Rex brought up this, new word to me, with willie providing the correct spelling,
resulting in a afternoon of reading white papers i.e. "Buckling of Imperfect Thin Cylindrical Shell under Lateral Pressure".
A number of thoughts on what eccentricity is from
http://www.bendtooling.com/e-k.htm, Rex and myself.
eccentricity – The deviation of the center point of tube’s inside diameter from the center point of its outside diameter; normally in tube milling the permissible level of eccentricity is specified by a wall thickness tolerance. This applies to the tube wall being thick and thin resulting in the strong (thick) portion of the wall weaving back and forth as you travel down the length of the tube moving the center of strength as you travel down the tube as compaired to the center of the tube. Not something we worry about but the well drillers with a ten mile pipe do worry.
Rex's great description of the center line being offset from the right angle shared endpoints or vertex of the angle. The problem is creating an offset of the diagonal tube centerline or new ray to the angle, eccentricity, creates a lever from the right angle vertex to the offset ray point making it easy for the diagonal tube to break the joint. Eccentricity force increases for compression loading. You should make a complementary angle i.e. 58 deg + 32 deg from the original 90 deg angle. panic can you draw this?
My definition of eccentricity can be applied to an X joint of verticle angles made in the bracing with one long tube and two short tubes and the definition is the offset distance between the two centerlines of the short tubes. Either (1) the offset centerlines with parallel planes like the two rails of a railroad track or (2) non parallel centerlines creating two angled joints or knee joints. panic can you draw this?
Would an offset less than the thickness of the tube wall be OK?
The two main parameters that control the static strength of un-stiffened knee joint under compression loading are the joint angle and the diameter thickness ratio. It is found that the joint strength is more sensitive to a change in joint angle.
My understanding is the joint should be directly across to make a straight line rather than making two knee joints or railroad track centerlines of the two short tubes.
To change the topic slightly back to the fantastic program by panic, thanks! Will the length of the tube change the force applied to the joint with shorter tube applying less force to the joint? Meaning we should keep the tube length short for this additional reason.
And just another term to think about: flattening – The reduction in the diameter of the tube as the lengthening outside radius pulls inwards while being stretched between the clamp die and pressure die. Reduction is mitigated by reducing drag at the point of bend.
I am changing a few of the tubes I have to make the cage stronger after this great thread!
willie can you tell us more about the gusseting? Every joint? Outside like the rule book shows or inside at the center? How big should they be?
Dean, thanks for the TibeMiter program by Giles Puckett of AU
saltfever brings up an important fact about welding skill and equipment. I think the cage should be made correctly with some additional money spent having a pro weld the few joints that cannot be done buy the crew.
Geo
