Thread: Weight on Directional Pulleys
12-22-2005, 08:34 PM #1
Weight on Directional Pulleys
I was on a drill the other day where some guys were practicing pickoffs. I am a confined space tech so i am familiar with rope and lowering systems. We were setting up a lowering system to lower the rescuer and the victim once they had picked the victim. The issue that I had was that they wanted to put a directional pulley on a guardrail. I said that that was not a good idea due to the fact that OSHA only requires guardrails to withstand 200lb force applied to it in any direction. Now this was a substantial rail, but you dont know what the rail is engineered for, but I do know what the minimum standard is. They said that a directional pulley doesnt take any wieght at all which I dont believe. I know it all depends on the degree of angle as far as weight on it, but I would have to believe that a directional on a guardrail would be carrying too much weight. Especially when you want a 4 to 1 safety factor with 2 guys at 400lbs that would be a factor of 1600lbs. My view is using a directional on any kind of guardrail is a gamble, but I could be wrong. What do you think??
12-22-2005, 09:13 PM #2
The directional should theoretically take all the weight of the rescuers pulling and the load on the line.
Are we talking highway guardrails or guardrails around a platform or something similar? I would assume it's in an industrial setting. I would trust a welded guardrail system to an extent but it is one of those things you would have to see before comitting to.
12-22-2005, 11:25 PM #3
they are guardrails in an industrial setting and they were bolted to the floor.
12-23-2005, 02:19 PM #4
- Join Date
- Jul 2001
If you look at the angle formed by the rope as it passes through the redirect, you can use this to calculate the force on the redirect. The formula is:
2(cosine(angle/2). Cut the angle in half then get the cosine of that and multiply this by 2.
With a 90 degree angle you will have roughly 1.4 times the system force at the redirect. The smaller the angle gets, the higher the force is. This is usually confused with the anchor sling angle forces where a smaller angle creates less force.
Another thing to consider is in what direction the force is applied. The guardrails are rated for a lateral force exerted upon them. In many cases a well built guardrail can take a lot of force in a vertical or near vertical direction. Again if you look at the angle formed by the rope you can find the resultant force. Draw an imaginary line through the halfway point of the angle and that is your direction of force. Another visual indicator is the direction the carabiner points. A lot of teams will back tie a rail, thinking they are beefing it up. In reality, they are often making it more prone to failure because the resultant force is applied inward toward the catwalk as well as the back tie. A front tie to a beam or other structure would be more beneficial.Sometimes, in order to make an operation idiot proof, you must remove the idiot!
12-24-2005, 05:02 PM #5
- Join Date
- Apr 2001
Use the formula resqtek gave and create a little cheat sheet for use. (I have one written in my FOG) With concern to this angle remember with anchor straps anything under 120 degrees is good (reasonably anyway), with directionals anything under 120 degrees is bad (Force on directional is equal to load and continues to grow the smaller the angle).
With concern to the rail. Obviously, the lower the anchor point is the stronger the rail will be. Remember if the direction of pull is in shear to the bolts, and the anchor is low, you are looking at more than the 200lbs. If you have a FEMA FOG you can look up shear strengths of bolts / nails. To reiterate what Halligan 84 posted, I would have to take a good look at it to make a determination.
12-28-2005, 08:10 AM #6
- Join Date
- Dec 2005
- Brisbane, Queensland, Australia
Good info all here, have a talk to those boys who reckon a directional pulley takes no weight, they're setting themselves up for some shock loads sometime soon if thats what they think.
Get 'em to set up a high point redirection and really think about the forces they are applying to move that load, and where those forces are felt, I guarantee they'll figure it out and never forget the lesson.
01-04-2006, 12:22 AM #7Originally Posted by Squad1LT
01-04-2006, 12:31 AM #8Originally Posted by jmatthe2
Unlike the vector force on a two-point anchor (or highline), which grows exponentially toward infinity as the angle approaches 180, the load on a directional can be no more than 2x the load - it does NOT continue to grow.
So it is simplistic to state a rule of thumb (bad angle) for directionals. But both the load multiplication (up to 2x) and the resultant vector must be taken into account when rigging a redirect.
Typically, the advantages of a redirect outweigh the disadvantages, as long as those factors are considered and accounted for.
A fixed steel pipe guardrail, bolted to concrete, usually works fine for a redirect, but a temporary guardrail on scaffolding, would be inappropriate for that use.
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