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SS Bar Bending

SS Bar Bending

SS Bar Bending

(OP)

I have an application where I am specifying a SS for a 1" bar ~5' long that will be subject to a bending load (i.e. supported on the ends and loaded in the middle).  I am looking at 303 (drawn to 100,000psi), 416 H900 temper & 17-4 H900 temper.  These materials are being evaluated based on corrosion, machinability, appearance, tensile strength & hardness for my application.  What about bending?  When I compare Modulus of elasticity's for each of these they are about the same in MPa.  

Does this mean each bar will bend the same amount given identical loading below the yield strength?

RE: SS Bar Bending

To a first approximation that's correct. Modulus is dependent on composition, not structure. The differences in composition are minor from a modulus point of view. The cold drawn bar, however,should be stress relieved to give a clear yield point. Without stress relieveing you'll get a little creep at room temp at less than the 0.2% offset yield.

Michael McGuire
http://stainlesssteelforengineers.blogspot.com/

RE: SS Bar Bending

Yes, E is very nearly the same for all the alloys you mention, and for all steels as well.

If deflection is important to you, as it seems to be, fatter bar (or tube) will work better, even in less expensive alloys.

If you could describe your application in more detail, you could get a _lot_ more help with it here.  

Same charge as always.



Mike Halloran
Pembroke Pines, FL, USA

RE: SS Bar Bending

Just wanted to mention that there is no such thing as 416 H900. The H900 is an aging heat treatment and it is only for Precipitation hardening alloys such as 17-4PH, 15-5PH, etc. 416 is heat treat by a heating and then quenched and tempered just as low alloy steels such as 4340, 4230 etc.

The impact strength of 17-4PH, especially large size bar in the H900 and H925 conditions, may be very
low at subzero temperatures; consequently, the use of 17-4PH for critical applications at low temperatures
should be avoided.

RE: SS Bar Bending

(OP)
Thanks much for the terrific feedback, it is very helpful.  I have a high volume consumer product that I have only prototyped to date and I am trying to hone in on a material and manufacturing process.  One of my selection criteria was resistance to bending as I described.  Now that I understand that for a given cross-section most Stainless Steels will bend the same amout I can rule out "bending" as a selection criteria.

I am going to start a new thread stating all my requirments for this product as I need help with selecting a Stainless Steel, manufacturing process, and finishing process.

RE: SS Bar Bending

Since defelction is based on E and section modulus you will also find that heavy walled tubing is almost as stiff as bar.  Roughly speaking a 1" x 0.200" tube will have about 87% of the stiffness of a 1" solid, but it only has 64% of the weight.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: SS Bar Bending

(OP)
My cross section is a an octagon shape that is 1.0" flat to flat.  (4) of the (8) sides have a 0.9" deep by 0.35" wide channel machined into them.  The overall bar length is 62.0".  

Is E a function of cross-sectional area?  If the tube is that stiff there must be some other effect from the geometry.  What could I do to my "shape" to make it stiffer?  I thought increasing my tensile strength by going to different grades of stainless steel was the answer but I now know that is not the case!

RE: SS Bar Bending

pull your strength of materials book and look up calcualting the section modulus.
the section mod is based on geometry.  Knowing that and the materials modulus you can figure out the actual stiffness.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: SS Bar Bending

For this shape and length, have you looked at an aluminum extrusion?  You will get the final shape from the mill, (no milling, grinding, etc) aluminum is not as stiff as steel but you can put the metal where you need it and remove it from where it is less useful.  Do you have a maximum load and deflection to work towards?

RE: SS Bar Bending

The Al will be too soft and flex too much.  He has built around an iron based material, It sounds like he is stuck.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: SS Bar Bending

(OP)
Ed is correct.  I would like a tensile strength of 150,000 psi if possible (see my "Material Selection Challenge" thread).  I may be able to accept slightly less.  I would love to use aluminum from a forming standpoint but I need the weight, hardness & tensile.  

RE: SS Bar Bending

If the face channels are 0.9" deep, they have to meet each other.  I hope you dropped a zero there.

If you need 150ksi tensile, you're pretty well screwed out of stainless, too.


Mike Halloran
Pembroke Pines, FL, USA

RE: SS Bar Bending

(OP)
Yes, the channels are actually 0.09" deep, thanks.  

I found spec. sheets for 416 & 17-4 PH on www.alleghenyludlum.com that both show 150 ksi plus for these tempered at 900 deg.

RE: SS Bar Bending

I don't know the exact end use but 17/4 PH SS @ H900 isn't a good choice, especially if people are involved.  To use 17/4 PH SS I would not use it for an application like you propose unless it was heat treated at H1125.

