Save time? Use aluminum extrusion kits!

[Matt Adams]

Originally Posted by Tim Skloss

The Bosch extrusion is NOT a superior product. A local business that builds industrial robots out of extrusion has completely quit using the Bosch because it is notorious for vibrating loose. And on a FIRST robot that is too risky.

I guess that I have a different opinion on something like this. Assuming that the robot needs to compete in 30 matches over it's life time, that's 1 hour of run time. I'd believe that the need of FIRST robots are significantly lower than industrial equipment, which is running for 8, 12 or even 24 hours spans. If this aluminum extrusion was so bad that a robot chasis could vibrate loose in a 2 minute round, I don't think that Bosch would be in business!

Originally Posted by Tim Skloss

Furthermore, some of the numbers you quoted are funny. The pullout strenght of 300+ lbs for the 20x20 is simply impossible according to the tensile strengh claimed. The material will fail and release the fastener before you get to 300 lbs.

Tim, since you're a practicing engineer, I assume you made a quick napkin calculation about the pullout strength before you claimed that my numbers (from the manufacturer) were 'funny'. I'd like to compare some numbers with you.

Let's go ahead and take the above 20mm extrusion piece, and pretend to load in a 15 mm long fastener, and assume that the bottom lip of the extrusion that holds the T nut in is 1.5 mm tall. That looks pretty close to scale. Then we'll assume a 1700 N load on the bottom pulling it out. We'll assume the fastener (a T-nut) does NOT span to the edge of the V cutout, but rather is a 1 mm short on both ends and there's some bending going on.

Assuming the T-nut is 15 mm long, and for the sake of arguement we'll even assume the piece of extrusion is only 15 mm long too, so you just need to just bend the flaps down to pull it out, and not shear the edges like you actually do, and which would multiple the strength SIGNIFICANTLY. We'll treat the bottom flaps as two cantilevered beams, just to be safe

The transverse shear stress is 3*V/2*A
V is the shear force.
A is the area.
V = (3*1700N)
A = (15mm * 1.5mm * 2 sides)
Transverse Shear Stress = 31.875 N/mm^2

Bending stress = M*y/I
M = (1.5mm*(1700N/2 sides))
y= (.75mm)
I = (15mm * 1.5mm^3/12)
That gives me a stress of 226.66 N/mm^2

Since there's transverse shear AND bending stress, Mohr's circle comes into play here, so we'll find that the max stress from Mohr's favorite shape is:
226.66/2 + sqrt( (226.66/2)^2 + (31.875)^2 ) = 231.05 N / mm^2

With the tensile strength of the aluminum used in this extrusion being equal to 250 N / mm^2... it appears there's a factor of saftey of about 1.1. We won't go into some sort energy-distorsion or modified mohr for failure, I think my point is adequate as-is.

Since there were so many conservative factors in this entire calculation, I think that the pull out strength listed by the manufacturer is very apropriate.

I would enjoy comparing calculations that you did that say these numbers are "funny." However, I'd appreciate if you'd include the shear stress needed to rip it out of the aluminum, since that would really be required to make the case.

Originally Posted by Tim Skloss

Since in europe--where item originates--manufacturers claims must be absoluetly guaranteed, you will see very conservative estimates of structural strength. In our experience the material can be up to 3 times stronger than claimed in the catalog.

Bosch is a Global company that originated in Germany... not to be too nit-picky.

Originally Posted by Tim Skloss

Which is better for a FIRST robot? You have to weigh strength and reliability against your pocketbook... But remember the free market ensures that "you get what you paid for".

I agree. The numbers make this easy.

Good luck everyone!!

Matt