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Post by dougsanders on Dec 20, 2013 11:50:00 GMT -8
Looking for a simple substitute for the Prusik Minding Pulley (PMP) used for crevasse rescue.
Both Seattle and Everett, perhaps other Branches, require Basic climbers to purchase PMPs for crevasse rescue. It is a rarely used spendy tool added onto the sizable outlay made by the new climber. I would be nice to find a simple alternative. Also, it is possible that any climber may find themselves in need of prusik-minding on a pulley but no PMP.
We all know the problem, the gap between pulley side plates and rope is too large; the prusik jams into the pulley, stopping the raise or keeping it from 'minding.'
Although PMP's have a chamfered edge to 'catch' the knot I have never found this chamfer to be important. Any pulley will mind the prusik provided it's side plates are only slightly larger than the rope diameter. Any pulley, even PMPs, will fail to prusik mind if this gap is too large.
Prior to Everett's adoption of the PMP, Everett had tried a number of strategies to keep the prusik from being sucked up into the ratchet pulley. For a while, Everett integrated ATCs; however, they frequently added too much friction. Carabiners dangling from the rope were in vogue for a while.
It would be great if we could come up with a simple substitute to a PMP; something that uses an ordinary pulley and commonly used climbing gear such as prusik cord, carabiners and webbing.
One approach is to use a 'blocking hitch', placed between the prusik and pulley, that is bulky enough to catch on the pulley plates and tight enough to prevent the prusik from burrowing into the blocking hitch.
A girth hitch of 8 mm cordage works well as a blocking hitch but who, aside from some mountain rescuers, carries 8 mm?
More experimentation is needed with this other possibility: Take a single runner or seat prusik, double it to half it's original length and tie an overhand knot encircling (hitching) the climbing rope. There are 2 ways to orient this, only one works. Secure the loops at one end to the pulley carabiner.
Important characteristics of the PMP blocking hitch alternative: 1. Made from universally carried items such as webbing, prusik cord and carabiners. 2. Bulky enough to bridge the side plates and not be distorted and pulled by ratchet prusik into climbing pulley (like the short tail on half a double fishermans can be.) 3. Simple and fast, ideally not a new tie to learn.
Ideas? Thoughts?
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Post by jasonmartin on Dec 20, 2013 11:59:30 GMT -8
Perhaps the best option is to get rid of the pulley all together. A carabiner works almost as well in many applications. I only personally carry a pulley when I'm operating on a two-man rope team. The rest of the time, I just use a carabiner as a pulley.
One can easily mind the the friction hitch without a specialized pulley...
Jason
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Post by gregggagliardi on Dec 20, 2013 13:02:55 GMT -8
Some other options:
1. Use a Bachmann instead of a prussik. It's too big to get caught up in the pulley. Tacoma has used this system for years with good success. Its also easy to release if you need to lower the fallen climber or back off the system if the climber is being pulled into the lip. 2. Tie off the anchor, fixing the rope to the fallen climber. Use a Canadian Drop Loop System to haul out on a new line that is not entrenched. Add a Z to the C if you need it, 3. Invest in a high efficiency prussik minding pulley. Yes they are expensive but worth it (90% efficient versus 70% efficient (cheap pulley) versus 60-65% efficient (carabiner only). For maximum advantage use the high efficiency pulley on the moving part of the Z pulley system, and the cheap pulley at the anchor. The green REI pulley (not the blue one) is the cheapest one that is reasonably prussik minding.
I carry a Microtraxion. Expensive but it has multiple uses: high efficiency pulley, progress capture built in, useful for hauling, useful for rope ascension. It's very lightweight.
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Post by dougsanders on Dec 20, 2013 15:41:53 GMT -8
Arnor Larson developed the PMP in the 80's which was sold by Rock Exotica. He indicated that tests showed the carabiner was about 40% efficient while the PMP, with it's bearings, was 90% efficient. The loss of efficiency was because of the much greater coefficient of friction for aluminum and the much smaller tread to rope diameter (approximately 1:1 vs. 10:1 -for the large PMP.) Interestingly, a steel carabiner was 60% efficient because of steel's lower coefficient of friction.
Everett went to the prusik hitch because of ongoing problems with the Bachmann while practicing crevasse rescue on the glacier with the Basic Class. I can't remember for sure if the problems were with holding the mass or with the hitch jamming into the carabiner; I think it was holding the load after resets. The prusik cleared up the problem.
