Forums: Climbing Information: The Lab: Re: [patto] Cams retracted from ?% to ?%: Edit Log




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Nov 9, 2008, 12:58 PM

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Registered: Aug 22, 2005
Posts: 223

Re: [patto] Cams retracted from ?% to ?%
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C'mon guys, this is the Lab, not the beginner's forum. I wasn't looking for advice on placing cams, but for some meaningful explanations for the "useful range".

I made a graph with working ranges of my rack and according to different manufacturers' specs, it changes shape from "best rack in the universe" to "drop everything and get a real rack" (yeah, I know the one with send it to me for proper disposal...)
The useful ranges in the most plausible graph are varying from 10-25 % to 60-75 % opened, based mainly on intuition and experience with each type of SLCD, but this is way different from the manufacturers' specs and this intrigued me.

Now, you all the engineers from the Lab can you bring some light on this ?


Edited to add this:

Today I received an answer to my questions from a DMM engineer and I think it might be of some interest to you too:

"Firstly I do find it slightly odd that we talk about 1/3 and 2/3 optimum range. We would, as you suggest be better to align ourselves with Wild Country on this, not only because we use the same cam angle but also the EN Standard requires we test our cams at 1/4 open and 3/4 open, the strength rating we give is based on the test in these two positions. We will look at changing this when we reprint the instructions.

The real expansion range of the cams is from fully retracted to fully open. However there are practical problems that the less experienced user may not have appreciated. As you know if a cam is fully retracted in a crack then it is well nigh impossible to remove. This is because you need to retract the cams from the contact surface to remove them. Also cams have a tendency to walk into cracks. Obviously the experienced user can place a cam in a narrower crack and still get it out.

Similarly as you get towards the fully open position there is a judgment call to make with the rock type that the cam is placed in. If it is very soft rock then as the cam engages the rock it can open significantly further, ultimately to a point where the cam would fail. Again experience is probably the best teacher to the user.

This explains why we are fairly conservative when we talk about 'preferable expansion ranges'. Other manufacturers such as Metolius go a stage further by colour dot marking the sides of the cams to indicate the preferred/optimum expansion ranges.

Other issues that are relevant are the aspect ratios of cams. The aspect ratio is the relationship between axle length and the cam 'height'. On a small camming unit the axle length is far greater than the cam height which makes the cam very stable. On a large camming unit the reverse is true and the cam is less stable. Ideally the optimum scenario is when the axle length is equal to the cam height. Also the aspect ratio changes within the expansion range of a camming unit. It the retracted position its aspect ratio is more stable than when the cam is fully open. When the cam is less stable then there is more of an issue of the cam moving or walking in the crack. This could in certain situations affect the holding power of the cam.

Finally, to add to equation is the cam angle. The greater this is then again you tend to lose stability. DMM use 13.75 degree cam angle. BD use a slightly larger one which helps, along with the twin axle design to increase their expansion range. This instability is more noticeable on larger cams. For the cam angle to work well the spring pressure should be even throughout the opening range as this will give the best fit for cam angle and rock. As I mentioned before rock type is very important. Rough rock engages the cams well, where as smooth rock may not even engage the cams on parallel or marginal placements and the cam may just pull straight out. This is especially problematical on polished limestone and slate.

As you know cams are complex bits of kit and their operation is a skill that is acquired through experience. In our instructions we have to educate the user into the correct operation of the devices in a clear and concise manner and if we err on the side of caution you can perhaps see why. Our instructions are guidelines for use. As it says in the instructions, there is no substitute for instruction by a trained and competent person."

The letter has also a footer:"Any views or opinions are solely those of the author and do not necessarily represent those of DMM Engineering / International Ltd."


(This post was edited by verticon on Nov 15, 2008, 11:42 AM)



Edit Log:
Post edited by verticon () on Nov 9, 2008, 1:08 PM
Post edited by verticon () on Nov 9, 2008, 11:00 PM
Post edited by verticon () on Nov 11, 2008, 2:43 AM
Post edited by verticon () on Nov 15, 2008, 11:42 AM


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