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fracture
Nov 11, 2004, 1:05 AM
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This recent thread about how far to retract cam lobes got me thinking, and leaves me with a question for any physics/cam gurus (yes, I'm talking about you, david.yount :D).
Why are we taught to make sure that all the lobes on a cam placement are retracted the same amount?
From what I understand, since they have a constant angle, the outward force by the cam is the same no matter how retracted it is placed. Does this not apply for some reason if different lobes are retracted different amounts? If so, why does a different outward force on different sides of the cam make the placement less secure (will it potentially flip out due to lesser friction on one side or something?). Or is the concern something about the propensity of the placement to walk?
Take a look at the black diamond camalot instructions if this is confusing. I'm refering to their pictures 7 and 8. They say if you place it this way "it may not hold a fall". I'm interested in why.
I seem to spend most of my trad climbing time falling on cams (:lol:); so it's helpful to try to understand how the hell they work. ;)
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fracture
Nov 12, 2004, 12:34 AM
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I went ahead and asked Black Diamond. Looks like it's not an issue, really (at least not how I thought it was). :D
In reply to: From: Jeff Maudlin
To: Jordan DeLong
cc: bdmo@bdel.com, Kolin Powick
Subject: Re: Camalot Placement Question
Hi Jordan
The examples given are extreme and the warning fairly ambiquous. They
show the upper and lower limits of retraction and expansion. Of course
some offset in a placement is acceptable, desirable and undoubtably
inevitable...as long as all 4 cams are retracted within an acceptable
range. What we warn about is 2 cams extended (not retracted at all) and
2 cams retracted. This is not an optimal placement and "may not" hold a
fall. It's somewhat possible this placement may also hold a fall but the
point being if this is what you're presented with you might strongly
consider a different piece or placement. 4 cams are always better than
2 right? I hope that clarifies it some. Thanks for asking about this.
Jeff
On Thu, 11 Nov 2004, Jordan DeLong wrote:
>In the camalot manual it says not to place camalots where all four
>lobes are not all retracted the same amount. It says if you place
>them with offset lobes, that they "may not hold a fall". This is
>in the text refering to pictures 7 and 8.
>
>Why not? Why is such a configuration potentially not going to hold
>a fall? Or does it just say that, and it's really an issue of how
>the cam may walk?
>
>I have trouble understanding how this is any different from a WC
>offset or a offset alien placement.
>
>Thanks,
>--
>Jordan DeLong
>fracture@allusion.net
>
Jeff Maudlin (jeffm@bdel.com)
http://www.blackdiamondequipment.com http://www.biblertents.com
http://www.scarpa-us.com http://www.franklinclimbing.com
Black Diamond Retail, Inc. (801) 278-5533 (phone)
2084 East 3900 South (801) 278-5544 (fax)
Salt Lake City, Utah 84124
DISCLAIMER: Unless otherwise indicated, this correspondence is personal
opinion and NOT an official statement of Black Diamond Equipment Ltd. |
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tchamber
Nov 12, 2004, 12:53 AM
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Good post fracture. Quite informative, something I've wondered for a while as well. It seemed to me that they implied if one set of lobes was say 25% retracted and the other say 75% retracted that your piece might blow. I'm glad to know that isn't the case (although I think I'll still stick with "proper" placement whenever possible).
I'm also quite impressed that BD contacted you so quickly. Very cool.
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mattm
Nov 13, 2004, 12:27 AM
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I'll take a stab at this one...
I believe it has to do with spring tension and the coefficient of friction between the cams and the crack walls. In and ideal placement where both sides are retracted the same amount, the springs thusly push outwards on the crack wall an equal amount. This equal "pushing" results in a similar coefficient of friction on the cams. That way, when a load is applied to the axle, all four cams (because of equal frictional forces) "activate" and apply equal pressure outward and the same "instant". If the cams were not set evenly the outward force of the cams due to the spring would not be equal (spring force F = kx where k is the spring constant and x is the distance the spring has moved - greater compression equals more force) and thus the "upward" frictional force at the cam/rock interface would not be equal as well. This MIGHT result in the cams not engaging the rock at the same time and in an extreme example MIGHT cause the less compressed cams to slip and thus the unit will not hold.
A rough example of this could be stemming between two walls with your legs - you would want to keep both legs equally extended because if you leaned to one side you will more likely fall as one side has a greater downward force (this kind of works but fails to address the constant cam angle)
i think that all makes sense...
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corpse
Nov 13, 2004, 2:05 PM
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Has there been any real testing on this spring tension stuff?
