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!
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 3V/2A
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))
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.
Bosch is a Global company that originated in Germany… not to be too nit-picky.
I agree. The numbers make this easy.
Good luck everyone!!