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Post by jimnelson on Dec 21, 2013 10:40:00 GMT -8
While the efficiency of the pulley is better than a carabiner and the weight insignificant; the simpler method offered by Jason might make some sense for the clubs basic mountaineering course.
Use of the pulley w/prusik and other ratchets could be part of additional Crevasse Rescue curriculum in the intermediate course, or any modular or stand alone Glacier Travel Courses.
Currently the Crevasse Rescue taught in the basic course is a fine beginning suitable for large group glacier travel, but there is more to know. The basic crevasse rescue curriculum focuses on building a raising system and not much more. You pass a fairly specific test at basic, and then because there is nothing additional with the intermediate course you are basically assumed to have full glacier travel knowledge.
btw there are a number of small pulleys that mind a 6mm cord pretty well some as low as $15
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Post by gregggagliardi on Dec 21, 2013 11:55:18 GMT -8
90% efficiency is only possible with a high-quality bearing type pulley. The best I know of is the Petzl Mini. True, it is expensive. As Jim pointed out there are a number of less expensive pulleys that work fine with 6mm cord. The best have prominent square shoulders that prevent binding. They still need some attention, but then again the entire anchor system needs to be monitored.
Practice with a minimal set-up as suggested by Jason is a good way to learn, save money and also know how to set-up a system if someone loses or forgets a pulley. That learning transfers nicely to rock climbing where pulleys are often not available for rescue. A good round spine carabiner is not much less efficient than an inexpensive pulley. Thus if one is going to carry a pulley it is wise to carry a more expensive efficient one.
Our experience with the Bachmann at Tacoma has been good. It needs to be well dressed and cinched up on a short loop of cord. If the loop is too large this tends to cause the wraps to unravel when the biner contacts the pulley. A short loop of 6mm cord just large enough to wrap the biner with only a little cord left to reach the anchor works best. If you want the best possible Bachmann, tie it with Sterling Hollow Block. Jim introduced me to this cord a while back. This stuff is amazing, You can tie a three wrap prussik around your ice axe shaft on bare metal half way down the shaft and hang from it without it slipping.
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Post by dougsanders on Dec 22, 2013 9:01:36 GMT -8
Based on unsatisfactory experience, over the course of many years at their Basic Class crevasse rescue outings, Everett decreased their jamming and slipping issues by evolving from Bachmann to 6mm prusik, and later, from pulleys to PMPs.
Over the decades a variety of materials and pulleys were used in a variety of conditions. While everything can work some of the time, the most consistent satisfactory results were obtained with 6mm prusik and Rock Exotica mini-PMPs. Notes: The mini-PMPs were the only PMPs used during the initial years. Not sure if there is a difference with other PMPs, which are becoming more common, or if it was the side plate gap on the mini-PMP that made it work. The Class ropes are pretty fat, say, 10.5mm.
With respect to force, a 50% efficient pulley on a 2:1 is similar to a 1:1; the amount of force needed to raise the load is the same. Where there are multiple pulleys, the influence of each pulley's efficiency carries through the system. With 90% efficient pulleys a theoretical 3:1 becomes an actual 2.71:1.
This thread was established to discuss techniques, using commonly carried materials together with any pulley, that will assure minding the prusik and holding the load. Ideally, the resultant technique would also work on smaller diameter glacier ropes and with smaller diameter prusiks.
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Post by jimnelson on Dec 22, 2013 11:39:34 GMT -8
Help me understand why a self tending pulley is important? More important for a single rescuer, but can,t you also tend the prusik to keep it from jaming?
I feel like a good ratchet is important for small team glacier travel, but perhaps it should be part of a continuing glacier travel curriculum beyond basic.
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Post by dougsanders on Dec 22, 2013 15:56:33 GMT -8
Here are a few thoughts on why a self tending pulley is important. Some of these come into play during circumstances a bit more complicated than what we practice in, illustrating flexibility. There are probably others.
Everett, and I presume other Branches and organizations such as the Boealps,WAC, etc. teach the 3:1 mechanical advantage system in their Basic Climbing Courses crevasse rescue system. The system assumes that 1 team of 3 will set it up, a prudent and conservative assumption. After all if 2 teams were available we would just pull the person out.
