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: elastic disc stress
: elastic disc stress -- sean	Post Reply	Top of thread	Engineering Forum

Posted by: jem.cadd ® 01/15/2011, 08:48:46 Author Profile eMail author Edit

Ha Ha, nice brain teaser. The next step is to prove why the x-sect perimeters cannot be equal.

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: elastic disc stress
: elastic disc stress -- sean	Post Reply	Top of thread	Engineering Forum

Posted by: Pinkerton ® 01/14/2011, 19:37:03 Author Profile eMail author Edit

Hi Sean, I am somewhat confused. First off, a disk cannot have Fillets, it will have external radii to represent that hot dog profile. Fillets are the domain of internal corners. If there is no pressure then why would there be Stress? Is there pressure on the outer surface of the sphere? If so, is it uniform, partial or point load? is this something that is lowered into deep water etc and the water pressure is applied to the sphere loading up the polymer at the external corner radii? Is the disk trying to move out of the sphere? Are you talking about Stress due to shrinkage of the sphere during manufacture or what? Maybe it is me, but I do not understand what you are trying to evaluate. Also, how are you adding the Polymer? If it is in an injection molding machine, how will you hold it in position? You will be surprised at the speed, pressure and force generated by the incoming plastic material. Dave Modified by Pinkerton at Sat, Jan 15, 2011, 09:39:27

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: : elastic disc stress
: : elastic disc stress -- Pinkerton	Post Reply	Top of thread	Engineering Forum

Posted by: sean ® 01/17/2011, 10:07:33 Author Profile eMail author Edit

Alright Dave, I will attempt to clarify for you. First off, my apologies on the use of the term "fillets" (semantics and used by my CAD software to refer to both inside and outside radii, technically referred to as rounds - I hope that helps you.) To clarify further - and this I believe is the true source of the confusion - the sphere is hollow, thin walled, flexible, and of an elastomeric material so that it can stretch to take the shape of the outside surface of the disc. Because the cross-sectional perimeter length of the disc matches the internal circumference of the sphere, the sphere must stretch to envelop the disc. Because the sphere must stretch, it experiences stress and strain (thus there is no need for gas or hydrostatic pressure for stress to appear - the same is true for materials under tensile load.) I am trying to evaluate how much stress is experienced by the sphere, it is useful in evaluating forces applied to the disc. Don't worry about how I will be getting the disc into the sphere, I will worry about that. Sean

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: : : elastic disc stress
: : : elastic disc stress -- sean	Post Reply	Top of thread	Engineering Forum

Posted by: RWOLFEJR ® 01/19/2011, 14:51:19 Author Profile eMail author Edit

I don't know about anybody else... but now I'm really, really, curious as to how you're gonna get that donut into the rubber ball. I get what you're trying to calculate... You won't be stretching the entire ball since you're only contacting it around the outer edge of your donut. Your value will be some amount less (good bit less) than the force required to expand the entire ball by the same amount. Somebody correct me here... I could be wrong... I'm not familiar with the modulus of elasticity of rubber etc. and don't have the time to fool with this much today. I think you should get a pretty close number if you can figure how many psi to expand entire ball to diameter of donut... then percentage of total square inches that will be in contact with donut. If ball has 100 square inch i.d. surface and 100 psi to move it to diameter required... and say donut contacts 25 square inches then 1/4 of pressure or 25 psi. (I'd guess 25 would still be a bit high... probably some loss in pressure on donut since remainder of ball will also move some to accomodate donut.) But fairly close. You'll have other variables that will weigh pretty heavy on getting an accurate number besides this. Mainly any variation in wall thickness of ball and durometer. But again... I NEED... to know how you're gonna get that donut in the ball. Good luck, Bob

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