|
Mail
Order:Need
Advice?
These notes are intended to assist the
climber who has already gained some knowledge of ice climbing
and is fully aware of its risks but still wishes to progress
further up the grades, and indeed the mountains. They
are brief and not designed to be comprehensive in any
way. Ultimately climbing is a dangerous sport and claims
many casualties each year. One of the guiding principals
of British climbing and mountaineering is that it is the
individual climber is responsible for his or her own safety.
If you cannot accept this then this site and probably
climbing in general is unlikely to suit you. May we refer
you to this
very interesting site instead!
|
 |
Myths,
Cautions & Techniques of
Ice Screw Placement
|
|
a
summary of two years of research by
|
|
|
|
For
years manufacturers and "experts" have been
telling ice climbers to place ice screws in certain
ways, explaining that the holding strength will be
improved in these situations. Over the last two years
Black Diamond has diverted several hundred of its
batch test samples towards investigating these placements
and answering some fundamental questions. What length
of screw is strongest? What angle should the screw
be placed? When should the screw be tied off? What
is "good" ice?
| HOW
WAS THE RESEARCH DONE? |
To
investigate these issues Black Diamond designed
and built special steel cells, or buckets, that
allowed us to load ice screws in our Universal Test
Machine using actual ice. These ice cells are made
of 1 .3 cm thick steel walls, 6 cm deep, with a
surface area 5 cm high and 3.5 cm wide. These cells
are prepared by filling with normal tap water and
allowing to freeze at 10±5 degrees below
zero centigrade. Freezing takes about 72 hours during
which time the ice expands, cracks, bulges, and
overflows the cell.
The top few inches of really bad ice is chopped
off and a new layer of water is added and allowed
to Freeze. A screw is placed in one half of the
cell and pulled to failure at 12 cm per minute.
The screw generally fails by cratering the top 5-8
centimeters of ice away from the screw, bending
of the screw and finally, either levering the hanger
off the head of the screw, breaking the tube of
the screw, or pulling the screw out of the ice.
A second screw is placed in the other half of the
cell near the first screw's hole and similarly pulled
to failure (yes, the ice is shattered and broken
from the first test). After the tests are completed
any broken-off screws are removed by chopping them
out with an ice axe. The cell is placed back into
the freezer and water is added to the old test holes
and allowed to freeze onto the remaining ice. The
cells are used repeatedly in this fashion up to
20 times before the entire cell is allowed to thaw
and be 'regenerated". As can be inferred, this
technique of ice preparation is highly variable
and unpredictable (not unlike real world ice), yet
has yielded conclusive and supportable resuIts.
The trends observed in Black Diamond's static lab
tests were similar to those recorded in static tests
by REI Engineers in glacier ice and by Craig Luebben
in Ouray and Boulder Canyon ice. This suggests that
the trends observed in laboratory ice are similar
to the highly variable conditions and results found
in the field.
| WHAT
LENGTH OF SCREW IS STRONGEST? |
I ntuition
holds true in that long screws are stronger than short
ones. However, the variation is relatively small for
dense solid good ice. This conclusion does not hold
true for "bad" ice - ice that is detached,
cauliflowered, chandaliered, aerated, hollow, and/or
slushy.
Table 1
shows the number of samples, average, standard deviation,
high, and low values obtained duringroutine
batch testing that Black Diamond has conducted in
ice during the last 2 years.
| Length |
22cm
|
17cm
|
13cm
|
combined
|
| #samples
|
86
|
113
|
63
|
262
|
| Average
(Ibf/kN) |
5006/22.3
|
4853/21.6
|
4279/19.0
|
4764/21.2
|
| Std.
Dev (Ibf/kN) |
1299/5.8
|
1433/6.4
|
1275/5.7
|
1377/6.1
|
| High
(lbf/kN) |
9074/40.4
|
8333/37.1
|
8269/36.8
|
9047/40.2
|
| Low
(Jbf/kN) |
2748/12.2
|
1979/8.8
|
2387/10.6
|
1979/8.8
|
|
Table 1: Batch test strength results
for Black Diamond Ice screws placed in solid
dense ice.
|
All
this data is from screws placed within 5 degrees perpendicular
to the ice surface. As can be seen from Table 1 there
is a very large spread in the data, reflective of
the fact that not all ice is equal.
Does this data mean that you should always place the
longest possible screw? Mostly, but not necessarily.
There are several factors to consider. Assuming that
the ice is thick enough for any screw, the placement
should be strongest with the longest screw. How much
surface ice did you have to remove to uncover good
ice and how much good ice is left? How pumped are
you while trying to place the screw? Is this the only
piece of protection you are likely to get for some
distance? How far out from the ground or belay are
you?