Depending upon circumstances there will be 1 or 2 pullers. If there is one puller (both climbers have gone into crevasse and landed on a snow bridge or at crevasse bottom; or, 2nd is injured) then it is unlikely the single puller will be able to both raise and tend the prusik.
The slope may be steep/icy or there maybe obstacles (including crevasses) that keep the 2nd puller from being able to position themselves to both tend the prusik and help raising.
Puller(s) may not be able hold the load during reset do to icy rope, steep slope or slippery gloves/mitts. The self-tending prusik serves as an automatic belay, redundant to loosing grip on the rope. As such it guards against the puller(s) slipping, dodging icefall, or involuntary letting go due to lightning, avalanche, etc.)
If the prusik is not tended correctly and it jams into the pulley the extrication becomes more complicated and time consuming. At worst, it stops. A properly sized prussik (1" gap) on a self-tending pulley reduces inefficiency during resets. There is no risk of clothing or fingers getting drawn into the pulley; or, freezing fingers as a wet rope runs through them during tending.
Do to injury, poor technique, forgetfulness the tendor may not properly management the tending. In mountain rescue team rope systems, improper tending of prusiks is the most common foul up; tending requires uninterrupted attention.
If one of the 2 pullers must locate to help with edge transition, while the other continues to pull, there may not be anyone in position to tend.
Probably the best argument is the self-tending set-up frees one person in a challenging situation that may present itself in many ways.
Finally, familiarity and confidence with the self-tending set-up in crevasse rescue is a really helpful to someone who goes on to additional small party rescue techniques or SAR, allowing them to focus on other stuff. It is consistent with a serial building block approach to growing climbing skills. Teaching it at the Basic level has been going on for decades across several organizations.
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Post by gregggagliardi on Dec 23, 2013 19:42:50 GMT -8
Check this out. Note the large Bachmann locker and the relatively smaller anchor biner. Also note the short prussik loop.
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Post by gregggagliardi on Dec 23, 2013 19:47:04 GMT -8
Again I tried to post an image (less than 1mb) but no luck.
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Post by jasonmartin on Dec 30, 2013 14:03:42 GMT -8
I don't see anything here that convinces me that a self-minding pulley is important. One need only attend the prussik and there won't be any problems.
I do feel that having a pulley (with a good level of efficiency) on each member of a two person team is helpful, but anything beyond that is really uncommon amongst guides...
Jason
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Post by gregggagliardi on Jan 15, 2014 16:37:09 GMT -8
Doug's post and the responses to same have prompted me to do some exploring and experimenting. I think found something that this group will find interesting. The trick is not the pulley, but the hitch.Some hitches are better pulley/carabiner minding than others. First, here is what I tried in the way of friction hitches, with a without pulleys, with ropes of different diameter (8mm to 10.5mm), with cord ranging from 5mm to 6.5 mm, tied in a closed loop or tied as a split tail,arborist style:
Girth hitch style friction hitches (traditional Prusik, Kleimhiest, Hedden), although they grab well, all perform fairly poorly. The tend to get caught in the pulley even if it has square shoulders designed to make it "prusik minding". The problem is that the coils are difficult to uncoil once they are loaded. They may jam in the pulley or if they don't jam they may not fully uncoil, introducing enough friction to undermine the pulley's mechanical advantage. When used without a pulley (carabiner only), they may pull through the carabiner and thereby totally foul up the system. This latter problem can be partially overcome by running the rope across the narrow end of an HMS carabiner. A particularly good solution is to run the rope through the special end of a carabiner that is designed to trap a belay device (BD and DMM among others make belay biners with this trapping mechanism that is designed to prevent cross loading. This works great on systems that don't employ a pulley, but these specialized carabiners offer no help when a pulley is attached to them.
Generally the best behaved (pulley/carabiner minding) hitches are members of the French Prusik family. These are helical hitches that simply coil around the rope. The best known example is the "autoblock", which is often used as a rappel back-up. The most serious problem with the "autoblock" is that it (a) may not grab (b) it may slip under heavy load and (c) if it encounters any obstacle above it will "tend" (start uncoiling and releasing) even when this is not desired.