RE: SS Bar Bending

Must be a really high end consumer product, to afford a long exotic bar like that.

Mike Halloran
Pembroke Pines, FL, USA

RE: SS Bar Bending

(OP)
unlesyd - Why not use 17/4 PH SS "if people are involved"?  The bar will generally not be grabbed by peolple but occasional hand contact could occurr.  Why H1125 instead?

The bar will be used for a high quality fitness product.

RE: SS Bar Bending

The reason for my recommendation is based on our experience with 17/4 Cond A and some recommendations by Armco after some catastrophic failures of same.  Our failures were in  shafts and special fasteners that were fabricated from 17/4 Cond A despite recommendations to the contrary.  People were involved in one of the shaft failures and the recommendations from Armco, incorporated into our standards, were that 17/4 Cond A was not to be used in open machinery, such as ours, where personnel were in proximity.

By analysis we found that the published physical values for 17/4 Cond A were quite variable.  We found early on that if your design required ductility and toughness you had to heat treat.  The recommendation that H1125 heat treatment was put forward by Armco early on to maximize the physical properties for shafts and fasteners.   

17/4 doesn’t machine or cold work very well in Cond. A

Personally I’ve never seen or heard of a recommendation to use 17/4 in Cond A in a dynamic component.  
  

RE: SS Bar Bending

If you really need that strength level use 17-7 in the RH950 condition, but the TH1050 should be fine.
I just don't seet he reason to go to that expense with this product.
Either 201 or a lean duplex in the cold finished condition should be enough.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.
http://www.trent-tube.com/contact/Tech_Assist.cfm

RE: SS Bar Bending

(OP)
Thanks again for all the great input.

SS is chosen for it's high quality appearance and resistance to corrosion.  The 3 original reasons I was looking to increase tensile of the bar above our SS 303 prototypes were as follows:

1) Less bending under load (I now know Modulus is the driving factor)

2) Increased surface hardness.  I have 1/4" wide bearings riding over the flat portions of the bar.  Bearing is SS 440c @ 58-65Rc.  Each bearing is loaded up to 50 lbs ontothe bar.  The 303 began lifting (small chips) quite severely well before 40,000 cycles.

3)  This bar will be lifted during exercise.  I must guard against fracture if awkwardly dropped onto the floor.  It will take abuse.  A large safety factor is prudent.

For these reasons I am targeting a 150,000psi tensile.  I may be able to go under this but must run cycle testing and analysis to validate.

My thread has wondered a bit but I need help defining a cost effective material and process to produce bars in volume.  (see Material Selection Challenge thread)

Thanks to all!!!!  

RE: SS Bar Bending


Regarding your #1:

Deflection of a beam (channel or tube are hollow beams) is more a function of the Moment of Inertia (I) than the modulus of the material.

Example:  an I-Beam . . . increase its stiffness by increasing its height rather than the web width.

Reason:  I= cube(h) * b * 1/12

Another way to look at deflection is through bending equations like :

y = P * cube(L) / 48 EI

for a simply supported 3 point bend.  

So, deflection is inversely proportional to I and E.  To decrease y, increase I or E.

But you can't control E since steel is 30,000 ksi and most stainless steels are 33,000 ksi and aluminum is 3,000 ksi.  You can always control the Moment of Inertia through part geometry.

Strength and striffness are not the same.

You rule out aluminum alloys that would require only a modification to part geometry or the addition of a supporting internal steel channel.

There are filled and reinforced polymer materials that are sometimes more cost effective to manufacture after capital amortization.

If shiny is what you want, why not chrome plate steel?  There is also an acid chloride zinc plate that is just as shiny as chrome and adds the corrosion resistance value.

Regarding #2:

Ever think of placing a less frictional material between the rolling part and the tube?  Acetal (tradename Delrin) is used a great deal to reduce wear on moving parts, which by the way is the definition of "bearing".  You can machine a rod of acetal to have an ID equal to your rolling parts OD.  50 lb-f is not sufficient enough to cause compressive failure, as long as the acetal is thick enough . . . see Moment of Inertia above.

Regarding #3:

On what formula are you basing the need for a strength of 150,000 ksi??

Remember that strength based failures are one of only three types:  tensile, compressive, or shear.

In bending a beam is placed in a state called flexure.  This is a composition of tenile (along the bottom beam axis) and compression (along the top beam axis) and shear (on a continuous profile, usually at the center).  Failure is usually shear.

Have you thought about conducting a shear and bending moment diagram on your part from end to end?  You'll have to know the formulas for shear and bending moment.  One does involve a differential.

Mohr's Circle is another way to determine max shear.

Find a Strength of Materials book or ask for assistance from other Mechanical Engineers.




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