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Here is how I made a set of fitness rings using grey Sch 40 PVC conduit. I was inspired by this Instructable and various comments posted on it:
http://www.instructables.com/id/How-to-make-PVC-gymnastic-fitness-rings/
Rather than using regular PVC pipe, I decided to use grey conduit as it is designed to be bent when heated up. I also used fine-grained "play sand" because it can be compacted better than most sand you would find on the ground (unless you live at the beach), resulting in fewer air pockets and presumably a reduced chance of the pipe kinking. I have a small oven that is not large enough to fit the lengths of pipe so I describe the steps to compensate for that.
Overall, this process was much simpler than I had anticipated. Once all the materials were in-place, it only took me about an hour to actually make the rings and they seem very strong.
http://www.instructables.com/id/How-to-make-PVC-gymnastic-fitness-rings/
Rather than using regular PVC pipe, I decided to use grey conduit as it is designed to be bent when heated up. I also used fine-grained "play sand" because it can be compacted better than most sand you would find on the ground (unless you live at the beach), resulting in fewer air pockets and presumably a reduced chance of the pipe kinking. I have a small oven that is not large enough to fit the lengths of pipe so I describe the steps to compensate for that.
Overall, this process was much simpler than I had anticipated. Once all the materials were in-place, it only took me about an hour to actually make the rings and they seem very strong.
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Materials:
2 x 27.5" sections of 1" diameter grey rigid schedule 40 PVC conduit (Home Depot)
2 x 12 foot lashing straps (Harbor Freight Tools $3.99)
wire (if your oven is small)
Gorilla Tape (or duct tape)
Tools:
PVC pipe cutter (or hacksaw)
tape measure
an oven
a 9.5-inch inner-diameter cylindrical object (like a bucket or a trashcan)
More information on bending and cutting PVC pipe can be found here:
http://www.backwoodshome.com/articles2/sanders94.html
2 x 27.5" sections of 1" diameter grey rigid schedule 40 PVC conduit (Home Depot)
2 x 12 foot lashing straps (Harbor Freight Tools $3.99)
wire (if your oven is small)
Gorilla Tape (or duct tape)
Tools:
PVC pipe cutter (or hacksaw)
tape measure
an oven
a 9.5-inch inner-diameter cylindrical object (like a bucket or a trashcan)
More information on bending and cutting PVC pipe can be found here:
http://www.backwoodshome.com/articles2/sanders94.html
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These rings are awesome. I've put together several pairs over the years and have never had any problems with them whatsoever. Pullups, muscle ups, dips, pushups, pronated knee tucks. Same PVC tubing and cheap auto tie downs.
Steve
http://scrabblecheat.com
if the working static load is ~330lbs that's fine, however i assume you intend to swing and do pullups and the like from these. that's a *dynamic* load, and not something the buckles are designed for, and something you don't know the webbing rating for.
there's a decent chance these will never fail for you, however your gear does not produce a system that is designed to sustain a dynamic load, and that's not a safe thing. if you read the package of your straps i'll wager that there's even an explicit warning along the lines to not to use for lifesafety uses or to support people.
http://www.bstorage.com/speleo/Pubs/rlenergy/Default.htm
In it, it states an example:
In the example of Figure 1, a 180-lb person takes a 5-ft fall, yielding 900 lb-ft of energy. From the graph we see that this energy results in a load of about 1900 lbs and a rope elongation of about 1 ft. This exercise shows, without the use of equations, that the dynamic loads on ropes and belay anchors are the consequence of rope characteristics.
I weigh 150-lbs and each strap I have is rated to support a static load of 1000 lbs. Since there are two straps, given I applied equal force to both, together they can support a static load of 2000 lbs.
