20mm Round Rail Strength??

[albertsteptoe]

Hello

Can i please ask your advice.

My machine will be 40" long, 28" wide.

I would like to use 20mm Hardened Round Rails for the supports but worried they may bend under the load?
The maximum load weight on the lower rails will be approx 7Kg - 8Kg.

I have spoken with two dealers, one said the rail will not bend and the other said they will bend up to 1mm.

The 20mm rails are half the price of the 25mm rails when you consider the costs of the pillow blocks and bearings, well they are in the UK.

Any help will be really appreciated.
albertsteptoe

[ger21]

They'll actually sag under there own weight, with no load on them. Unsupported rails typically yield poor results. Supporting them along there length and using open type bearings is a much better option. Something like these.

http://www.vxb.com/Merchant2/merchan...rMotionSystems

[steve323]

Rail strength is proportional to the diameter^2 so the larger rails are 56% stronger. There are flex calculators that will show how much flex you will have. Although, I agree with ger21 that supported rails are much better. You loose a small amount of bearing surface, but gain a huge amount of rigidity.

My system has a 40" span with 1" round rails. The flex is not too bad, although I have not measured it. If you want to go with unsupported rails, it is critical that the ends are very solidly supported. Loosely connected end supports will allow the rail to flex by simply bending in the middle. Solidly supported rails have to bend in 3 places, so there will be significantly less flex. Also, I can't imagine less than 25mm rails with a 40" span.

Steve

[irving2008]

Hello

Can i please ask your advice.

My machine will be 40" long, 28" wide.

I would like to use 20mm Hardened Round Rails for the supports but worried they may bend under the load?
The maximum load weight on the lower rails will be approx 7Kg - 8Kg.

I have spoken with two dealers, one said the rail will not bend and the other said they will bend up to 1mm.

The 20mm rails are half the price of the 25mm rails when you consider the costs of the pillow blocks and bearings, well they are in the UK.

Any help will be really appreciated.
albertsteptoe

The deflection of a single 20mm rail securely endsupported (so there is no flex in the endsupport) under a load of 8kg centrally placed is 0.3mm. Two rails will deflect 1/2 that amount when the load is midway between the rails. In practice your gantry would have pillow blocks which would further spread the load, for example 2 pillow blocks on a single 20mm rail, 150mm apart would reduce the deflection to 0.25mm when the load is central.

Clearly for a router aiming for 0.05mm accuracy this is no good, but it may be acceptable in some circumstances.

In general you should use support rail for X and Y where the Z-position is important. Unsupported rail for positioning where Z is either on or off (e.g. a plotter) is fine.


I use 16mm unsupported rail carrying a 250mm x 250mm MDF table on a 500mm x 500mm PCB mill and with a load of 1kg centrally on the table the deflection is 0.02mm which is fine for the purpose (few PCBs weigh a kg anyway!)



Albert, if you havent already done so, come join the UK CNC community

[albertsteptoe]

A BIG Thanks to Steve, Ger21 and especially Irving for a brilliant well detailed reply, really appreciated.

I think you have answered my question in full.
I will only be milling out plastics and light metals, holes and such like.
And as Irving said, the load will be well distributed across the pillow blocks area.

I will indeed also join the UK CNC Community, thanks for the invitation!

Thanks again for everyones input.
Albertsteptoe

[Brenck]

Does anyone have a dealer catalog to acurately calculate this flex? Some of them show precisely the amount of flex on each situation....

Im thinking on use a 12mm rod for my z axis, on total it will be 9 inch long, both ends the suppor will have 5/8''... i wonder if that wont be a good thing to do, or i might use a 16mm for that....

But I dont know I just wanted to calculate precisely how much i will need... and not just simply overkill my needs...

Do you guys think that the hollow rods flex less the then solid ones? I dunno but they might...

[nonubbins]

Here is a simple beam deflection program you can download free.
http://www.geocities.com/richgetze/

[Brenck]

I tried that software but its for construction more for construction and I beams...

I got a catalog from skf that got everything you got to know...


To bad the pdf got 7000kb i cant post it....

I wanted to know if i calculated right...

I got a 0,018 flex in a 20mm round bar with a 250N load on a 800mm gap... The force on a carriage 152mm long.... I think its a lot less then i expected so i dunno if i calculated right...

I took a print screen from the page that tells u how to calculate

[Brenck]

Irving2008

how did you calculate the deflection of the shaft? Somehow i cant calculate or im doing it wrong on the skf catalog...

I will have a 800mm rail lenght rigid clamped on both ends by a clamp that is 15mm... so the gap it will be 770mm ( +-30'' )... The load it will be 25kg ( 250N ) centerload, i will use 4 pillowblocks, 2 on each rail... The pillowblocks will be 150mm apart...

If you want to use my numbers to give me an example it would be nice, but if you can teach me how to calculate I apreciate even more...

Thanks

[irving2008]

Irving2008

how did you calculate the deflection of the shaft? Somehow i cant calculate or im doing it wrong on the skf catalog...

I will have a 800mm rail lenght rigid clamped on both ends by a clamp that is 15mm... so the gap it will be 770mm ( +-30'' )... The load it will be 25kg ( 250N ) centerload, i will use 4 pillowblocks, 2 on each rail... The pillowblocks will be 150mm apart...

If you want to use my numbers to give me an example it would be nice, but if you can teach me how to calculate I apreciate even more...

Thanks

I never rely on catalogs... to many simplistic assumptions.

I use a program called Framework, its a bit of pig to use but once you get the hang of it its quite effective.

Alternately you can use a formula in a spreadsheet, have a look here. The formula you want is for Fixed-Fixed with point load, 6th one down. If you need more help, just ask.

[Brenck]

Jee somehow my calculations are wrong... I tried to calculate with that formula from the website but im getting a number like... 3 digit...

I will get the result will be on mm? or microns?

Check my last post and look at the values that i need to calculate...

I gotta be doing something wrong while i calculate...

I would be really thankful if you could teach me how to calculate, i dunno if im calculating something firts that im not supose to... or something like that...

Im suppose to calculate that V formula right?

Thanks a lot

[harryn]

The deflection from the mass is only part of the deflection - there are also forces from moving things around and the cutting forces.

These forces can easily be 2 - 4 X the force due to gravity alone - and often are spikes - making planning for them that much harder.

I went through the same questions, thinking a 25mm / 1 inch dia rod / pipe should be quite stiff. Then I spent an hour at home depot flexing various rods, shafts, and pipes. Unsupported rod / pipe doesnt' get really stiff until you get closer to 2 in dia / 50mm. Seriously, you can easily flex 1 in dia rods and pipe with your hands.