My recommendation is to place the length that makes
you most comfortable in terms of protection level
and pump factor. When you are close to the ground
or belay you have the potential to generate more force
on your protection. Make these placements as bomber
as you possibly can. In other words, use longer screws
when close to the belay or even equalize two placements
if you are unsure of the quality of the ice.
|
|
|
Figure 1: Angle convention for testing
ice screws.
|
| WHAT
ANGLE SHOULD YOU PLACE THE SCREW? |
This
question was investigated by placing screws to the hilt
at various angles. Perpendicular is chosen as the reference
and labeled zero degrees. The conventional "place
the screw at a 10 to 15 degree angle against the direction
of pull" is a negative angle. Placing the screw
in the direction of loading is labeled a positive angle,
see Figure 1. As can be seen in Figure 2 there is a
very dramatic change with angle of placement. What we
observed is that placing the screw in the direction
of loading is significantly stronger. In fact, at 15
degrees from perpendicular the screws are over two times
stronger when placed in the direction of load than when
placed against the direction of load. The data included
in Fig. 2 is a compilation of all three lengths of Black
Diamond ice screws using the highly variable test conditions
described above. It is amazing that such a strong trend
exists in such a variable experimental setup.
A general trend shows that screws are stronger when
placed in the direction of loading. For pound-in type
protection this trend is opposite, as expected.
 |
|
Figure 2: Failure load versus angle of
placement. See Figure 1 for a definition of
the placement angle.
|
| SO,
WHAT DO THESE PLACEMENT ANGLE RESULTS MEAN? |
Should
you place your screws 10 to 20 degrees in the direction
of pull? The answer depends on several factors:
First, is that these results are only for Black Diamond
Tubular Ice Screws. In other words, for these results
to hold true the screw must have well spaced, external,
high-relief threads that resist pullout. For pound-in
protection you must always place them at an angle against
the direction of loading in order to provide any level
of holding strength. (See Table 1.)
Second, what is the ice quality? Is it detached, hollow,
slushy, and/or rotten? If so, and this is the only option
for protection, it may well be better to rely on the
potential hooking/lever resistance ability of the screw
rather than the holding power of the threads (an ice
hook may be better in these conditions).
If the ice is shaded, thick, dense, and 'good' the placement
will be stronger if placed in the direction of loading.
Third, if there is any possibility that the screw will
melt out during the time you will need it to support
a load, you should not angle it in the direction of
the load regardless of the quality of the placement
or of the ice. I have personally seen screws melt out
so much that, in the time it took to lead and follow
a pitch, a screw at half height on the pitch could be
removed without unscrewing.
This occurred on Experts' Choice Center W16 in Canada's
Waterton Park which was south facing (full sun), with
the ambient temperatures below freezing.
Due to the variables of ice condition, aspect, and temperature
it is important to do what is needed for the specific
route, conditions, and protection options you have at
hand. Simply stating that all ice screws should be placed
at one particular angle is not sound advice and could
actually lead to failure of that protection.
Every leader must carefully consider every placement
and variable they can to optimize their protection system.
|
|
| Figure
3. Effect of exposed leverage on the
strength of a placement. The data points
represent averages of tests conducted
over several years with no record of what
the angle of placement was (approximately
10 samples per data point). The line represented
by Tie Off" is the average of four
tie off tests (7-8 cm of leverage) that
ranged from 2354 to 3299 lbf (10.5- 5.7kN).
This suggests that if the screw sticks
out more than Scm it should be tied off
to increase the strength. |
|
| WHEN
SHOULD YOU TIE OFF A SCREW THAT IS NOT PLACED
TO THE HILT? |
In
the limited testing we have conducted with tie offs
and various extensions clipped into the hanger, our
data suggests that it is best to clip in directly to
the hanger if the screw sticks out less than 5 cm. If
the screw sticks out more than 5 centimeters, tie it
off as you normally would. Figure 3 shows the effect
of increased leverage on the screw versus strength.
In the few tests conducted with tie offs the average
strength is near 2700 lbf/12 kN. The typical failure
mode of tied off screws is that, after about 1500 lbf/6.7
kN, the screw flexes and bends causing the sling to
slip to the head of the screw, resulting in high leverage.
Failure is generally caused by the sling being cut by
the edge of the hanger in the 2000 to 3500 pound (8.9
to 15.6 kN) range. For the majority of placements this
strength range is enough to hold typical falls. However,
with a high impact force rope and a high fall factor,
failure of the tie off could result. The best option
is to simply place a screw of the appropriate length
and not have to worry about tie offs altogether.
A word of caution relative to the tie off conclusions.
