press fit dowel pins 1

[proEdj]

When calculating the stresses created when press fitting dowel pins how important is it to stay within the yield stregth of the material? I am having a difficult time achieving this when using 316 stainless steel plates and pins. I have speced my pre drilled dowel pin hole all the way down to +.0000 -.0003, to garantee interference of .0001 in the worse case, but in doing so I cannot achieve a safety factor in excess of 1.0 for avoiding plastically yielding the material. Within reason I don't think this is an issue. The material still has some elasticity in the plastic region so my pin should not fall out following installation, however I think there is some uncertainty to this approach. I am using a yield strength of 30 ksi for 316 stainless, which is the minimum, so I may be acting too conservatively. Please comment if you have had experienced this issue.

 

[CMfgE1]

I think you are over engineering the problem.
If I read you correctly, You are trying to create a press fit so your pin does not fail either by falling out or breaking.
What is the application or purpose for press fitting the pin?
If you keep the principles simple it might be easier.
Ill assume your measurements are in american standards (inches) and not metric standard (millimeters). Because that would be an extremely close tolerance.

A .000 to -.001 tolerance for press fits is pretty standard in any dowel holes I have drilled. Mostly they were for locating a mating part. Sometimes used to position a part for machining. The Machinery's Handbook is an excellent reference for any machining tolerances. It will explain in detail the principles behind the design.


Good Luck

Quote: "Its not what you know, its who you know" - anybody trying to find a decent job

 

[ccw]

Hi proEdj,
Look at the % elongation to failure from some actual material tests or mill cert. reports. Some of the 3XX SST alloys perform out to and beyond 50% elongation and over a relatively wide temperature range. This should give you some comfort in working beyond initial yield with these materials.

We developed 304 and 304L freeze plug seals (LN2 quenched interference fits) with planned 0.010 inch interference in a 3.5 inch nominal diameter configuration without post-yield problems.

 

[unclesyd]

Is this a practical application or a design application using a computer simulation?

The diameter and tolerance you are calling out cannot be achieved by drilling alone. Y ou would have to size the hole by another means like reaming, broaching, ballazing, etc.

As ccw states the best way to use SS dowels is to use a drilled, good, and LN2.

 

[proEdj]

Ok. To clarify my situation a little further, I am drilling then reaming my dowel pin hole to be .2500''+.0000/-.0003. My dowel pin's diameter is .2501-.2503. This way in the worst case senerion I have an interfence of .0001. The most interference I have would be .0006. Using stress fomulas based on this maximum deformation a total stress of around 38 ksi results in the wall of the hole. Because this, exceeds the minimum yield strength of 316 stainless (30 ksi), I am concerned that the hole will plastically yield to the point where there will be no permanent compressive stresses on the pin, hence the pin will fall out if the plate housing the dowel pin will is turned upside down. I hope I was clearer this time around. Thanks for the input.

 

[unclesyd]

Do you have any idea of the load on the rolls?

What is the callout, grade, or standard on CI?


At your stress level I don't think you will have problem with the dowels falling out. As stated above I'll also say that you will find the min yield of your material greater than 30 KSI.
It's been a long time since I work with interference fits so somebody correct me if I'm out in left field.

You don’t need to worry about stresses on the dowel. The stresses on the plate diminish very quickly as you leave the hole. The interference pressure should be quite low in your case.

I have references to Shigley several places in my notes so you might get a copy. Some can help on the book as it was about 1987 that I scribbled these notes.

 

[arto]

Usually use 1.6x for bearing allow. - you would still have 30ksi elastic compressive stress, plus work hardening a bit to increase yield
With 316 pin in 316 hole you will get galling
use Nitronic 60 pin - Neverseez would probably squeeze out

 

[ccw]

Hi proEdj,

Let me rephrase the last statement in my previous response. We did have post-yield in the seal ridge area on the freeze plugs. (There was a visible impression or groove left in post-mortiem examinations of the adjacent sealing surface which means that the material exceeded yield and went somewhere); AND, there were no problems with loosing, or loss of seal, or structural integrity of the plug or the surrounding material.

Again, consider the post-yield performance of 3XX SST particularly the plate stock you are working with (50 to 55% elongation). How about this analogy? Buy some taffy candy. Buy a new shirt. Take the twenty or so pins out of the new shirt and force them into the taffy. Did the taffy split apart or show signs of cracking? Was there post-yield of the taffy? (Ans: yes out along the axis of the pins). Do the pins loosen or fall out? Does the taffy have good post yield performance, ie. high ductility?

Don't be concerned with those small local areas where (Mechanica?) shows stress exceeds yield unless it is a structure that undergoes numerous load and stress reversals. Even though there can be local flow of material immediately around the interference fit, there is a prevailing load back onto the pin generated by all of the elastically strained material around the hole, beyond the small area that shows post-yield stress. You can experimentally verify this by using a special strain gage for measuring compression strain. 1) Insert the pin, 2) apply the gage over the pin and hole and null out the instrumentation, 3) drill out the pin and 4) measure the residual strain in the gage.

Also consider this: "Ductile materials under compression loading merely swell or buckle without fracture, hence do not have a compressive ultimate strength." In addition, those troublesome safety factors you mentioned are not intended to be applied as a blanket to *static* loaded transition areas in highly ductile material where localized post-yield material flow can redistribute and reduce stresses in a safe manner.

 

[proEdj]

Thanks a lot everyone...that was very helpful.

proEdj

 

[jbel]

If you press fit a .2501" pin into a .2500" hole it will not fall out when turned upside down.

Press fit by definition means it needs to be pressed in and therefore interference will prevent the pin from falling out.

Go to Machinery's Handbook and it will give you just about all the practical information you need on th esubject

 

[corrosionman]

There are lots of dowel experts reading this topic. Please can I come in with a Dowel query. We accurately locate large bearing units by dowels something like Bissel pins except that the hole through the middle has a very slow taper and a tapered pin expands the dowel - - - it thereby creates a perfect fit which is sufficiently tight to lock everything together. (the tapered pin has a small threaded length at one end for pulling in the pin.

Does anyone know who makes these locking dowels.
proDjd I hope you do not mind me hijacking your thread with a query of my own ?
Regards DW

 

[ccw]

Hi corrosionman,
Punch in Carr Lane in google. Check under products/ alignment pins/ expanding pins, and see if that is what you are looking for.

 

[unclesyd]

Here is a source of the type dowel you require. The second site is the approch we use a lot.

http://www.sbwhistler.com/met.htm

http://www.spirol.com/apps/internet/spirol/website.nsf/DowelsBushings!OpenForm

 

[arto]

How about Superbolt radial expansion bolts?

http://www.superbolt.com/articles/machine_design_1997.htm

They might be able to make a blind hole version if you need one