Required wall thickness of a tube

[julieh]

I am not a mechanical engineer, but I am trying to play one on TV......we covered a bit of this in my electrical engineering classes, but not much and that was a looooooong time ago.

We are working to design a steel tube to be used in a spacer with a class 8 bolt fastening system. We are planning a 12L14 steel for the tube. We are trying to get the wall of the tube as thin as possible while ensuring that we will not experience crush or deformation at load. In general, we are looking at something that will be 12mm in length with an ID in the range of 10-13mm.

I have been checking in some of my reference texts and trying to search on-line, but I think that I am not finding the right terms to use to get to the equations that I need to get to my answer.

I believe that I want to have the wall thickness-to-diameter ration greater than 0.1 to ensure that I am working with a thick-walled cylinder. I suspect that I need to be considering Lame's solution...but I am not feeling confident about what direction to go. I also suspect that Young's modulus of steel is also a key factor...more so than the tensile strength of the material.

Can someone point me in the right direction? I am happy to do the math, but I'm not sure what equations/theories to be using. The more I research it, the more I feel I am getting conflicting or erroneous information and just confusing myself.

[PinkertonD]

Welcome Julie,

Firstly, I seriously doubt you will get 12L14 in tube form, but in the make-believe world of TV, I guess it can be conjured up. With it's high Lead content, the 12L14 steel is known for it's free machining qualities and is not a good structural or load bearing material. Given that, Grade 8 bolts to affix 12L14 to anything would be like using a two-handled cast-iron spoon to eat ice cream.

Next, we would need to know the load applied, the supports and the direction of the loads and stresses to be able to even offer a hint.

If this really-is for TV make-believe then why do you need to work the Math?

I have yet to be convinced that this is not homework.

[julieh]

Not to be flip. it is not homework...I just wish it were. I am an electrical engineer who has been pressed into service as a mechanical engineer in a Davenport/Brown & Sharpe machine shop. Dad was the mechanical engineer, but he passed away from a brain tumor a few years back, so it is left to me :(

We will not be purchasing 12L14 tube. We will be machining it from bar stock to gain a small internal feature required by the overall design. I would send a sketch, but we are not as far along the patent process as I woulcd

Do note that I did not reference a Grade 8 bolt....rather a class 8 (metric). Not sure it is overly relevant overall, but just want to be sure we are all on the same page.

We will have a class 8 stud, a tube that goes around the stud as a support (for the surrounding rubber material), and the needed washer/nut to go with the class 8 stud. (We produce millions of "insert" for injection molded parts every year....but I am not the engineer on that, the customer is. I don't know how they determine the needed wall thickness, and now we are involved in a design process.) The load is axial to the stud. Max torque load is defined by the class 8 designation (if I understand correctly).

[PinkertonD]

Hi Julie,

OK, my mistake on the Grade 8, but Metric would be Class-8.8. Given that you stated you were a neophyte in this arena of Engineering I assumed from the single digit that you meant Grade-8. It is not uncommon to get requests here where both units are mixed freely.

Given the lack of details, and I fully understand why, but it is impossible to give you accurate information. What you will need to do is calculate various wall thicknesses for your load until you find a suitable thickness that will carry that load with a sufficient safety factor built in to provide a useful wear life.

Flexing and fatigue should also be a consideration, especially with 12L14.

Since it is 12L14 it is very unlikely you will find charts to help you, so it is the pen, paper and slide rule.

I would start the calculation with the material being solid at the maximum acceptable diameter and work backwards removing internal material. When you are comfortable with the Stress failure figure, that will be your wall thickness.

[julieh]

Thanks. I will roll that around and see if I can get there from here. Although Dad gave me his slide rule and taught me how to use it (just for fun), I will stick with my trusty calculator.

In addition to the flexing and fatigue, we have to be concerned with lead and sulfur stringers causing splits in the longitudinal direction. However, we see 12L14 used in this type of application on a regular basis by our customers. We *will* have the option to change to another material if need be, but of course, price will suffer as a consequence.

Did you know that Ford references their metric class 8.8 fasteners as class 8? I think it is just to confound suppliers. When I questioned their fastener guru on this, he gave me a blank look and thought everyone should just know that they do it that way.

[PinkertonD]

I will stick with my trusty calculator.

I still have my original TI-59 calculator that I bought to replace my slide rule. In the early days I used to validate the calculator with the slide rule, just in case. Just have not had the heart to get rid of the 59, even though I bought a scientific calculator from Wallys the other day for 9-bucks and it does more than the 59.

Well, please drop back if we can be of any assistance.