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  • 2.14. Cable Analysis Overview
Cable Analysis Overview
Finding a suitable yarding layout is generally an iterative process carried out by the forest engineer. During this process several different combinations of parameters are evaluated for feasibility. The engineer may change the elevation of the landing, move the tail hold or try using a setup with a lighter carriage. The interactive results allow you to test various configurations until a good solution is found or a bad one ruled out. For any workable solution, critical values such as maximum payload as a function position can be printed and used as guidelines during production.
The Cable Analysis software models a real cable harvesting system in many ways; however there are limitations to the model and the results should be used with intelligence.
The most obvious limitation of the model is that it exists only in two dimensions: Station and Elevation. The haulback block, for example, can only be placed in the plane defined by the tower and the tailhold.
The dynamics of yarding a log out from behind a stump are unique to each situation; for this reason all calculations are based on a quasi-static model. In a quasi-static model no part of the mechanical system is accelerating; thus the forces are balanced. From the engineer's point of view this means that a flying load moving slowly (or standing still) is modeled well. A log breaking out of a pile and then falling off a cliff is not modeled well because the friction and accelerations create forces that are not taken into account. Breakout Force can be modeled by adding an estimated value to the load but note that the direction of the effective load will not reflect reality.
As of this printing, a partially suspended load is not modeled. All loads due to logs are considered to act straight down from the carriage as if the load were flying. In fact the ground profile is only used to find the carriage elevation given ground clearance. If the software is asked to find the carriage position for a given midspan deflection it may show it as being under ground; the tensions will be reported as if the load were flying.
The carriage is modeled as a point. For example the mainline is considered to join the carriage at the same point the carriage is suspended from the skyline. This assumption will cause very little difference from reality.
The equations representing the model are precise except that the load due to the cable weight is assumed to be evenly distributed in the Station dimension. For a tight cable this is exactly true; if the cable sags slightly its tension is much more than its weight (most working cables) but the approximation is good; if the cable has a large sag so that both ends slope steeply down it is a bad approximation. If the approximation is poor the cable tension is small and it does not greatly affect the whole system.
Many results are calculated using iterative methods. The accuracy of these results (if one assumes that the model is perfect) will be somewhere around 99.9%. For example if you set the clearance and calculate the maximum load you will find that the limiting skyline cable tension will be just under it's working tension (perhaps 49985lb instead of 50000lb). The software has terminated its search for the correct load when the skyline load was close enough to maximum. The tensions and deflections calculated for a given load are accurate to floating precision (6 significant figures).
NOTE: The results from the computer calculated Cable Analysis should be interpreted with knowledge of the limitations of the model.