QC LAB: THE RISKS OF REBORING OLD ICE SCREW HOLES

It’s tempting. You’re mid-pitch, wanting to save time and energy, and eyeing that nicely bored-out hole located conveniently at your hip. But the fateful question is: Should you ever rebore an old ice screw hole? BD’s Climbing Category Director Kolin “KP” Powick..

IN THE FIELD

As mentioned above, it’s pretty obvious when placing a steel screw into an aluminum hole, but it’s worth mentioning that in these examples, we’re doing it immediately one after the other. As in—no re-freeze time. We’re placing an aluminum screw, taking it out, and then putting a steel screw in right after.

It’s very likely that things wouldn’t be as obvious if you let the hole re-freeze a bit for a day or two, and then placed the steel screw.

CONCLUSIONS

Our one-data-point-of-each-test lab testing showed that placing the same diameter screw in the same hole, whether aluminum or steel, resulted in either the screw breaking or the placement failing at greater than 10kN, whereas, a smaller diameter steel screw in an aluminum hole resulted in the screw pulling out at significantly lower loads. (<10kN)

While placing the screws for these tests it was quite clear that placing a steel screw in a FRESH aluminum hole was obviously suspect and should be avoided. As holes are allowed to re-freeze slightly, it may become less obvious that there is a difference in diameters, increasing the risk of a suspect placement going unnoticed.

Intuitively, placing an aluminum screw (larger diameter screw) in an old steel screw hole (smaller diameter hole) is very likely similar in strength to placing it in fresh ice—though this wasn’t tested.

BOTTOM LINE

Know that lots of folks out there are now using aluminum ice screws and be aware of the likelihood and risks of reboring an aluminum screw hole with a smaller diameter steel screw.

Be safe out there and remember, when it comes to ice climbing, the leader shouldn’t fall.

DIFFERENT DIAMETER SCREWS

I don’t want to get into a physics and strength of materials lesson here because a) it’s complicated, and b) I learned this stuff 30 years ago in engineering school, so it’s been a while … But the short version is:

- Steel is stronger than aluminum – got it.
- Aluminum is lighter than steel – check.
- When looking at cylinders or tubes, if you’re reducing the strength of the material (i.e. using aluminum instead of steel), you need to increase the diameter of the tube in order to maintain strength.

There has been lots of unofficial, and some official debate on whether rebored holes are as strong as new holes, or “strong enough.” A very in-depth study on this (and several other interesting topics) was done by Stephen Attaway and one of my colleagues—a guide, science wizard and ice climber buddy, Marc Beverly. If you want to geek out, check this:

Ice Climbing Anchor Strength: An In-Depth Analysis

Two key takeaways as it pertains to rebored holes are in their conclusions:

1) Based on the variability of anchor strength observed in our test results, we must accept the null hypothesis that rebored ice screws are too weak to withhold a UIAA fall factor ALL of the time. They are, however, stronger than expected and compare closely with an ice screw placed in virgin ice.

2) Rebored ice screws are nearly as strong as freshly drilled holes. It is likely that any refreezing process that decreases the diameter of the hole over time is of benefit for a rebored screw.

This is good info, but I don’t really want to go too deep into their results—you can read the paper—rather, I want to discuss a development in equipment that has taken place since this study was done: aluminum ice screws. This study was carried out in 2008, using various brands of steel ice screws, because that was what was available at the time. The same screws were re-drilled into pre-existing holes created by that brand of screw. However, since then, several companies have come out with aluminum ice screws.

The reason I mention this is when looking at the videos of testing in the lab, the angle of placement appears to be opposite. That is because the direction of load in the machine is UP. Make sense?

At the end of the day, screws need to meet 10kN to pass required CE testing, and in the tests above where we rebored using the same screw, the results exceeded 10kN. However, when we used a smaller diameter screw (steel screw) in a larger diameter hole (aluminum hole), it resulted in the placement being compromised and the screw pulling out at reduced loads. (significantly less than 10kN)

It is worth mentioning that this was in super good, lab made ice—it doesn’t get much better. The strength of most ice screw placements is directly related to the quality of the ice.

IN THE FIELD

It’s very likely that things wouldn’t be as obvious if you let the hole re-freeze a bit for a day or two, and then placed the steel screw.

CONCLUSIONS

Know that lots of folks out there are now using aluminum ice screws and be aware of the likelihood and risks of reboring an aluminum screw hole with a smaller diameter steel screw.

Be safe out there and remember, when it comes to ice climbing, the leader shouldn’t fall.