"...does it really matter whether a Croll opens at 900 pounds or 1350 pounds?"
Bill, Those safety factors are not as over-engineered as many people might be tempted to assume. As an old time caver with a strong background in physics, you probably already know most of what I'm going to write here, but I think it's worthwhile to point out a few things for the benefit of those who haven't thought much about what equipment strength ratings actually mean. Lots of cavers today don't pay much attention to equipment strength ratings, because most of the time, they don't have to. They buy equipment designed for caving by Petzl and other reputable manufacturers, and they quite reasonably assume that these manufacturers design their equipment with adequate strength for its intended uses. As long as cavers use the equipment as intended (and sometimes even when they don't), they assume that it's safe without paying much attention to the actual numbers. After all, why would individual cavers worry about how much safety margin is adequate when expert equipment designers already have done that for them? Acquiring safe caving equipment wasn't always this easy. When many of us old timers started caving (1968 in my case), there was almost no commercially manufactured equipment designed for caving. We could buy a few items, such as carabiners, brake bars, and Jumar ascenders, that were deigned for rock climbing, but, mostly, we built and improvised our own vertical gear using parts and materials manufactured for different applications. There were no static climbing ropes made for caving, so, for long drops, we used braided rope made for marine and industrial uses (remember Samson rope). For rappelling, we used multiple carabiners with brake bars, and we linked the 'biners together using ordinary chain links purchased at local hardware stores. We made our own climbing harnesses from various types of nylon webbing, buckles, and threads. When Gibbs ascenders became available, many of us began designing and building our own chest pulley boxes for our own use, using whatever materials we thought would work. When we heard about other cavers beginning to use rappel racks, cavers in our grotto (the Florida Speleological Society in Gainesville) began buying steel rods and bending them on a homemade jig to make racks for our own use. In those days, we couldn't rely on commercial caving equipment manufactures to determine safety margins for us, so we had to rely on our own judgement to obtain information about product strengths and to decide for ourselves what was suitable for vertical caving. Most of us actually survived. >From those days, I recall looking up the strength ratings of various metal >products, such as steel chain, and of certain nylon products, such as ropes, >and finding that each product had two different kinds of strength ratings. One >was "breaking strength", which specified the force required to break a product >in new condition, and the other was "safe working load", which specified the >maximum load that could be safely applied to the product in normal use. The >ratio of breaking strength to safe working load was called the "safety >factor", which typically was about 4 or 5 for metal products and might be as >high as 10 for nylon rope. The safety factor was intended in part to >compensate for manufacturing variations and for gradual weakening due to >normal wear and tear. More importantly, if my understanding is correct, the >safety factor was also intended to prevent the product form being weakened by >repetitive application of excessive stress, such as by metal fatigue in metal >products or by similar stress damage in nylon rope or webbing. Whenever a >product was subjected to stress exceeding its safe working load, it might not >break immediately, but the excessive stress could weaken the material, >effectively lowering its breaking strength from then on. Repeated stresses of >this type eventually could cause the product to fail with a load significantly >lighter than the initial breaking strength. Thus, in normal use, it was the >safe working load, not just breaking strength, that we should avoid exceeding. I don't know what safety factor is considered adequate for a Croll ascender, but my understanding is that 4 or 5 is common for metal products. Suppose, for example, that it is 4. In this case, an ascender with a breaking strength of 1350 pounds would have a safe working load of 337.5 pounds, a load unlikely to be exceeded by most cavers climbing with normal amounts of cargo. In contrast, an ascender with a breaking strength of 900 pounds would have a safe working load of only 225 pounds (assuming the same safety factor of 4), a load that easily could be exceeded by the weight of a large caver with a heavy pack, or merely by the weight of a caver is some cases. Hence, with these assumptions, an ascender with a 900 pound initial breaking strength might be repeatedly subjected to stresses greater than its safe working load during normal use, causing it to gradually weaken and perhaps eventually to fail with a load much less than 900 pounds. This example considers only the typical case where one caver is climbing normally with an ordinary amount of gear. Significantly higher loads can occur in less typical situations, and I don't just mean dynamic falls. For example, consider certain rescue situations where an ascender might be subjected to the weight of more than one caver (e.g., rescuer + victim). Also, some rigging arrangements, such as tyrolean traverses, can place exceptionally high stresses on equipment. Whenever vertical equipment is used in these less typical situations, it is especially important to consider the actual forces involved to make sure the equipment is not overloaded. Substandard equipment is not necessarily dangerous if you know its limitations and avoid exceed them. The hazards occur when people mistakenly count on equipment being stronger than it actually is. Rod -----Original Message----- >From: Mixon Bill <[email protected]> >Sent: Feb 11, 2011 7:00 PM >To: Cavers Texas <[email protected]> >Subject: [Texascavers] fake Petzl gear > >I suspect that the safety factor in the strength of real Petzl gear >over what is really needed for normal use is so great that the fake >gear is not really dangerous. For example, does it really matter >whether a Croll opens at 900 pounds or 1350 pounds? You should never >take a fall on an ascender anyway. Still, of course, if you think >you're getting Petzl gear, you should get the real thing. Just don't >get paranoid about relying on somebody's Petzl gear because it might >be fake. -- Mixon >---------------------------------------- >A fearless man cannot be brave. >---------------------------------------- >You may "reply" to the address this message >came from, but for long-term use, save: >Personal: [email protected] >AMCS: [email protected] or [email protected] > > >--------------------------------------------------------------------- >Visit our website: http://texascavers.com >To unsubscribe, e-mail: [email protected] >For additional commands, e-mail: [email protected] > --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
