Cantilever loading

[drax]

I recieved a question that I not sure how to answer. I was hoping that one of you can help me out. We have a 5.5 " OD, 3.25 I.D, 7.0 " tall porcelain bushing that has a connection on top in which a user would connect electrical cables to and the question is What is the permissible cantilever loading (lbf) on the bushings. Does this mean the max. moment applied to the bushing to prevent breakage.

thanks in advance

 

[drawoh]

drax,

Some questions for you...

How strong is the porcelain? What is the maximum permissable stress (rupture modulus?)?

How acceptable is it for this thing to fail? Is a failure going to be an annoyance, or is it going to kill somebody?

How much control do you have over the maximum moment someone will apply to the thing? Will all access to the thing be by highly trained personnel who systematically read and follow all your documents? Will some idiot use your cantiliver loader as a chin-up bar?

How are you attaching the cables to your bushing?

The stress calculations sound simple to me unless your cable attachment is weird. The real problem is working out safety factors.

JHG

 

[TVP]

Cantilever bending on a ceramic material like porcelain is not considered good design practice. The formula for stress is as follows:

&[ignore]sigma[/ignore];_b = (M_b * e) / I

where M_b = maximum bending moment
e = distance from neutral-axis to the outer surface

For cantilever bending, M_b = (F * L) / 2

F = applied force
L = length
I = 0.049 * (d⁴ - d_i⁴)

 

[Viktor]

Do not forget about dynamic effect – how the load is applier/released. Viktor

http://viktorastakhov.tripod.com

 

[kenvlach]

Please define the geometry of "a connection on top in which a user would connect electrical cables to."
Do you have a stub, a hole, a circumferential groove or what? What size? What is the radius of curvature where the "connection" joins the main body?
It seem to me that the stress concentration at this point, plus material properties and loading, will determine your failure.

From looking at a microstructure of porcelain (W. D. Callister, p. 423 (1997)), I suspect there is so much variability in properties (due to porosity, grain sizes of the various constituents) that only way to get properties is to contact the manufacturer. And just maybe, they will have tested a statistically significant number to failure.

 

[sancat]

Correct me if I am wrong, but porcelain behavior is similar to that of a glass, and therefore like most other fragile materials, there is great uncertainty on the value of rupture sigma on the tension side. At least your ignorance ratio (safety factor) should be high, some glass situations call for factors as high as 1000.

 

[brad]

One other note--as this is 7.0" long and 5.5" diameter, this is too thick to qualify as a "cantilever beam"-- bending will not be the primary mode of deformation (shear forces will be far more significant).
Brad