pulleys. For the intermediate pulleys, will each one be 7.75 x larger than the previous one? Could I also experiment different pulley sizes by multiplying the initial pulley diameter by a root of 60?

For the minute to hour hand, would I have to work with a ratio of 1:1/12?

Thanks once again,
Georgie

Georgina-

Nice, and congratulations on doing with craft materials what master clockmakers have been perfecting for centuries. After the last exchange it occurred to me that I didn't suggest using roller-skate wheel bearings at the end of the sticks to keep the friction down. Once you have a whole bunch of sticks and pulleys, friction will likely be a problem.

Once you have the stick with the pendulum on it going, it will turn at some number of revolution per minute. That will depend on the length of the string & stuff. The important thing is to time that and see how many rev per minute it makes. You want to slow that down to 1 revolution per minute and that slowed speed will be your second hand. It might not tick evenly once per second but that's okay for a science fair clock.

Once you have that stick turning once per minute you can use any combinations of pulleys and sticks that divide the 1 rpm down to 1/60 rpm. The square root of 60 would make sense because you divide by 7.75 and do it twice. But yes, any other divisors of 60 would work. You could divide by 3.91, then again by 3.91 and a third time by 3.91 and get the 60 with three sticks instead of two. The end result of that is a stick that rotates 1/60 rpm and will give you minutes. Or by 10 and then 6, or...

Using that 'minute' tick and dividing down by 12 will give you hours, yes. Or if you'd rather, divide it down by 24 and your clock will display AM/PM as well.

Howard