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Say for instance you have a chain that has a safe working load of 200 lbs. If I wanted to hang an item from 4 corners, each with its own chain would I be able to hang 800 lbs? Or is there some other variable to factor?

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Another thing to consider is that the effective load on the chains is increased if the chains are not vertical, as would be the case if the chains are attached to the corners of the item, but are all attached to a single central attachment point above. Multiply the chain's working load limit by the cosine of the angle between the chain and vertical to get the adjusted limit. For example, if 1000 lb rated chains are at 45 degrees to the vertical their safe working load limit is reduced to 707 lbs. For overhead lifting where human safety is involved you should use Grade 80 or above. You would probably have to go to a specialty dealer to find it. Proof Coil, Grade 43 (Hi-Test) and Grade 70 (Transport Grade) are not rated for overhead lifting.

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The safe working load limit is generally 1/4 or 1/5 the minimum breaking strength. So there is a buffer and safeguard already built in if you are using the safe working limit number. However, you should confirm this with the chain's packaging or identification. There are cheap chains at the big-box stores that will have 1/2 ratios - do not use a chain that has a breaking strength of 200lbs to lift a 200lbs object.

As for hanging over humans or expensive property - the safe thing to do is to use chain meant for overhead lifting, and give yourself plenty of additional strength. Somethings to consider include a single chain (or attachment point) breaking which means the other 3 chains now have to support 800lbs. Also any movement will cause the load on each corner to vary. While you might think you will mount it and never move it - you never know who will jump up and try to hang on it; or an earthquake will shake things up.

Chain is cheap; people's heads are not. If you are supporting 800lbs by 4 corners, use 1000lb working load chain with grade-8 or AN-quality attachments.

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Well, there's the possibility of non-uniform loading, so probably not. And there's the pesky question of "What happens if this 800 lb. thing plummets?"

The fine print on most lifting hardware (chains, hooks, rings, etc) will state that it's not suitable for lifting or supporting loads over people; and hardware that is will be very conservatively rated, at least for that use.

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The termination hardware must have the same kind of ratings as the chain and properly attached to structure that will bear the load as well. Lag screws into wood don't count. Forged eye bolts penetrating the beam and supported by a nut and washer working against a large iron plate to spread the load come to mind. – Fiasco Labs Dec 17 '13 at 23:17

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