Figure 1: CE test spacing for 25% and 75%

EN 12276

Note: If a cam has a range of 5mm or less the CE standard requires it to be tested at 50% retraction vs the 25% or 75%, the Z4 #0 meets this requirement.

Figure 2: CE test spacing if total range of cam is less than 5mm

% STRENGTH REDUCTION

– To show the difference in strength between 25% and 75% the % strength reduction will be determined using the following formula.

RATINGS

– The marked rating is the load which the manufacturer claims the product can consistently meet under test conditons. The minimum allowed rating outlined in the CE standard is 5kN (1124lbf). When looking at the markings on the sling of a cam there may be two ratings listed; one for active strength (the normal way that a cam is placed) and one for passive (referring to the cam being used as a nut/stopper). For Black Diamond cams, only the dual axle versions are rated for passive strength.

Figure 4: Markings that display ratings for holding force and passive strength found on the cam sling.

Now this doesn’t necessarily mean that if a cam is rated to 8kN it will fail at 8kN. Ratings are determined and calculated using what is known as a three-sigma calculation. By testing a statistically relevant population and determining the mean (µ) and standard deviation (σ) the lower three sigma bound can be found. From this it can be said that 99.7% of failures will be at a load above the lower bound. This is the starting point for determining the published rating. After taking into consideration the various use cases of the product the rating could be lowered further.

Lower Bound = µ - 3 * σ

Lower 3 sigma calculation used when determining ratings

Figure 5: Trend of published CE ratings of C4’s & Z4’s as cam size increases

CLASSIFICATION OF CAM SIZES

For the purposes of this article certain cam sizes will be labeled small, medium and large, the breakdown of specific sizes is listed below.

1. Small – Cam sizes Z4 #0 through Z4 #0.4
2. Medium – Cam sizes Z4 #0.5 through C4 #2
3. Large – Cam sizes C4 #3 through C4 #8

THE TESTING

No matter what size cam you’re placing, you should always aim to make the best placement possible, however, this isn’t always possible. There are variations in the rock, you have a limited number of cams to use along the pitch and sometimes you would just plain rather have something instead of nothing.  But how much do those funky placements affect the strength of the cams at either end of the spectrum?  

As with all QC Labs it is important to remember that we are dealing with a small sample size and testing in pretty-much ideal conditions: the test setup is accurately measured and tested with steel file plates to ensure as little variation as possible.

To keep it simple, we narrowed it down and decided to perform two different sets of tests:

TEST 1:CE STANDARD TEST

- Following the CE standard - the Z4 #0, C4 #7 & #8 as well as variety of mid-range cams.

1. To determine the % strength reduction when comparing testing at 25% retracted vs 75% retracted, and …
2. In order to be able to directly compare small to large sizes in both standard CE test configurations

TEST 2: NONSTANDARD ASYMMETRICAL TEST

 - A nonstandard asymmetrical test of only the larger C4’s.

 - To show the realities and limitations of larger cam placements.

TEST 1

CE STANDARD TEST – RESULTS

#1 Camalot – rating 14kN – tested at 50% retracted - Broke at 15.5kN

The following table shows the results when tested to the CE standard test set-up (for simplicity not every size of cam is shown):

Table 1: Testing summary for various Z4 and C4 cam sizes. – tested to standard CE testing protocol

*Not required by the CE standard testing but added to table for completeness

CE STANDARD TEST - ANALYSIS

When looking at the results in table 1, there are several key takeaways:

1. Overall, the ultimate strength is higher than what is typically seen in a normal field loading scenario. This is good. It means when placed well, cams are strong enough to hold a whipper.
2. In the medium sizes tested, the difference in results between the 75% retracted and 25% retracted configurations are negligible.
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3. However, as the cams get bigger, the % strength reduction between the two testing scenarios increases.  
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4. In the case of the Z4 #0, due to the limited range, the standard specifies that it is only tested at 50% retraction, which of course doesn’t allow for the % strength reduction calculation. The fact that this size is not tested at the 25% retraction exemplifies its sensitivity to low retractions and the need for precise, secure placements.  