This data is very limited. There is a very large variation
in any given test setup. It is possible that the trends
stated above lead us to the wrong conclusion. More testing
is underway to verify these trends from a more controlled
experimental setup and statistical analysis on a larger
data set.
In
our experience most ice is bad to varying degrees. Good
ice is relatively difficult to find on modern vertical
or overhanging routes. Good ice generally is found on
lower angle slabs where the ice forms thick solid flows
(blue, green, or clear ice). If the ice is hollow, layered,
slushy, aerated (white appearance with obvious air pockets),
chandaliered, cauliflowered, etc. screw strength should
be suspect.
What is the surface ice like? Does it dinnerplate easily?
If so you must clear this surface ice off. My experience
has indicated that dinnerplate conditions occur most
often right after or during rapid temperature changes
and usually at colder temperatures. Does the ice have
running water under or on it? If so, you may have to
worry about the screw melting out due to the water.
In general I have found that warmer ice shatters less
and is in general capable of holding higher forces.
From talking with climbers who have fallen (including
myself, unfortunately) protection that held was more
often in warmer conditions than in colder conditions
and the protection that failed was almost always in
cold brittle ice or horrible thin or slushy ice.
A final word of caution relative to equalizing
two screws. Ice generally fails horizontally. Placing
two screws horizontally also increases the force on
the screws due to the triangle force multiplier (American
Triangle). Place the screws vertically with one screw
above the other. This will give the best possible
chance for both screws to hold.
Below are excerpts from a letter sent by Alex Lowe to
Chris Harmston relative to the above information. Alex
was formerly the Quality Assurance Manager of Black
Diamond. Some of the research reported here was from
early work that Alex conducted while he worked for Black
Diamond: |
"Chris,
good work on the screw research. Having read it and
thought about the mechanics of pulls along the axis
of the screw as opposed to loading such that shearing
through the ice plays a role, it makes intuitive sense
that a screw placed at a positive angle should indeed
hold better, but only in ideal ice conditions - that's
the big qualifier. Of course determining what constitutes
"ideal ice conditions" is the art and essence
of placing ice gear. I felt you made this clear in your
article. My personal conclusion from your tests is to
place screws at a positive angle when I feel the ice
is 'very solid'. Obviously some rather ill defined terms
in that statement! 'Very solid' will remain an intuitive
assessment. But here are some attributes I associate
with 'solid' ice.
1. Appearance: Ice that is clear (glassy) in
appearance usually contains less air, thus having greater
density. Grayish, opaque ice is often shot through with
air bubbles and thus is less dense and has less ability
to support a screw placed at a positive angle. (At what
declining density though does positive angle yield the
advantage to zero or negative angle?) The effect your
ice tools have had on appearance as you've climbed toward
the screw placement is also informative. Pick holes
will reveal brittleness, plating and layering and other
indicators that affect my assessment of ice quality.
|
|
|
|
.2.
Sound: Low density ice often receives picks without
creating much surface deformation. High density ice,
because the dense ice must be displaced upon pick
entry, often reveals a conical pit and other surface
distortion where the pick enters. I would be more
inclined to place screws at a positive angle in the
latter situation.
3. Torque required to turn screw: How hard
is it to turn the screw once you've made your initial
assessment and begun to twist it in? If I don't need
the mechanical lever-arm advantage of the hanger to
turn it in, I would not feel comfortable placing at
positive angles. I would remove the screw and place
at zero or negative angle.
4. Layering I consistency of force required to turn:
As I turn the screw I feel for consistency of resistance
to turning. If I feel the screw break through into
less dense layers I would remove and place at a negative
angle.
Again, these are general and intuitive indicators
I use. In the end, there are absolutely no absolutes
in assessing ice quality. I find this to be one of
the appeals of ice climbing, wherein the art of the
sport lies. Protection on a typical ice lead will
still run the gambit of Spectres, tied off ice sickles,
pins, positive angled screws, negative angled screws
and most importantly the confidence and experience
to climb through unprotectable sections sans pro.
Prudence, trust in gut feeling and experience are
the requisite essentials for leading and protecting
ice safely. The research you've done is great and
becomes additional weaponry in my arsenal of criteria
and options available in the back of my mind as I
seek to make an ice lead as safe as I can. To apply
the positive angle technique across the board would
be folly but to add it to ones repertoire of savvy
techniques adds certain advantage."
©
Copyright October, 1997
Black Diamond Equipment, Ltd.
Alex Lowe
Reproduced
by kind permission of Black Diamond Equipment Ltd
|
|
Winter
climbing is a constant battle with one's equipment! |
|
Unknown |
|
|
|
|
