MLA Citation: Bloomfield, Louis A. "Question 1392"
How Everything Works 23 Oct 2017. 23 Oct 2017 <http://howeverythingworks.org/print1.php?QNum=1392>.
1392. I want to support a group of bird feeders on a horizontal cable, one end of which will be fastened to my house and the other end of which will run over an 8 inch pulley attached to a large tree. That end of the cable will be attached to some concrete blocks which must be heavy enough to keep the horizontal cable taut at all times. The idea is to prevent the cable from snapping when the tree moves in high winds. It's already done so twice, even though I left what I thought was adequate slack in the line. I guess this sounds like a Rube Goldberg solution, but I can't think of any other solution. How much should the concrete blocks weigh? — HS, Burk's Falls, Ontario
Your solution should work nicely—the pulley and weight system should protect your cable from breaking because the weights should maintain a constant tension in the line. As the tree swings back and forth, the weights should rise and fall while the tension in the cord remains almost steady. Obviously, if the rising weights reach the pulley the cord will pull taut and break, so you must leave enough hanging slack.

However, if the tree's motion is too violent, even this weight and pulley system may not save the cable. As long as everything moves slowly, the tension in the cord should be equal to the weight of the weights. But if the tree moves away from the house very suddenly, then the tension in the cord will increase suddenly because the cord must not only support the weights, it must accelerate them upward as well. Part of the cord's tension acts to overcome the weights' inertia. Just as a sudden yank on a paper towel will rip it free from the roll, so a sudden yank on your cable will rip it free from the weights. If sudden yanks of this type cause trouble for you, you can fix the problem by coupling the cord to the weights via a strong spring. On long timescales, the spring will have no effect on the tension in the cord—it will still be equal to the weight of the weights. But the spring will stretch or contract during sudden yanks on the cord and will prevent the tension in the cord from changing abruptly either up or down. The spring shouldn't be too stiff—the less stiff and the more it stretches while supporting the weights, the more effectively it will smooth out changes in tension.

As far as the weight of the weights, that depends on how much curvature you want in the cable supporting the feeders. The more weight you use, the less the cable will sag but the more stress it will experience. You can determine how much weight you need by pulling on the far end of the cable with your hands and judging how hard you must pull to get a satisfactory amount of sag.


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