huangyhg

lifting frame
what is the difference between using chain members (as shown in the spreader beam in the attached picture) and steel square tubing which are welded to the yellow beam and another horizontal beam at the top (no picture), for a lifting frame? what are the factors that it depends on?
why choose one over the other?
thanks!
picture shown has chain(flexible tension only
a solid welded frame would be abit bulky to have lying around. intuitively, the chain is going to be heavier than the tube, 'cause there are more breaks in the load path.
possibly you could design a rigid frame with lugs at the intersections (rather than welds) so the thing could be folded flat for storage.
either way it'd need to be designed, which is a good thing 'cause it puts food on the table.
1. the chain will be lighter, so the pick load will be minimized
2. the chain is designed to rigging stanards which require a higher fs than std struct work.
3. the lifting lugs & chain require less fabrication costs than a frame.
and you can buy the chain apparatus already made up and certified for a specific load.
richard a. cornelius, p.e.
my thoughts:
1) i never use chain in an above-surface lifting scenario, since chain is only as strong as its weakest link. i always use synthetic slings or swaged-end steel cable slings specifically designed for lifting. if the load is likely to be submerged for any period of time, i.e., a sub-surface marine environment where corrosion will be a factor, then chain is acceptable, since the biggest concern in corrosion is the surface area (lots in a cable, but much less in a chain).
2) the chain will require a greater amount of inspection when the lifting assembly comes up for its periodic re-certification. the steel tubes generally only require nde of the welds -- very easy to do and people can do this anywhere.
3) as drc1 states, the welded tube will be lighter -- always a benefit, but it's probably more bulky as mentioned by rb1957.
4) for me, though the biggest problem for adding the welded tubes (i believe that was what you're talking about) would be that you have to be certain of the metallurgy of the spreader bar. since you're talking about a finished product from an outside manufacturer, it's unlikely that you're going to get mill test reports and a chemistry analysis of the steel. it's pretty likely that it's a mild steel, but there's certainly no guarantee of that. you can have sub-surface weld cracks if you're welding a mild a500 to a piece of a514 plate that was formed in a brake. it's pretty difficult to guess the type of steel you're dealing with by looking at it.
5) for welded lifting components, in the past, i have done something similar where i designed the square tube to accept two plates on one end and act as the sling with a properly-designed pin in double shear between the welded tube tension member and the spreader bar. this would avoid your problem of welding to an unknown type of steel. on the crane hook end, i brought them together to a point where an anchor shackle held them all together and accepted a master link that went over the crane hook.
6) any time you're welding square (or round or rectangular) tube to be used in lifting, always weld it closed so that no moisture can attack from the inside. you can't inspect inside, so don't leave it open to corrosion.
7) if you're doing dynamic lifts, i.e., off the back of a heaving boat onto a stationary crane, your safety factor will likely need to be higher than it would on a static lift, so you will need to look at some other design specs to come up with an appropriate safety factor.

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