Pull tested another one of those breakaway sheaves today that's for a bigger sheave and a different design. Hook is rated at 4k and the sheave is as well. 
29,383lbs and the metal tore. There was a 1.5"X1.5" .120 tube that's welded to 3/16" material. 
	
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	View attachment 6467
Previous one I did straightened the hook out at about 28k but either way that's a far greater force then it will be exposed to. 
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