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Eccentric Cam Ascenders

These ascenders are asymmetrical, and can be classified as left-handed and right-handed. Holding the ascender vertically with the open side of the shell towards you, a left-handed ascender has the cam to the left of the rope channel while in a right-handed ascender the cam lies to the right. Many of the handleless ascenders come only in one version. Most handled ascenders made in both left-hand and right-hand versions; naturally one normally acquires a pair of one each.

Cam angleThe eccentric cam ascenders use a toothed cam which is spring loaded towards closing. Most eccentric cam ascenders appear to use an equiangular spiral as the basis for the cam design. The equiangular spiral is a logical choice for two reasons. First, the locus of rope-cam contact points for different rope thicknesses is a straight line segment. This segment, if extended, passes through the cam pivot, and makes an angle with the horizontal equal to the expansion angle of the spiral. For small variations in rope diameter, the elevation of the rope-cam contact changes very little. These features simplify the structural design of the ascender. In addition, this arrangement results in a nearly constant cam to rope pressure for different size ropes. These two features help make ascender performance relatively insensitive to the diameter of the main line.

The cam teeth are essential for proper operation . Abrasion of the cam teeth is a problem, particularly with some of the softer cam materials. An ascender with worn cam teeth may not hold; neither will one whose teeth are caked with mud or ice. Cave mud appears ideally suited for ruining the performance of this class of ascender, so extra care should be taken to keep ropes clean. The teeth appear to have little or no effect on the life of the ropes the ascenders are used on. As weight is applied to the ascender, the teeth provide the friction necessary to keep the cam from slipping down the rope. Since the shell tends to slide initially, the eccentric cam closes until further closure is prevented by the thickness of the now distorted climbing rope. At this point the ascender stops its downward motion and begins to support the load. Note that there is an inherent slippage in the operation of these ascenders. This creates some loss of climbing efficiency, although the loss is usually small.

Most of these ascenders have a problem with horizontal and diagonal ropes such as one might find on Tyrolean traverses. The attachment point is located at the base of the ascender, it exerts a torque about the rope channel when loaded. This can generate substantial forces between the rope and the rope channel. Since the rope channel is open on one side, the rope may be forced out of the rope channel, even if the cam is not fully open. Because of the large forces, the rope can sometimes pass through the small gap between the cam and the shell or frame. The accepted method of preventing this involves clipping a carabiner through both the lower attachment hole (if it exists!) and the sling, and then clip this carabiner around the main line. This keeps the ascender from rotating so the large torque is not applied to the rope channel. THIS IS NOT A COMPLETE DESCRIPTION SO DO NOT TRY THIS WITHOUT INSTRUCTION!

All my comments are oriented towards using these devices for their design purpose. Unless I specify otherwise, this is limited to a single person plus equipment ascending ropes within the 9 to 11 mm. diameter range. Comments do not apply to ascender abuse, such as use in rescue hauling systems.