Two other members of the French Prusik family that are ideally suited for mountaineering applications, not only as reliable friction hitches but also as progress capture devices, are the Valdotain Tresse (VT) and the Marchand Tresse (MT). The VT is just a short length of cord with an "eye" on each end, created by tying a figure 8 or overhand knot. It is coiled around the rope several times and then the two legs are braided several times around the rope and the two eyes clipped to a carabiner. When loaded this hitch grabs tightly, especially the braids, which literally bend the rope. This is best shown in a picture, which I am willing to post here if someone can tell me how we do it in this new version of Proboards. The MT is just a short loop of cord, essentially a hero loop, tied in the usual manner. It is coiled around the rope several times, exactly as one would do with an autoblock. The difference is that the loop ends are not immediately clipped to a biner or harness but instead braided 2 or 3 times around the rope before clipping to a biner.
The VT and MT work on the same basic principle, which is the principle behind finger handcuffs. The more you pull away from the coils the tighter they get, yet even a small amount of force in the opposite direction causes the coils to slightly open easily releasing the lock. Arborists have for years used these hitches for rope ascension. These hitches will work with carabiners as well as pulleys. The typical arborist system for ascension uses locking biner attached to the belay loop of the harness. The cord used to tie the VT is called a split tail and is sold as such by numerous companies that supply arborists. One interesting property of the VT is that it can be tied in cord that is nearly the same as the rope. It is tied above the belay device, braided end towards the belay device.
So why should an arborist's technique ascending trees be useful for crevasse rescue? This is best understood by way of not only a photo but a short video clip. I will try to describe it in words. The key is in the set-up procedure. Regardless of whether it is a VT or MT one end (call it the eye) is first attached to a carabiner. Next the rope, either on its own or first run around a pulley is clipped to the carabiner. Finally the second eye is clipped to the carabiner. The resulting configuration as seen on the fat end of an HMS biner is: eye, rope (or rope in pulley), eye. Keeping the rope in the center of the two eyes is the key.
When constructed in the manner described what happens when the rope is loaded? Well the coils and braids tightly catch the rope. This is really impressive. Seeing is believing. When rope is pulled through the system across the pulley or biner (or DMM Revolver), the braids run into minor resistance and open, which then causes the coils to uncoil and allow the rope free passage through the system. Should the package suddenly drop, the braids and coils grab the rope and arrest the system.
After having played around with both the MT and VT I am convinced that these are the friction hitch/ progress capture mechanisms of choice for crevasse rescue systems based on minimal gear (no pulleys; or pulleys with or without biners serving as pulleys). Frankly, the only better systems that I know of are those based on devices that combine the pulley function with a rope capture function (e.g., Petzl Micro Traxion, CT Roll N Lock). These are great devices but they also cost about $100 with tax. A simple carabiner only system will work with some effort. If cheap pulleys are added, this may help a little, but not much. High quality pulleys (e.g., Petzl Omni) will substantially reduce friction and add mechanical advantage but at increased cost (2 to 3 times more expensive than cheap pulleys). Are these high tech tools worth it? Probably not for basic course students, but they are highly recommended for rope leads and trip leaders. In any event it is reassuring to know that good crevasse rescue systems can be constructed with no more than accessary cord, biners and one or more haulers.
I know that visually following a narrative is frustrating. The points made can be immediately "grocked" with pictures and video. I am more than happy to provide these visual aids to the forum as a whole (if I can figure out how to do it) or via email. Send your request to gagliardi@nventure.com.
Note to Gene: I think that we ought to open this forum up to arborists, cavers and canyoneers. These communities have much to offer climbers and vice versa.
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Post by gregggagliardi on Jan 17, 2014 10:27:12 GMT -8
Here is a video that shows how to tie a Valdotain Tresse (VT) Other knots used by arborists www.treebuzz.com/pdf/0505_geneology.pdfwww.treebuzz.com/pdf/climbing_hitches.pdfThe Machard Tresse is a loop and tied similarly to the VT. We know the Machard as the "autoblock'; the English call it a French Prusik. When there is enough cord left to braid it several times around the rope it is the Machard Tresse (MT)
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Post by jimnelson on Jan 17, 2014 13:41:01 GMT -8
Thanks Gregg, I really enjoyed the articles about the geneology of friction hitches.
I wanted to mention another ratchet or PMP. The petzl Tibloc, no pulley but very effective and a personal favorite.
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