If I weighed 180 lbs, let the straps slack 5-feet, held on to them, and jumped off of a ladder, these straps would be able to support that. My arms would undoubtedly be dislocated if I hung on. Since nothing I will ever do on these rings will come anywhere close to that, I deem the straps to be safe.
look at the packaging for your straps, or even a little warning tag on the strap themselves and tell us that no where on it does it say anything about using it for supporting people, climbing or the like.
you're also not addressing the hardware, the clips. those are not designed to maintain on a dynamic load. something like a locking carabiner is.
as i said, you'll probably never have an issue, BUT, you have not designed a system that is safe. (or as safe as an engineered system designed to support a moving body ever can be)
brilliant use of molding for forming the pvc though. and well documented instructible.
pounds = mass x acceleration due to gravity
Pounds and newtons are just units of force:
http://en.wikipedia.org/wiki/Newton
Dynamic loads need to account for possible acceleration greater than the force of gravity, such as putting on the brakes on your car (see the first paragraph of my first response). That's why these straps state that a static load is not to exceed 1/3 of the maximum force of 1000lbs because of the fact that accelerating and decelerating could cause the load to exert more force (presumably, up to 1000lbs in most tie-down situations that these straps were designed for).
My own link, in the section you described, concludes:
"The probable end result is that there is a negligible reduction in chance of rope failure and a significant increase in chance of victim injury from high deceleration loads."
as a result of making stiffer ropes. They are actually stronger but can cause someone who is belaying to decelerate faster, increasing the risk of injury or death. As they say "it's not the fall that kills you -- it is the sudden stop". They were trying to say that "shock strength" is misleading:
"Probably the most harmful result of this situation is that "shock strength" has encouraged us (and equipment manufacturers, through competition) to make everything stronger."
As for the clips, they are designed to hold alot of force when pulling the strap parallel to them (which is where most of the force on the rings comes from, such as for muscle-ups, pull-ups, push-ups, etc.). I would never go belaying with them obviously since the force can be exerted in a multitude of directions (which is why carabiners are used). I wouldn't feel comfortable swinging on them like a swing but I think the amount of sideways force is minimal.
It is good of you to question the safety of lashing straps for this application. I would not use them if I didn't understand why they are rated for 1000lbs and how they could fail. You bring up a good point about the clips as I'm not entirely sure how they could fail if they experienced a great amount of sideways force. They are designed to clamp down tighter when force is exerted on the strap normally. Like I said though, I won't be swinging on them like a swing -- I'll swing on them like gym rings.
you do not have documentation about elongation or shock strength. various materials exhibit dramatic differences to different types of force.
if you are moving, you are providing a dynamic load. simply swinging, it'll be similar to a static load, but is not.
while you're probably not placing a falling "shock" load on them, you are not placing a simple load in one direction on them.
you have not commented on any warnings provided with the straps.
you're very probably fine. were i doing the same thing, i'd sew the straps into loops and put a rated link between them. still not a rated system, but it places stress in a known manner on a known material.
pondering this further, is there a reason to not use a length of chain that will fit through the pvc? possibly threaded through an old bike innertube to remove pinching of fingers above the pvc grips? I imagine that wire robe would wear the pvc significantly.
i've always gone with 10:1 so if it says it'll hold 1000 lbs you can safely support 100 lbs. that's with non-life safety loads, and straight lift loads.
if things are going to be dynamic, or support a person i double that, although i've *carefully* operated at as low as 6:1 for temporary purposes (a lower rated attachment while getting something properly located, or as a lift line for something that won't get hurt by the line failing, and a line that won't slice me wide if it fails) keep in mind that using multiple supports at angles changes the forces on each point, so you want to add a further derating factor, 20% at 30 degrees comes to mind, but i look that up to be certain when i need it.
again, i think your biggest risk factor in this system is the buckles. i barely trust those things when loading a truck, and when they fail, they can *launch* parts. (remember that knots will significantly weaken things, but a proper splice will do so *much* less)
It starts to soften at about 170F. I'd use 300F, but if it didn'tsmell up, then ur prob fine.