I decided to do a field test of my safety line setup. It's the only way to quantify the actual performance and to provide some real data for all the interested parties. Note that I haven't read the Safety Project thread since my last post and don't see a reason to do so.
Most of the test results can be seen from the pictures. I used a spare length of line identical to the line on the roof, it was a backup line. It is now useless for that purpose but will still serve for tree falling.
I strung the line between two trees in my driveway at the same 45 foot distance and at the same approximate tension, about 50 lbs. I used the lifting pole on my Land Rover with a pulley mounted on the end and hung a total of 180 lbs of iron from it using the same type of line used to clip on the safety line. I weigh 172.5 lbs fully clothed as of this afternoon so this fairly approximates the situation on the roof.
I measured the distance from the center of the line to a nearby tree as a static no load reference, about 14 inches. I then raised the load to perform a static test. The line stretched so that the distance to the tree became about 100 inches. Subtract the original 14 inches and that is slightly over 7 feet. When I am rigged on the line on the roof I rig it so that when at the very edge of the roof the line is deflected about 1 foot. This is a negligible amount and one can deflect the line one foot by pulling with a finger. It does however subtract from the total static load distance that would exist if I fell off the roof.
In the worst case, at the center of the line span, I would be hanging off the edge of the roof by 6 to 7 feet. I would expect to wait no more than a minute or less for my wife to show up with a ladder.
Judging by the way the line responded to the dynamic load test it will stretch permanently after dynamic load by enough to increase the deflection by about one foot. It seems that I was somewhat conservative in making the posts able to fold in order to absorb shock load. I don't think it is necessary. It does still serve the purpose of preventing the brackets from tearing out of the ends of the roof although I don't think that is likely either.
I haven't done the math to calculate the tension on the line under static load but the deflection ratio is about six to one so 180*6=1080lbs is close. With 1800 lb test line this gives an 800 lb safety margin. In actual use no free fall is possible before the line takes load so shock loading with multiple gees cannot occur.
Regardless, I ran a simple dynamic test that undoubtedly placed at least 2 times the load on the line, twice. It performed just fine and shows no sign of failing other than the expected permanent stretch.
My judgment is that the safety line I have designed and built is entirely adequate for the intended purpose which is to perform maintenance on the gutters and first row of shingles on either side of the roof. It isn't intended for use at the end of the roof and different rigging will be used for that.
Pictures follow with a video of the dynamic test. I expect my server to be clobbered by this post so if not all the pics show up please try again later.
Comments are welcome.
Link to the dynamic test video. It is one megabyte so shouldn't be too long to download on dialup.
Most of the test results can be seen from the pictures. I used a spare length of line identical to the line on the roof, it was a backup line. It is now useless for that purpose but will still serve for tree falling.
I strung the line between two trees in my driveway at the same 45 foot distance and at the same approximate tension, about 50 lbs. I used the lifting pole on my Land Rover with a pulley mounted on the end and hung a total of 180 lbs of iron from it using the same type of line used to clip on the safety line. I weigh 172.5 lbs fully clothed as of this afternoon so this fairly approximates the situation on the roof.
I measured the distance from the center of the line to a nearby tree as a static no load reference, about 14 inches. I then raised the load to perform a static test. The line stretched so that the distance to the tree became about 100 inches. Subtract the original 14 inches and that is slightly over 7 feet. When I am rigged on the line on the roof I rig it so that when at the very edge of the roof the line is deflected about 1 foot. This is a negligible amount and one can deflect the line one foot by pulling with a finger. It does however subtract from the total static load distance that would exist if I fell off the roof.
In the worst case, at the center of the line span, I would be hanging off the edge of the roof by 6 to 7 feet. I would expect to wait no more than a minute or less for my wife to show up with a ladder.
Judging by the way the line responded to the dynamic load test it will stretch permanently after dynamic load by enough to increase the deflection by about one foot. It seems that I was somewhat conservative in making the posts able to fold in order to absorb shock load. I don't think it is necessary. It does still serve the purpose of preventing the brackets from tearing out of the ends of the roof although I don't think that is likely either.
I haven't done the math to calculate the tension on the line under static load but the deflection ratio is about six to one so 180*6=1080lbs is close. With 1800 lb test line this gives an 800 lb safety margin. In actual use no free fall is possible before the line takes load so shock loading with multiple gees cannot occur.
Regardless, I ran a simple dynamic test that undoubtedly placed at least 2 times the load on the line, twice. It performed just fine and shows no sign of failing other than the expected permanent stretch.
My judgment is that the safety line I have designed and built is entirely adequate for the intended purpose which is to perform maintenance on the gutters and first row of shingles on either side of the roof. It isn't intended for use at the end of the roof and different rigging will be used for that.
Pictures follow with a video of the dynamic test. I expect my server to be clobbered by this post so if not all the pics show up please try again later.
Comments are welcome.
Link to the dynamic test video. It is one megabyte so shouldn't be too long to download on dialup.
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