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Roller Mill

Roller Mill

Roller Mill

(OP)
I am designing a two-roller mill press (imagine an old washing machine that wrings out clothes), and I need to make the rollers out of tube rather than keep them solid (for a variety of reasons).  My question is: how do I calculate how thin the walls of the tube that I make the rollers out of can be so that I can make the design as robust as necessary without the possibility of collapsing the tube while minimizing material in the roller?

I have all of the necessary parameters: roller OD, material properties, anticipated forces, etc.  I just need some general guidance on what calc. to use.  Thanks.

Dan

www.eltronresearch.com
Dan's Blog

RE: Roller Mill

Dan,
If you design to minimize the deflection in the tube due to the lateral forces exerted during the rolling (like a beam analysis), it should override the torsional forces, so the tube can handle it as long as the bearing/bushing attachment at the ends can handle it.

How much lateral deflection can you tolerate?

Ron

RE: Roller Mill

(OP)
I shouldn't have much lateral deflection at all since the rollers are only 16" long and 8" in dia.  I'm just concerned about putting a crease in the tubes if I thin them out too much.

Dan

www.eltronresearch.com
Dan's Blog

RE: Roller Mill

This may come down to trial and error.  I have seen well designed solid rollers get abused and bent or broken.

Many designs use backup rollers to help the main rollers.  

What are you rolling and what forces do you expect??

RE: Roller Mill

Dan,
What is the thickness of pulp (downstream) and the clearance of the rollers? (Yes, should be the same; however, the load required in the pressing will exert a lateral force...thus slight deflection).

Ron

RE: Roller Mill

Where is the "fuse" for the rock or other unmillable object that will inevitably find its way in?

Is it the roller itself, or something else?

RE: Roller Mill

You have all the parameters and loads for the design, all you need is the cookbook recipe to design it, right?  Why keep all of those important design considerations secret, but then ask how do I design this thing that you guys know almost nothing about?  It's a simple beam btwn. the bearings, with a drastically varying cross sectional stiffness and shape, with the center 14" loaded with a uniform line load, centered on and aligned with the shaft axis, plus some torsional and shear loading.  The thin wall thickness will give you fits and a secondary deflection.

Are you squeezing the water out of a extruded shape of pulp to end up with a fairly dry, and firm 14" wide strip of paper that's .03 - .06" thick.  How do you form the edges of this strip or prevent the extruded shape from expanding laterally when going through the rollers?  What is the compressive strength of the .06" paper strip material, right out of the rollers?  Where did you get or how did you arrive at ~200psi?

The rollers are both driven, and at the same surface speed, right?  So you don't introduce a shearing force into the paper.  You say 16" long and 8" O.D. with thinnest wall possible.  How do you know the roller material before you do the design?  Why 8" dia. and as light as possible?  Why not 6" dia. and much thicker wall?  You get a much better roller surface and a stiffer roller.  Then how do you put ends on this mechanical tubing, which are fatigue resistant, and provide for end bearings and a drive system, and won't defect to much?  I venture to guess that you will end up better off buying two pieces of a high quality 7" round stock, and machining your rollers out of them.  Or, go for thinnest tube wall possible and start a fatigue testing program, and get paper which is .06" at the center and .04 at each edge.  What's the reason for the thinnest possible pipe walls, because this fatigue testing program could get pricy.  

RE: Roller Mill

Hello Dan,

Why not give us the roller OD, material properties, anticipated forces, etc.   And, while you are at it, provide a sketch of the rollers and forces.  The calculations may be  relatively straightforward, but we need a little more information.  Thanks.

BA

RE: Roller Mill

As the OP suggests, this isn't a simple beam calculation of a tube section under load, but one where the local loading on the tube is critical to the design. Personally I'd use finite elements to assess the localised stresses and restraints to the tube, but as a hand calculation you might consider one half of the tube section as an arched beam under load.  

ex-corus (semi-detached)

RE: Roller Mill

(OP)
dhengr, I wasn't asking anyone to design anything. I just wanted to pick some brains about what essentially amounts to a tube-crushing calc. not a simple beam calc. (as corus pointed out).

Anyway, if anyone is curious this set of rollers is part of a retro-fit of a test rig in a partner's facility.  The rollers aren't driven, and the force of 200lb. is their industry standard.  I'm making the roller out of tubing for several reasons which I don't really want to get into, plus the current rollers are made similarly.  There's nothing too top secret or too bizarre about the materials, but I didn't add the whole of my proposed design because I don't want to turn a quick question into a Phase I project.  I did some FEA on my models, and it seems to match what experience tells me should work.  However, I just wanted to see for my own benefit what calcs. folks would use.

Thanks.

Dan

www.eltronresearch.com
Dan's Blog

RE: Roller Mill

Look in Roark under Plates and Shells. There's a case there of a curved shell with a central load. That might be similar to what you have and give you an estimate of localised stresses. Buckling is covered in the elastic stability section.
I think any hand calculation you do though will just give you a 'feel' on the true solution, which is more accurately obtained with finite elements (if used properly).

ex-corus (semi-detached)

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