Ball bearings for a "Newton's Cradle"

Hi,

I saw a home made Newton's Cradle once. I think that the builder just drilled the bearings and threaded the wire through. It didn't work very well as the balls didn't hang perfectly in line, so they went off to the side slightly, rather than swinging in line. Obviously needs to be constructed very accurately.

I think I would be tempted to drill holes and insert some cross drilled posts. Glue them in with Locktite ? Maybe also glue the wire into the posts so that the balls can't veer off to the sides.

Cheers Mark

I'm just worried about drilling the bearings.

I was under the impression they are case hardened and virtually impossible to drill.

Maybe if I broke the surface, it would be easier

If it comes to it, I can buy one from eBay for about 6 quid, delivered. But that's not the way I want to do it.

I hear that they're very practiced in that technique in Chile...

Regards, Tony

That would be a *WHOOSH* you just heard then.

together).

bearings?

The one that I have (sad person that I am) on my study mantlepiece has holes drilled in the balls, and the strings glued in. So far it has survived since the 70's including onslaughts by four children and now the next phase - grandchildren. !!!

AWEM

I attempted to silver solder a bearing to make a tooling ball, but by the time it was hot enough to melt, the surface of the ball was flaking. Drilling with a carbide drill was also not successful, and I ended up using a spark eroder. If you have a small company near by that does wire eroding (or toolmaking), they will be able to Hole Burn them for you. It will only take 5 minutes to set up and then seconds to actually put the hole through each ball.

I wouldn't go near the job with someone else's bargepole and especially if you can buy one for 6 quid. The chances of making it so the balls hang accurately enough to work properly is minimal IMO.

-- Dave Baker - Puma Race Engines

Turn some small 'cups' from brass, or stainless if the colour matters to you, drill these crossways for the wires and cyano them to the balls. The closer you get the cups to matching the radius of the balls the stronger the joint. I've seen ready made ones done this way. I would grind a tool with a round nose especially to make the cups, after that it's a simple turning and drilling job.

Greg

Excellent idea - I'll substitute epoxy for the cyano, and then it'll gap fill.

Thanks very much

You're welcome, there is cyano that will gap fill when used with an accelerator but epoxy is simpler.

Greg

Better still, grind a small flat on the balls then you don't have to machine a cup, plus the grinding marks will make a better key for the adhesive.

[snip]

This picture is of a larger example :-

I've seen a better photo in print, and the balls used are plastic 10-pin bowling balls. From that evidence alone, I'd guess that "dead hard" is not required.

Conservation of momentum is what's required. Come to think of it, isn't that what snooker balls do best ? (and they are easier to drill than steel BBs).

Snooker balls :.))

I can do that!

I thought dead hard would be required to transfer the momentum through the row - otherwise deformation would lower the efficiency.

But perhaps not.

'Fraid not. Momentum is always conserved in 'all' collisions in which no external forces act on the bodies concerned. There is however a significant difference between elastic and inelastic collisions. In the latter some of the kinetic energy is converted into heat, sound, deformation of the objects etc. In the former it isn't. The difference between elastic and inelastic collisions manifests as a difference in conservation of kinetic energy.

So what we actually want here is the best possible conservation of kinetic energy - not conservation of momentum which would happen even if the balls were made of plasticene and stuck together on impact. In turn that implies we need highly elastic collisions and we measure this with the coefficient of restitution (relative difference in velocities before and after impact). The perfect material for a Newton's Cradle would therefore be balls made of diamond. Glass or hardened steel balls perform acceptably well at somewhat lower cost and with a lesser tendency to shatter on impact :)

Come to think of it, isn't

I don't have figures but I suspect the coefficient of restitution of colliding snooker balls is somewhat less than glass or steel ones. A balance has to be struck between attaining the ideal coefficient of restitution and not shattering under severe impact.

-- Dave Baker - Puma Race Engines

My daughter's comment:

"I want one"

anyone know where I can cadge half a dozen bowling balls?

Seriously - that would be a cool (k3wl?) garden ornament.

Ebay ??? It would definitely look good in a garden !!!

I was thinking about making a giant 'Magic Tap' to be run from my Lister D, but haven't found a source of clear pipe big enough. Anybody got any ideas ?

Cheers Mark

On the r.c.m group a while ago there was some nut making a windchime out of propane cyclinders - how about 5 of those 15Kg bottles suspended side on?

:-)

Steve

you use super balls you can just use screw in eyes. :rolleyes

-- olmo

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