Analysis: Pulley Efficiency, Base Systems and Their Relationship
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 Dec 19, 2012 Cool study Coming from somewhat scientific background I am more comfortable with breaking down system efficiency to each individual element - if you setup consists of 3 pulleys I would attempt to evaluate efficiency of one and look at how that correlates with the overall efficiency, and you did evaluate efficiency of separate elements but I could not figure out how to combine them. You claim +/- 2% error on measurement - this sounds like instrument error? How about statistical variances of measurements? - you did take more than 3? I did not spent a whole lot of time contemplating your argument regarding low loads being less efficient than high loads - perhaps different levels of deformation of rope under different loads have something to do with it? - higher deformation, more surface area, more frictional losses. Would be interesting to see if anything changes if you change thickness of tensioning rope. amariusJoined Feb 23, 201214 points
 Dec 19, 2012 My head caved in. SuperclimberJoined Mar 7, 20091,481 points
 Dec 19, 2012 amarius wrote: I did not spent a whole lot of time contemplating your argument regarding low loads being less efficient than high loads - perhaps different levels of deformation of rope under different loads have something to do with it? - higher deformation, more surface area, more frictional losses. That is the characteristic of sintered bronze impregnated bearings (Oilite is the best known make), they start to release the lubricant at high pressures which is why they are used for low-speed, high load applications. And for general information for 20kN, the bit the rope runs on is the pulley or sheave, the bits each side that hold it all together are the cheeks and the entire object is called a block. Put a rope through and it and the two combined are a tackle. Correct terminology is everything, except perhaps for slackliners trying to make tight lines! Jim TittFrom GermanyJoined Nov 10, 2009350 points
 Dec 20, 2012 If the loads shown are those actually measured by the cell attached to the post when 100 lbf are applied to the free end of the rope, then the problem with the analysis is that the tension in the slackline is actually higher by 100 lbf than the force measured--assuming that you are pulling towards the post, which is, to first approximation, what is seen in the pictures. The effect of underestimating the tension in the slackline is to underestimate the actual mechanical advantage by 1: proportionally more so for the systems with lower advantage. That would explain why the efficiency of the simple systems is lower than the efficiency of some of the most complex systems (which have higher losses). It would also explain the large apparent increase in efficiency of the complex systems built on the 5:1 base system: 2*(295+100) > 649+100. brentaFrom Boulder, COJoined Feb 2, 200677 points