I am designing a carriage that is going to be cantilevered over a conveyor belt. I've attached some files (the motors and linear bearings are not shown). A steel weldment is used in the files I attached. The weldment is a little complex and heavy. I was wondering if it might be better to have a couple of weldments that bolt together, or an aluminum assembly that bolts together (with thicker plates). Which would be better in this case? If anyone has good experience with welded vs. bolted connections, I would be interested in what you have to say. Also, does anyone know some good ideas for this type of cantilever in general?
Thank you,
xrhp
522899.pdf522738.pdf
You haven't been clear as to what your problem is, or the weights, or the sizes, or the application, or the loading, or the duty cycle, or why it seems complex (looks simple to me), or why that's a problem. Other than that, we've got all we need! Just razzing you. More info please...
I would also help to see an illustration showing only the specific weldment alone, without all of the mechanical and bolted attachments.
In my original post, I forgot to mention that the pdf attachments are 3D pdfs- if open them in adobe (not in the browser), click on the image once and you can rotate it. Also, there are two pdf's. One is of the general assembly, and the other is the weldment alone.
The weldment will extend over a conveyor belt and be raised up and down on THK rails over a range of about 2". The weldment will weigh about 77 lbs and be cumbersome to install, especially with respect to getting it on the THK rails and aligning them. It would be simpler if the cantilever arms could be removed. As for loading, at some point during maintenance someone will stand on this (guaranteed)- all other loads will be smaller. I'm hesitant to give more details because my company might object to me revealing too much. I'll include a crude drawing of the weldment later to give you an idea of the size.
Thank you,
xhrp
Here is a basic drawing.
522899_drw.pdf
You say it would be simpler if the cantilever could be removed. OK, what's keeping you from doing that? You don't know how to design it? You also say someone will stand on it. (I feel your pain there.) So the real load isn't the normal load. Its the normal load plus an overweight maintenance guy, his tools, and any force he can create. Have you done a force vector analysis to determine actual linear bearing loading? What exactly is your question here?
After viewing the weldment, if you are concerned about weight, I am curious as to why youe have made the horizontal arms .75 in. thick and the stiffeners only .375 in. when the primary stiffening elements are the tapered stiffeners.
As to your problem of someone standing on the arms (and I realize these are "real world" types of problems, you might discourage (prevent?) this by increasing the height of the vertical plate above the horizontal arms to allow giving the tapered stiffeners a steeper downward angle and also extending the stiffeners to the ends of the horizontal arms. This would generally discourage anyone from trying to use this as a standing platform.
As to the construction of the this item, a weldment is definitely the most straight forward and lightest method for its fabrication.
jboggs,
I was just wondering if anyone had any practical experience with using a cantilever like this and knows what kind of problems I might run into bolting it vs. welding it. As far as the load on the linear bearing, if a 300lb person stands on the far end, the force on each bearing would be about 750 lbs- the maximum for the bearing is 8630 lbs.
JAlberts,
The idea behind making the horizontal arms so thick was to allow plenty of thread engagement for the tapped holes. Thank you for your input and suggestions.
I've done thousands of bolted and an equal number of welded structures. They each have their benefits and weaknesses. The most common error I see in both is due to a lack of complete analysis of all loads, stresses, and deflections. Based on the structure I see in your drawings, it appears that even if you make it "bolted", you would really just be bolting one welded structure to another one. I will say this: bolts are good in direct tension. Welds are not. Try to arrange the geometry of as many of your joints as possible so that they are loaded in shear rather than tension.
I agree with with all of the items in Bogg's above post except for his comment regarding the "weakness" of welded connections in tension. The weld in any correctly designed and welded connection is always stronger than the base metal of the structure. Based upon his comment I have tom assume that he has never done any piping fabrication design because in these designs bolted flange connections in place of a direct welded connectio are only utilized where the piping routing limits the size of the piping sections being installed or as necessary to provide for the later disconnection for servicing, replacing, etc of an installed component.
On the subject of the thickness of your extention arms to "allow sufficient thickness for the tapped thread", in this case I would suggest that sizing your arms' thickness based upon the loading requirements and using a nut with the connecting bolt is a better solution and allows you to minimize your overall structure weight and save the heavier fabrication components for where they most needed for strength.
Jalberts - Thanks for your comments. My recommendation to avoid welds in tension is based on my own personal experience witnessing the results of failures of tension welds, in multiple cases. In one that experience involved testifying in a court case in which a fork truck driver was critically injured when a trailer he was unloading tipped over while he was inside due to the failure of a tension weld in one of the support legs. In another a worker was very nearly severely injured when an overhead crane system welded to building structure came tumbling down. (Luckily I did not design either joint!) In both cases your statement about a "correctly designed and welded connection" is absolutely correct. I have just learned over the years that while technical principles are always true, human behavior is consistently variable. After a few decades of doing this stuff we all develop our own list of "preferred practices".
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