I don't think spring pressure really affects the "biting" of the cams, as spring force is sooo small in comparison to the force the cams are extering during a fall. I don't think it really affects the "reaction time" of the cams engaging in the crack either - the force of a fall, to cam, is very hard hitting, brutal and fast - a dinky spring that can be
The only case where I heard spring pressure comes in handy, is to help prevent a piece from walking - but then, to me, the answer is a longer runner, not a stiffer cam spring.
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monopocketmojo
Nov 13, 2004, 2:24 PM
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i absolutely agree with corpse. when you think about the forces generated during a fall, that tiny spring couldn't possibly be what keeps the cam in place and exerts the outward forces. what would really does that is the force the lobes of the cam exert on the rock that it's making contact with (which is why cams have features on the lobes and are not completely smooth) due to their attempted expansion. the springs probably have a minor part in providing the initial force outward to keep the cam in place when it is not holding a fall (not only to prevent walking but just to keep the lobes pushing outward in order for the cam to stay in place).
when the fall forces are exerted on the cam, the cam is pulled in the direction of the fall. since the cam is angled down, as it is pulled, the friction against the lobes stops them from sliding across the rock, so the lobes try to expand, putting more force against the rock it is in contact with. this force is both outward (perpendicular) to the cam and slightly downward (in the direction of the fall), to provide the force needed to hold the fall. it is that outward force from the lobes, not the spring tension, that provides the force needed by the cam lobes to counteract the force of the fall.
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fracture
Nov 13, 2004, 3:32 PM
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In reply to: i absolutely agree with corpse. when you think about the forces generated during a fall, that tiny spring couldn't possibly be what keeps the cam in place and exerts the outward forces.
Right. I don't think mattm is right.
In reply to: what would really does that is the force the lobes of the cam exert on the rock that it's making contact with (which is why cams have features on the lobes and are not completely smooth) due to their attempted expansion.
I believe the original friends had completely smooth sides. I don't think cam teeth really change the coefficient of friction. I recall seeing a quote somewhere from Ray Jardine blaming the teeth on marketing; dunno if that's true though.... :lol:
The new C4's seem to hardly have any teeth, btw.
In reply to: when the fall forces are exerted on the cam, the cam is pulled in the direction of the fall. since the cam is angled down, as it is pulled, the friction against the lobes stops them from sliding across the rock, so the lobes try to expand, putting more force against the rock it is in contact with. this force is both outward (perpendicular) to the cam and slightly downward (in the direction of the fall), to provide the force needed to hold the fall.
Almost.
It puts an outward force, yes, but what that changes is the maximum force from static friction. If the downward force (from the fall) is greater than that maximum friction force, then it will slide out. Static friction is an upward pointing force (opposite to your fall).
I don't know if the force from the springs just add into the outward force (probably; I guess in real life it depends how fast the springs expand).... But it seems like it shouldn't matter; the point of a cam is that the normal force (against the rock) is proportional to the load applied, and thus there should always be enough force from static friction if the coefficient of friction is higher than whatever is supported by the angle of the cam.
This seems consistent with what black diamond said as well: I don't think it actually matters if the cams are somewhat offset. (And I have fallen on at least one such placement, btw).
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climbhoser
Nov 13, 2004, 5:15 PM
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As per the JArdine quote...the teeth are a marketing gimmick, but they do have a functional value. Mostly their value is in anti-walking. When the rope pulls up against a cam it can push it directly back into the crack. With the teeth sometimes this is more difficult. However, in Indian Creek this wouldn't help.
And yeah, what BD said, just as long as all four lobes are past 50% then it doesn't matter where they are in this range. Well, actually it does, but ONLY for BD cams.
This is because they're double axled. What happens is if you place a BD cam in a horizontal or a diagonal crack falling on it will result in changing the intercept angle, and CAN cause the cam to fail. A lot of people don't know this, so with BD cams just ALWAYS make sure they're as cammed as humanly possible when in these sorts of situations. This may also lead to part of their warning about cam lobe retraction, as even if the situation is, say, thre lobes 85% retracted and 1 lobe maybe 60% retracted in a horizontal. If the 60% lobe is on the upward side of the cam then in the event of a fall the intercept angle will change and it will actually be something like 20% retracted (depending on a lot of things, of course), and might cause the cam to fail.
It's why the double exle design is sketchy sometimes. Oh, and if you don't believe me I have a Malcolm Daly quote on it for ya
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thegreytradster
Nov 14, 2004, 5:46 AM
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Let's consider the case where you have 100% uneven cam placement. What have we done? Turned a cam into a Tricam at its maximum extension. Not a good idea!
