Stress concentration on a tube. 1

[Biggadike]

Here is my problem:

A rotating steel/aluminium tube is under a known (high)load which is caused by a weight (casuing oscillating stress). The tube has longitudinal (parallel to axis) ob-round holes milled into it.

Does anyone know the stress concentration factor of the ob-round holes in such a situation?

I know that the tube is now not now a pure form but short of FEA, it would be good to get some first order calculations.

 

[unclesyd]

You might try:

http://www.x-calcs.com

They have series online calculators that may deal with your situation.

Design of Weldments by Omer Blodgett
This is a very handy and outstanding book and doesn't cost much ($12.50). 500 hardcover pages of formulas and diagrams.

https://ssl.lincolnelectric.com/foundation

 

[Biggadike]

I had a look at xcalcs.com (its unhyphenated) and interesting though it was, it couldn't answer my problem. I already have a great deal of data on beam calculations, the problem with this situation is the stress concentrations (and loss of strength) due to the longitudinal slots.

 

[unclesyd]

Sorry for the missed direction. I'm still trying to revive my memory as to where I saw some discussion on the subject of "lightening holes". I thought the book by Blodgett had some info on holes in tubes, but it is for slots.

What makes this hard for me is that one time we had several machines operating with oblong slots in hollow steel shafts. Unfortunitly I didn't work on the shaft design, just the ends.

The SIF's of oval/oblong penetrations for branch connections are used in piping analysis. As I remember the SIF can become quite large and are a big consideration in fatigue analysis. If only we had rotating pipe.

One question, can you give any information concerning the size and tube speed. If as you say it is oscillating it's going to fail due to fatigue. Just how long it will last is the question.

 

[Biggadike]

The basic arrangement is this.

A steel or aluminium tube is held on supports 759mm apart. The OD is 69 DIA and the ID is 57 DIA. The tube has 6 ob-round holes measuring 15 x 141mm. They are in three opposing sets. One is central and the other two are symmetrically placed starting 150mm from the support. The sets are 90 degrees offset and aligned to the central axis.

The load is 500kg and the rpm at max load is 36. The material unwinds giving a min load with a max rpm of approx 200 with a load of approx 90kg.

The failure will be by fatigue as you say. I know that nitriding or similar surface finsihes will improve this but without some figures to go on I can't change the design (as I have to) with any confidence of performance.

 

[prex]

Hey, if I understand correctly your description, in the middle section of the pipe you have two quite long slots 180 deg apart, so that the pipe section there is formed by two unconnected half circles: did you consider this in calculating the basic stress (apart from SIF)?

prex

http://www.xcalcs.com

Online tools for structural design

 

[Biggadike]

prex, you are spot on. The situation is structurally very unpredictable (without FEA) and I am sure that the slots are in triaxial stress with the forces trying to 'peel open' the slots. Most SIFs are based on plane stress as far as I know.

What I am trying to do is get some kind of first order calcualtions which I can compare to the performance of a known and tested design and therefore have a benchmark for assessing proposed future design changes.

It actually gets even more complicated because this tube is supporting a reel (of paper) which may in turn give some resistance to bending in the tube.

My normal method is to assume worst case for everything and design from that but in this instance that would just tell me that the design I have deosn't work - it does work although it will eventually fail by fatigue.

It may be appropriate to work out the second moment of area of a 'c' shaped beam and go from that.