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Topic Date User

Cam failure discussion 24-Apr-2015 At 1:28:58 PM patto

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On 24/04/2015 Macciza wrote:
>Cant agree Patto - look at the equations at the end of the standard cam
>models for parallel vs flared cracks. As the flare approaches the cam angle
>more friction is require to hold it .
That is not what the equations suggest. In all cases tan(α) < μ is the friction requirement.

>Where do you get the 2x figure from?
The total flare of a crack is 2x β.

>Are you suggesting that deformation is the main reason cams fail in flared
>cracks?
The maths shows that in ultra flared cracks when β > α no grip initiates the cam doesn't hold at all in fact spring pressure will probably push the cam out! When α < β and friction is good {tan(α) < μ} then the cam will hold on placement. However lobe compression reduces the contact and can result in placement failure.

>Back on topic, some of the variables for a cam to hold include the angle
>of the cam, and the material that the cams are made from (and finish etc),
>This will account for varying results for different brands in identical
>conditions. Different materials and finishes have different frictional,
>compressive and shear strengths, as do the rock they are placed in. Basically
>BrandX may work perfectly well where BrandY would simply fail.
>These factors may account for much of the variance between peoples experience.
That doesn't say much. Why back away from objective statements towards subjective?

Going back to the mathematics, bigger cam angels are better for flares but are worse for glassy cracks. This is why aliens are so good in high frictiion granite, along with softer lobes they hold flares well. Totems combines the advantages of outward force (grip) along with good flare behaviour.