All the talk of springs and things; the springs have nothing to do with holding power! That's determined by a mathematical relationship of cam angle and generally equals about 1.5 X the downward force applied horizontally. That can have some unexpected consequences. I once took a fall on a smallish cam, (well placed) that held the fall quite nicely and then watched helplessly as the cam squirted slowly out of the crack as I was hanging on it. The 1000 or so ft-lb of impact translated into 1500 ft-lb of grip in the crack and was plenty to hold the fall. The 200 lb of static pull only translated into 300lb or so of "grip" on the crack and it wasn't enough, sending me sailing to the next piece.
Teeth? A marketing gimmick I like and would have probably prevented that plunge!
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euroford
Nov 14, 2004, 4:29 PM
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i thought the springs and teeth prevented 'cam bounce'?
:D
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monopocketmojo
Nov 14, 2004, 5:13 PM
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In reply to: Let's consider the case where you have 100% uneven cam placement. What have we done? Turned a cam into a Tricam at its maximum extension. Not a good idea!
how do you figure this? i'm not sure i can picture what you're saying about the 100% eneven placement. Does that mean all the lobes are different, that one side is fully cammed, the other fully extended?
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thegreytradster
Nov 14, 2004, 6:18 PM
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one side fully camed the other fully extended
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climbinginchico
Nov 14, 2004, 7:00 PM
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In reply to: i thought the springs and teeth prevented 'cam bounce'? :D
Of course they do! :righton: :lol:
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mattm
Nov 14, 2004, 11:17 PM
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In reply to: Has there been any real testing on this spring tension stuff? I don't think spring pressure really affects the "biting" of the cams, as spring force is sooo small in comparison to the force the cams are extering during a fall. I don't think it really affects the "reaction time" of the cams engaging in the crack either - the force of a fall, to cam, is very hard hitting, brutal and fast - a dinky spring that can be The only case where I heard spring pressure comes in handy, is to help prevent a piece from walking - but then, to me, the answer is a longer runner, not a stiffer cam spring.
I'm gonna have to disagree with you on this - I re-read my post and nowhere do I say the spring force has to do with holding power - the point I was making was that the springs ARE necessary for keeping the cam in place BEFORE you fall on it. If there were no spring the only thing keeping those cams in place is the cams own weight (and some rope weight I'd guess) if you even lightly bumped that cam up you'd no longer have any outward force (no downward and thus now outward force) and the cam would just fall out. If you were lucky the cams would catch on the way out and hold but that's a little dicey I'd think. Springs CREATE the initial outward force (and yes, VERY small holding force) that keeps them in palce. As soon as you load the cam on it's axle the camming forces far exceed the springs initial outward pressure.
EDIT: Giving more thought to it and drawing a nice little vector diagram - I'm gonna ding myself for not considering some equal and opposite force rules. DAMN - mechanical physics was too many years ago - anyway - The unequal spring loading idea - yeah - just ignore that one. I DO stand by the idea that spring tension is important in keeping the cam in place so that it can load properly. Another way to think of it... Say you have an imaginary crack - one side consists of rough, squamish granite that is very grippy with cam lobes (high friction). The other side of the crack consists of table top smooth marble (Low friction). You place a #4 friend in the crack but its not the greatest placement and is fairly tipped out. You'd be more concerned with the smooth, marble side because the friction on that side is less and therefor less likely to hold INITIALLY. Stronger springs would push harder on the wall and increase that initial friction making the marble side more secure. Once the cam is loaded, the force on both sides far exceed the spring forces, allowing the cam to hold greater loads.
brain hurts - going climbing now
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mattm
Nov 14, 2004, 11:21 PM
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In reply to: i thought the springs and teeth prevented 'cam bounce'? :D
T1ish
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chriss
Nov 15, 2004, 12:36 AM
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I think waht BD is refering to in the section you mention, is that when fully extended the cam is at the limit of it's rotation. Look to the section on how the cams hold the applied force, in the first part of the paper.
Someone brought up the difference between 1 axle SLCDs and the Camalots. One of the biggest differences is in the range of cam rotation. The Camalot uses the axles to stop the cams from overrotating. Limiting the largest part (largest radius) of the cam from going above the axles (holding the cam vertically). This is regardless of where the other cams are.
Single axle cams have the "cam stops" on the upper part of the cams. So the (single axle) cams are limited by the postion of the opposite cam. The largest part of the cam can rotate until it is completely opposite the opposing cam. The cam stop moves with the opposite cam.
Check this with some SLCD cams in your hand. Hold 1 cam compressed and then move the opposing cam through the full possible range.
