A friend of mine recently told me about a potentially dangerous situation he encountered while belaying with the SUM belay device from Faders. This was not the "keep the handle free" issue which is discussed in the instructions from Faders and can be mitigated with the rubber "belay positioner" that now comes with the device. I've used the SUM for a couple years, and I love it, so I was anxious to check into this problem. I attempted to recreate the situation and, unfortunately, was successful. It all has to do with the fact that the hole in the SUM, through which the karabiner is clipped, is oval rather than round. To function effectively, the SUM needs to be able to rotate freely around the karabiner tubing by way of this karabiner attachment hole. The instructions from Faders point out this need.. "To function properly the device must be able to rotate with respect to its attachment point to the karabiner (HMS) and the central piece must be able to rotate with respect to the fixed side-piece." Now the problem....
If the karabiner being used with the SUM has tubing with a cross-section that is oval (or flattened) the karabiner tubing can be significantly wider where it rounds corners than along the relatively straight sections. A karabiner like this can be easily clipped into the SUM, will allow the SUM to rotate freely at the upper (relatively "straight across") portion of the karabiner, BUT won't allow the SUM to rotate at the corners of the karabiner. Specifically, if the SUM rotates down with respect to the karabiner tubing, then slips sideways across a corner of the karabiner tubing, then ATTEMPTS to rotate back upward such as would happen with the force from a falling climber, the karabiner tubing can become jammed in the oval SUM attachment hole preventing any further rotation of the SUM with respect to the karabiner. In this situation, if the direction of pull on the rope is just right (meaning wrong), the SUM won't lock the rope. Although a belayer may be able to manage the position of the SUM on the karabiner when lead belaying, it is much more difficult to do so if "top-rope" belaying. Typically, when "top-rope" belaying, the belayer has both hands on the rope (neither hand on the SUM).
My simple solution was to find a karabiner that has tubing with round cross section and to make sure that the SUM rotates freely around all portions of the karabiner tubing, including the rounded corners. Surprisingly, the karabiner that has been marketed by Faders for use with the SUM can exhibit the problem I've just described. I've emailed Faders about this.
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