CE STANDARD TEST – FAILURE MODE DIFFERENCES

CE STANDARD TEST – CONCLUSION

Placing a large cam with minimal retraction can significantly reduce the strength of the unit due to the tendency for the lobes to buckle. Placing a micro cam with minimal retraction can also significantly reduce the strength of the unit due to the tendency for the lobes to invert. While the failure mode is very different for both sizes of cams, their sensitivity to “tipped out” placements remains nearly the same.

CAMS GET WEAKER AT 100% RETRACTED—FACT OR FICTION?

As a side note it is worth mentioning that as the cam gets closer to 100% retraction the stronger it will be. It is commonly overheard that a cam is weaker when placed at full retraction. Some call this condition being “over-cammed.”  This is not true. The closer that a cam is placed to 100% retraction, the stronger it is. The downside to placing a cam at full retraction is that it might be challenging if not impossible to remove without damaging the cam or rock.

TEST 2:

NONSTANDARD ASYMMETRIC TEST

During the development of a new product here at BD, there are many tests in addition to the CE standard tests that are created and completed in order to validate a product. These tests are performed to better understand the effects of potential misuse scenarios that may occur in the field.  One such test involves holding the cam at an angle that is off axis from the direction of pull to see what happens.  All the previous testing in this article has been done under ideal conditions and placements.

So, what happens to the strength of large cams when they are placed off-axis from the direction of pull?     

This second round of testing was done on the larger size C4’s (#5, #6, #7, #8) and placed at roughly 30 degrees off from vertical at a spacing of 50% retraction with the retraction of the lobes uneven. In many cases the cam will attempt to orient toward the direction of pull but in the case of this test the cam was held out of line to represent the worst-case scenario.

NONSTANDARD ASYMMETRIC TEST - RESULTS

NONSTANDARD ASYMMETRIC TEST – RESULTS ANALYSIS

From earlier testing it was already determined that as the size gets larger the instability increases.  The following points are key components to the large C4’s decreased performance during uneven loading:

• Placing the cam in this orientation only amplifies the unit’s tendency to buckle out of plane.

1. The % retraction has a major role in the cam’s performance in this scenario. As the amount of retraction increases the cam’s stability will also increase improving the cams ability to hold higher loads. These tests were done at 50% which isn’t worst case scenario but ideally the cam would be at a higher % retraction if used in this way.

• Starting with the C4 #5, the strength decreases at a constant rate to a load that is significantly lower than what is seen during proper placement testing.

1. As the lobe size increases the moment force on the lobe also increases contributing to the lower failure loads.
2. During uneven loading certain lobes will be subjected to higher forces than others. Due to this any slight irregularities can initiate the buckling quicker than if the load was evenly dispersed across all four lobes.

NONSTANDARD ASYMMETRIC TEST – CONCLUSIONS

Large cams are more vulnerable to strength reductions than smaller cams when placed off axis from the direction of pull. It is essential that large cams are placed such that they are oriented in the direction of loading, with the individual cam lobes retracted as evenly as possible, and retracted as much as possible.

CONSERVATIVE RATINGS

At BD, we consider more than just the standard CE requirements when assigning a rating to the cam size. The results of our internal tests are also heavily considered.

When looking at the testing results above, the loads are well above the 5kN rating (for the Z4 #0 & C4 #8) and the 8kN rating (for the C4 #7). However, because of the known limitations and sensitivity to placements, a conservative rating was chosen to convey the need for the user to strive for a more optimum placement when possible.

BOTTOM LINE 

It is very unlikely that every placement in the field is going to be perfect, and for some sizes that is more acceptable than for other sizes. While every piece should be placed with care, extra caution should be used for cam sizes at the far ends of the spectrum:

Sufficient cam retraction and symmetrical loading are the keys to a good placement.   

Figure 7: Our cam instructions highlight a few suspect placement scenarios to be aware of.

Even though the size of the large cams can give the appearance of a strong, secure anchor, their ultimate strength performance is similar to a micro cam and needs to be treated as such before deciding to take the big ride out of some gnarly off-width.

Be Safe out there,

HG