Both single and double axle SLCDs are perfectly safe if used correctly. Someone also hints that the single axle design is safer than the double. The cam range issue mentioned above make the Camalot extend out to its maximum range if it should ever need to and load like a chock.
chris
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fracture
Nov 15, 2004, 4:00 AM
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In reply to: What happens is if you place a BD cam in a horizontal or a diagonal crack falling on it will result in changing the intercept angle, and CAN cause the cam to fail. .... It's why the double exle design is sketchy sometimes. Oh, and if you don't believe me I have a Malcolm Daly quote on it for ya
I'm definitely interested if you can provide more information on this.... That quote, or even an explaination of how the angle would change would be neato.
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catbird_seat
Nov 20, 2006, 6:13 PM
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In reply to: I think waht BD is refering to in the section you mention, is that when fully extended the cam is at the limit of it's rotation. Look to the section on how the cams hold the applied force, in the first part of the paper. Someone brought up the difference between 1 axle SLCDs and the Camalots. One of the biggest differences is in the range of cam rotation. The Camalot uses the axles to stop the cams from overrotating. Limiting the largest part (largest radius) of the cam from going above the axles (holding the cam vertically). This is regardless of where the other cams are. Single axle cams have the "cam stops" on the upper part of the cams. So the (single axle) cams are limited by the postion of the opposite cam. The largest part of the cam can rotate until it is completely opposite the opposing cam. The cam stop moves with the opposite cam. Check this with some SLCD cams in your hand. Hold 1 cam compressed and then move the opposing cam through the full possible range. Both single and double axle SLCDs are perfectly safe if used correctly. Someone also hints that the single axle design is safer than the double. The cam range issue mentioned above make the Camalot extend out to its maximum range if it should ever need to and load like a chock. chris
Not having the mechanical engineering background, I am at a loss to adequately express this in technical terms, but I've noticed this just from empirical observation. Just take a Friend and, without retracting the cam lobes with the trigger, push it into a crack. Do this several times and note how often the cam lobes are equally retracted. Now do this same test using a double axled Camalot. What do you find?
(This post was edited by catbird_seat on Nov 20, 2006, 6:13 PM)
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chriss
Nov 25, 2006, 12:46 AM
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I think I understand your description, but I don't see the "advantage". Spring tension would not vary that much in either style SLCD. I think the shape and surface of the crack sides would have a greater influence than the springs ever would.
chris
And this thead was 2 years old.
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catbird_seat
Nov 25, 2006, 2:06 AM
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Spring tension is irrelevant to the discussion about whether cams can be pushed. In the double axle cam, the lobes "flag" back from each axle. In the single axle cam, if you push one lobe (by contact with the side of the crack) the spring acts on the other lobe to push it the opposite direction. This does not happen in a double axle cam.
If you don't believe me, then you should go experiment with each type of cam yourself and make up your own mind.
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chriss
Nov 25, 2006, 7:11 PM
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catbird_seat wrote: Spring tension is irrelevant to the discussion about whether cams can be pushed. In the double axle cam, the lobes "flag" back from each axle. In the single axle cam, if you push one lobe (by contact with the side of the crack) the spring acts on the other lobe to push it the opposite direction. This does not happen in a double axle cam. If you don't believe me, then you should go experiment with each type of cam yourself and make up your own mind.
You are correct about the movement of the cams. And you are right, again, it is not the springs.
You said.
"This advantage is that because the springs act between each lobe and the axle base, rather than one lobe against the other, there is a greater tendency for the lobes to remain at equal retraction, even though one may be careless in placement by "pushing" the cam into the crack, rather than "placing" it. "
The "advantage" you speak of is in the cam stops, not springs. However be careful of the flexible shaft. This can bend when pushed and allow even the double axle cams get out of "position"(for the lack of a better word).
Pushing a SLCD into a crack is not the right thing to do, placing it is. But everybody does it sometimes, me included.
chris
edit for typo
(This post was edited by chriss on Nov 26, 2006, 5:15 PM)
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catbird_seat
Nov 26, 2006, 6:42 AM
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In the Camalot each axle doubles as a stop for the lobe on the opposing axle. In all others the stops are machined into the lobes themselves.
I'm not saying that pushing a cam is a good idea. It is better to place your cams, but I maintain that the main reason people like then is because they are more forgiving in their placement than other cams when pushed.
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jonathan.gaillard
Oct 14, 2010, 8:05 PM
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I wanted to revive this thread because of the discussion on the last bit. I did a bit of searching and didn't find a whole lot of information on this subject, perhaps I missed it.
It would seem there is an advantage when a camalot walks. With a single axle cam, I have noticed that the pair of lobes that are walking can invert together (one inverting the other retracting) in some circumstances. I think camalots would be much less likely for this to happen because the lobes are pushing off the axle. I only own single axles though, could someone confirm or deny this?
This also being responsible for being able to be pushed easier of course, however I would be more interested in them keeping a better "form" while walking...
Thanks guys !
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