Pipe in Bending With Roller Forces. What should I be checking for.

[Colfax]

I have a pipe that is fixed to a base plate and the base plate is anchored to a concrete floor. There are a set of rollers at the top of the pipe (the end furthest from the anchored end) and a second roller set on the opposite side of the pipe anywhere from 48" to 70" below the top roller set. Bending and deflection on the pipe is a no brainer but I am having trouble determining if the roller forces could exceed the bearing strength of the pipe or could cause the pipe to cripple at the roller location. This is especially a concern on pipes as big as 30" OD and wall thickness as small as 5/16". Another issue I need to solve is how much the pipe will indent and how much the rollers compress so I can calculate the force required to move these rollers about the pipe. I know I can get the deformation of the rollers from Hertz contact stress equations but the pipe is what I am having trouble with. I'm not asking for a solution, I'd just like to be pointed to some references that may address the issues I am having so I can fully understand what I need to do. A drawing is attached to illustrate what I am babbling about. Any responses/ constructive criticism would be greatly appreciated. I'm not sure if the image uploaded or not so let me know if it didn't and I will try again.

Thanks!
Andrew

 

[robyengIT]

drwg attached : none

 

[Colfax]

I tried attaching again. The pop up window said it was attached successfully but it also said that the first time.

 

[Colfax]

drawing attached

 

[dhengr]

Colfax:
Your sketch should be standing vertically, and not cantilevered horizontally, I trust? It is an improved design in that it has four tandem rollers on walking beams (equalizer beams) at each load/couple ring location. These will distribute the reaction loads to the pipe better than only two rollers would. Is this actually some sort of a cantilevered pillar crane which can rotate 300̊, or some such? If so, you need a thrust bearing at the top or thrust rollers to take the gravity load into the pipe. Or, if horiz., you would need some sort of a track for one set of four rollers to run in so the unit can’t move left or right in your sketch. With a 30" dia. pipe, you should be able to get inside to weld in an interior reinforcing ring if you wish. You will likely want a top/cap pl. which will act as a stiffener at the upper rollers, to keep the pipe round. And, you will probably want welded external stiffener/running rings, on which the rollers run and wear, rather than wearing the pipe itself. One of these can also be designed and act to take the gravity load into the pipe. The pipe thickness needed to accommodate the base bending moment and load transfer is usually sufficient to take care of the top roller loads as long as you do something to prevent pipe buckling at these point loads, and take care of the other mechanical and wear issues.

 

[racookpe1978]

You've distributed your crushing (kinking) forces oonto the pipe wall by trying to use 4x rollers on each of two connections. But, the center two rollers of each pair will transfer 88-90% of the sideways force: two at the top in compression, two at the bottom in compression as well, but on the other side of the pipe.

Check to local shear and kinking forces on the pipe wall for that condition.
If forces are too great, use longer rollers. The outside two rollers will help, but they will "spread" away from the pipe with increased sideways force, not be forced into the pipe wall.

At 30 inch dia, the relatively thin pipe wall will "almost" act as a flat plate. A smaller dia pipe will act like a cylinder.

 

[Colfax]

Dhengr, you are correct, the pipe does stand vertically and there is a slew bearing on top of the pipe. I have a couple different setups that I chose basically from aesthetics only. I have a 2 roller set up, top and bottom, for light loads on small dia. Pipe with relatively thick walls compared to the diameter. I have two roller sizes to choosee from and I have a 4 roller setup for the small roller and larger roller. I guess what I am looking for is a mathematical/stress approach to determine what setup to use. Is there a certain term or criteria I should search for to find this information? I know if the pipe deforms or the rollers deform “too much”, the force go move the rollers becomes greater. A reference to get me started would be much appreciated. Thanks for the replies!

 

[dhengr]

Colfax:
You already have the slewing bearings, they are the sets of two or four rollers we have been talking about. What you still need is some rollers or a thrust bearing at the top to take the vert. gravity load down into the pipe column. At the top cap pl. a center pin (smaller dia. pipe) and larger dia. roller bearing, along with a thrust bearing can take the place of the top rollers we’ve been talking about. Don’t over do it with choices such as varying roller sizes and dias., except in fairly wide load, pipe dia./thk., boom canti. length ranges, etc. All you may be doing is adding drastically to number of different details for essentially the same parts and assemblies, and to the parts inventory needed. An oversized roller, dia. and width, just works a bit more gently for a lighter load, but you can buy them in quantity and the whole roller, side plates, equalizing beams assembly can be fabed. and sub-assembled in some quantity too. Design for the worst condition in a given range, then back off until things become impractical or uneconomical. You may be able to buy 100lbs. of thicker col. pipe cheaper than design and detail for a slightly different roller.

Roark’s book “Formulas for Stress and Strain” has sections on rings, pipes, columns, all with a wide variety of load conditions, which can be superimposed to make up the total design/stress condition, for a good first shot at hand calcs. It also has a good list of ref. matr’ls. for each section, for further detailed reading and theory. Get some good Engineering Mechanics, Strength of Materials, Theory of Elasticity, and Machine Design textbooks as general refs. I don’t think you will find a specific book or std. on these jib cranes, maybe some industry or ASME stds. and guidelines. It sounds like you might be thinking of making these jib cranes in some quantity, so despite my usual haranguing about always going directly to some software, some parts of this problem might lend themselves well to some FEA for a better understanding of the stress/strain/buckling problems in specific areas.