Flatbed winches

This can be calculated by simple torque to force conversion. Those units "foot-pounds", "inch-pounds" etc actually mean something. That's one thing I remember vaguely from my physics class.

So you got 34" winch bar, level to the ground, and a 200-lb person applies all his weight at the end. You'll achieve 200 pounds x 34 inches = 6,800 inch-pounds.

The spool of winches (where you feed the straps through) are what, about 2" in diameter or 1" radius? And the effective radius increases as the strap rolls around the spool as you tighten. So let's say the effective radius of 1.25 inch, with a few layers of straps around the spool.

Both the winch bar and the winch spool rotates around the same axis, so you simply divide the torque generated by the winch bar by the effective radius of the spool, and you'll get the tension.

6800 inch-pound / 1.25 inch = 5,440 pounds (inches cancel each others out). That's pretty close to WWL of 4" straps. Of course, it's neglecting losses from friction, straps stretching, the load deflection, etc.


That said, honestly, I think you're over thinking a little. What keeps straps from loosening is tension from stretching, just like nuts and bolts. As long as there's enough tension not to come loose, it's fine even if you haven't achieved WWL.

It's kind of like tightening a structural bolt to 45 lb-ft of torque, where the spec calls for 50 lb-ft. It's not likely to cause any harm.

To further apply this analogy, try tightening a 1/2" bolt to 100 lb-ft into a threaded 1/8-inch thick aluminum plate. You'll strip the threads in aluminum. You can only torque as much as the material can take it. Bagged sand/concrete, cardboard boxes, or anything fragile are the same way. You'll damage the load before you achieve WWL. Bagged sand probably won't get damaged, but after a few miles, the sand will reshape itself and the straps loose tension. Again, the key is to apply enough tension to prevent the straps from loosening and keep the load secure.

 

It's even more fun when you have large pieces that like to walk around on you and there's no way to strap them down where they won't move in one direction or the other.

 

hands down these are the best winch bars out there
http://www.morganmfg.com/tr_cat03.htm


you can get them from dawson tarp for less than 50$

broke my wrist when a cheap china bar broke on me...never again..

my 42" bar has busted a few winches and straps though...... lol