a question about springs

A simple spring won't do that.

Can you provide any more details of what you want?

I wanted to make a table with holes on top and cylenders below the holes with a springs in it so that whenever u put a pack of CDs on it one pack alwaysremain on top of the table but after putting the second one the first one goes down and the second takes its place.

That might be a bit difficult to do with an open top. But it would be really simple to do if you think (conceptually) about how a magazine for bullets works. Or a kids PEZ candy dispenser. That is assuming that you want the packs of CDs to push back up as they are removed.

If you do not need it to be repeatable, you can use various coatings on springs to make them rigid to a predictable load.

Once the critical load is exceeded, the coating breaks, and it becomes just a spring.

Such a thing might be most easily acomplished by putting tie bolts or bars across the spring or by other means capture it and retain it in a partially compressed state.

As you load such an assembly, the spring will not compress apreciably until the preload is exceeded.

Have FUN!

TT3

would u give me more info?

Arresting cables would be the easiest way to accomplish this. Picture a coil spring. Partially compress it to your desired pre-load. Now attach cables from either the bottom coil or the spring support to the top coil or load plate. The cables will hold the pre-load and the spring will only further compress under a load exceeding the pre-load.

Another option would be to have your load plate be captured on a set of guide rods or runners. Placing a top-stop on your guides that restrains the spring in a pre-loaded state would accomplish the same effect.

In either case, be sure to label your final product with a captured-energy warning. Anyone disassembling the device could be injured be a sudden release of the pre-loaded spring.

Good luck!

Register and I'll think about it.

Easy to do in hydraulic springs

Pressure relief halve relieves at the set pressure and then onwards it may be controlled. Of course you need a bit of hydraulic circuit to do it.

You're kidding.

GA! Navy messes use a similar technique where the top plate stays slightly below the top of the container so that ship motion doesn't dump the plates.

Back to the OP: All you need is Hooke's Law and a uniform weight for the CDs.