Steel beam for x axis?

[flanajb]

Hi,

My original plan was to use 100*50mm*5mm box section for the x axis rails. I am now thinking that using I beam section is the way to go ?

I think it will deflect a lot less and provide more material for milling on the surface

102 X 44 X 7KG S275JR - Rolled Steel Joist - ParkerSteel

Thoughts ?

[PaulRowntree]

I would have thought that the rectangular tube would be about 2x stronger : the height and thickness of your I beam vertical are about the same as the tube, yet the tube has two vertical walls instead of 1. Most of the strength against deflection comes from the vertical sections.

Of course, it is easier to use nut/bolt combinations on the I-beam, but with 5 mm thick material you can tap either case. I don't know if the manufacturing tolerances for straightness are different for the two sections

If you want to contain a lead screw inside the axis itself, C-channel might be an alternative.

Check out the engineering calculator websites; they post the std formulas, and it is easy to calculate the moments of inertia of each section. Some of them have formulas adapted for the specific sections too. If the X rail is supported along its length I don't think strength will be a serious problem, regardless of what shape you use.

[Darylb]

Originally Posted by PaulRowntree I would have thought that the rectangular tube would be about 2x stronger : the height and thickness of your I beam vertical are about the same as the tube, yet the tube has two vertical walls instead of 1. Most of the strength against deflection comes from the vertical sections..

The moment of interia for an I-beam is higher than that of a rectangular section as more of the cross section is farther from the neutral axis. Now this is only if the cross-sectional area's are the same. That means an I-beam will be stiffer for the same amount of material. If you are talking about an I-beam and Rectangular section with the same dimensions and the only difference is the second vertical member of the rectangular section, then yes, it will be stiffer than the I-beam. I-beams are just a more efficient use of material therefore that's why you see them in structures.

[asuratman]

You can use H beam 150X150 the same as I used, then welded some ribs on it. It is very strong and nice to see. I installed linear on top and bottom, balscrew and motor in the chanel. Thanks.

[PaulRowntree]

Originally Posted by Darylb The moment of interia for an I-beam is higher than that of a rectangular section as more of the cross section is farther from the neutral axis. Now this is only if the cross-sectional area's are the same. That means an I-beam will be stiffer for the same amount of material. If you are talking about an I-beam and Rectangular section with the same dimensions and the only difference is the second vertical member of the rectangular section, then yes, it will be stiffer than the I-beam. I-beams are just a more efficient use of material therefore that's why you see them in structures.

In this case the material is thinner for the I beam web than the rectangular tube wall (4.5 mm vs 5 mm), so I think the I-beam would be lighter, and the strength would suffer as a result.

[jsheerin]

The closed tube would be torsionally stiffer than an I beam as well. I'm not sure exactly how much this would increase the stiffness of your frame, but it couldn't hurt.

[flanajb]

Originally Posted by asuratman You can use H beam 150X150 the same as I used, then welded some ribs on it. It is very strong and nice to see. I installed linear on top and bottom, balscrew and motor in the chanel. Thanks.

Hi, would you happen to have a picture you could post ?

Thanks

[asuratman]

Hello Flanajb,
I am out of town now, I will be back in 2 days. I will take picture and post them here.

[kiwibrick]

100x50x5RHS (box section) - 10.3kg per m
Ixx = 1.53x10^6mm^4
Iyy = 0.511x10^6mm^4
Zx = 30.6x10^3mm^3

102x44x7RSJ
Ixx = 1.53x10^6mm^4
Iyy = 0.078x10^6mm^4
Zx = 35.4x10^3mm^3

Ixx - simply put, the resistance to deflection in the direction of the major axis (up and down)
Iyy - resistance to deflection in the direction of the minor axis (side to side)
Zx - Strength, or how much load it can take before failing

So the 2 members have the same Ixx value so will deflect the same amount for a given load, the RSJ is ultimately stronger ie. can take more load before failing (if adequately restrained) and the box section has far greater resistance to deflection side to side (over 6 times).
The box section is also considerably heavier per m.
The side to side strength won't matter if it is held from moving sideways by the table.

[jsheerin]

If we're talking about a gantry beam, Iyy would normally be in the plane of the table, so for example if you cut something, you would have a force trying to bend the gantry beam in the x/y plane which Iyy would come into play with resisting. Ixx would resist vertical forces due to drilling. In reality of course it is more complicated due to the forces being applied at a distance from the center of the beam. In any case, I don't think your statement that Iyy won't matter is accurate if I understand how you're setting up your coordinate system correctly.

[kiwibrick]

I forget that X is the gantry and Y is the table.
I just really wanted to get some actual numbers in here more than anything.

[PaulRowntree]

Originally Posted by kiwibrick 100x50x5RHS (box section) - 10.3kg per m Ixx = 1.53x10^6mm^4 Iyy = 0.511x10^6mm^4 Zx = 30.6x10^3mm^3 102x44x7RSJ Ixx = 1.53x10^6mm^4 Iyy = 0.078x10^6mm^4 Zx = 35.4x10^3mm^3 So the 2 members have the same Ixx value so will deflect the same amount for a given load, the RSJ is ultimately stronger ie. can take more load before failing (if adequately restrained) and the box section has far greater resistance to deflection side to side (over 6 times). The box section is also considerably heavier per m. The side to side strength won't matter if it is held from moving sideways by the table.

I am surprised that they have the same bending moment vertically, but thank you for that correction! Could you please point me to the site where the mmoments are tabulated for various sections? I use the simple engineering calculators, but the I-beam has an shaped inner web that must help quite a bit.

Cheers!