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Old May 28th, 2006, 11:18 AM   #71
Wissenschaftler
Papa Smooth
 
Join Date: 03-28-06
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Default More on winching safety

Just a couple more thoughts on winching safety.

Never step on or push against a winch cable that is under tension. The increase in strain (force) in the cable is increased far more than you might think. If the cable is already near its strain limit, it could snap with deadly results. To get an idea of the added strain from just a small amount of force being applied perpendicularly to the cable, get about a 4 ft. length of really stout fishline. Hang a small weight---just 3-4 lbs---in the center of the line and taking an end of the line in each hand, see how much force you need to exert to bring the line back to perfectly straight, from the “V” shape caused by the small weight. The small weight represents someone stepping on a winch cable and the force needed to pull the line straight again represents the increased strain on the cable. You can see why tightrope walkers need to be so careful!!
If you’re so inclined, run some numbers:
the Tension = (Force/ sine of angle)/2.
E.g. A 175lb person steps on the middle of a winch cable that is under tension and displaces the cable 5 degrees (as measured from either secured point). This increases the strain in the cable by (175/.0872)/2 or about 1000 lbs! (the “2” is there because we have two lines---kinda like a single pulley with one rope going around it---each end supports half the load). In fact, if you pulled the cable out 90 degrees (totally fictitious here) you’d end up with (175/1)/2 or 87.5 lbs, just what the tension would be in each rope of the pulley system if you hung a 175 lb weight on the two ends of a rope suspended over a pulley. The tightrope walker would be better off to have some “sag” in the rope.

Another one: Throw a “winch blanket” over the cable when winching.
When Captain Kirk told Scotty to fire up the Impulse Engines, Scotty was, in effect, applying an engine Force for a length of Time or (Force X Time). A fellow by the name of Isaac Newton said that this was what caused a change of momentum----which is what they called the product of an object’s mass times its velocity. Since the mass couldn’t change, the velocity changed when a force was applied for a length of time. Thats why the expanding gasses in a rifle barrel increase the velocity of a bullet more in a long barrel than in a short barrel----the Force acts on the bullet for a longer Time in a longer barrel. So: Force X Time = (change in momentum----considering here only the object’s velocity). More importantly for us, Force = (change in “velocity")/Time---(of course the mass is still there but for this purpose we're ignoring it because it remains constant). Everybody knows that the LARGER the number in the denominator of a fraction, the SMALLER the piece of pie (or anything else). If a winch cable snaps, the Force doing the snapping is going to propel a hook & cable at deadly velocities (acceleration = Force/mass if you’re interested). What we need to do is make "TIME" in the denominator as LARGE as possible and still be practical about it. The easiest way is to throw a blanket over the cable to absorb the energy that produces the Force-----make the change in momentum of the cable/hook take as long a TIME as possible. Increased TIME = reduced FORCE = safety! Padded dashboards, airbags, carpet vs. hardwood floors when a wine glass is dropped. Think of how much “Force” your foot needs to exert on the brakes when you compare running right up to the stop sign or braking way back from the stop sign, i.e. you change your momentum by exactly the same amount either way, but the TIME it takes makes all the difference in how hard you push on the brake pedal-----all the same principle.

Just my three-cents worth and probably more than you wanted to read.
Papa
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"If you can read this, thank a teacher.
If you're reading this in English, thank a veteran."

Last edited by Wissenschaftler; May 29th, 2006 at 10:51 PM.
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