How much force can a person pull on a rope?

[kevinz]

Can anyone quote me an authoritative source for the amount of force an average person can exert pulling on a horizontal rope? I found this source: http://www.lewmar.com/webcat/technic..._tech_ref.html which says 75lbs. Is this accurate? Any authoritative sources? How about peak force when heaving together?

I ask because I just broke a 5/8" piece of nylon webbing by five guys pulling on a 4:1 tackle. The nylon sling was tied in a cow htich, and oddly enough broke in two places, about where the two legs went under the loop of the cow hitch. All five guys hit the deck, which was probably lucky, because the 10-15 lb double block ended up behind them all; it must have flown over their heads.

75lb x 5 men x 4 mechanical advantage = 1500 lb
Breaking strength of 5/8 inch tubular nylon webbing = 1500-1850 lb (from various sources).
Haven't been able to find a source for how much a basket hitch (essentially a cow hitch) reduces breaking strength.

Thanks for your suggestions.

-Kevin

[Brion Toss]

Hi Kevin,
Most hitches will weaken rope about 20%; I don't know about webbing. Possibly the failure was sequential ó rapidly so ó as otherwise one would expect the doubled portion to be smallest. Hitch aside, there were in any event likely some localized deformations in the webbing.
As for the max force, a typical hauler can readily pull a sustained load of 75 pounds, and the limiting factors are height and traction. That's why a team of relatively light people, pound for pound, will usually beat a team of relatively heavy people in a tug o'war: they can put more feet on the ground, thus more effective traction.
The other important factor here is impact loading, something that a well-coordinated heaving gang is [i]designed[i] to apply. You might have doubled the sustained purchase at peak loads, something that would have been enough to overcome block friction and materials elasticity.
I think the lesson here is the need for a suitable safety factor in all hauling applications, which is to say a usably precise knowledge of what the loads will be. Clearly, that way-strong-seeming gear wasn't nearly strong enough.
Fair leads,
Brion Toss