Aluminum Jib Track Maximum Capacity

[Colfax]

I have an aluminum jib track design that I need to find the maximum capacity for. If it was an I beam or a more standard track this would be simple but it is a patented enclosed track and I can't find any help on this subject. I have attached a PDF showing the design. Figure 1 just shows the boom set up, Figure 2 shows the track profile (highlighted in blue, black arrows show the load) and figure 3 shows the end plate on the end of the track. I believe the capacity is going to be limited by how much the bottom "flange" spreads open. Deflection is simple but could anyone point me in the right direction or tell me what to search for to calculate how much the part of the track the trolley rides on will separate or open up? In figure 2, I used the red arrows to try and show what I mean by separate. I know at the right and left ends of the boom in figure 1 will have zero separation and it will be at a maximum when the trolley is loaded at mid span but I don't even know where to begin to calculate this. Any help/direction would be much appreciated!

 

[SwinnyGG]

This is a tricky one for sure.

Couple of thoughts..

-It appears to me that the design of the trolley is such that friction forces between the wheels and their contact areas will counteract the spreading behavior- meaning that the trolley itself will take some of the load, in tension and bending of the axle, as the tracks try to deflect. This is an interesting behavior.

-It also appears that what you really need to know is which failure mode occurs first- does the beam buckle due to the moment load, or does the jib open up and let the trolley pull out?

-It also appears to me that in order for the trolley to pull itself out of the bottom of the boom, the tracks will have to deflect a LOT relative to their section dimensions. Can you skirt the difficult stress analysis by determining the strain required to allow the trolley to exit the bottom of the boom? If you can get yourself to that number, you can then work backward and determine which failure mode dominates.

 

[Colfax]

Thanks for the response. I'm not concerned with the track opening up to the point that the trolley drops out but I would like to find a way to calculate the distance the portion of the track the trolley rides on spreads open. I'm concerned with the push pull force of the trolley increasing as the bottom spreads open further. The designer tells me the trolley performance decreases after the track spreads more than 1/4" but I don't even know where to begin to try and calculate this. If the boom was free to open up at the ends it would be simple but since the outward deflection is 0 at the ends I don't know where to begin.

 

[SwinnyGG]

If the boom was free to open up at the ends it would be simple but since the outward deflection is 0 at the ends I don't know where to begin.

That definitely complicates the issue. FEA is your friend here if you have access.

 

[thaidavid40]

Since this is clearly a proprietary system, what does the manufacturer have to say to your questions? Most of these types of members are qualified not by theoretical calculations, but by manufacturer's testing. It is likely that they have some test results which would guide your investigation. Have you asked for their help yet?
Dave

Thaidavid

 

[Colfax]

jgKRI-Already used FEA to evaluate the system. I was just hoping to learn a way to do this by hand. Thanks!

thaidavid40-This is just a design. We have similar jibs with close to the same track design but this was designed with a newly proposed heavier aluminum extrusion, so no test data for this particular design yet. The only thing I do not know how to check by hand is the spreading open I mentioned previously. I guess for this, I will have to rely solely on FEA.

 

[MotorCity]

Colfax,
For the spreading open effect, I think there are two forces that control this. The first is any lateral loading of the wheels on the track. This could occur perhaps due to any wobble of the load during transport, misalignment of the track, etc. The second force affecting the spreading would be any eccentricity of the wheel load on the rail (I am assuming that the wheel load does not perfectly align with the centroid of the shape). This could cause a torsion which would result in a twisting of the section (i.e. a spreading effect between the two parts).