What is the stress on the hauling part when lifting a 4,200 lbs. weight using a threefold purchase rove to advantage? (Allow 10 percent of the weight per sheave for friction.)
• Threefold purchase rove to advantage and how many parts of line support the load • How to compute ideal mechanical advantage (MA) of a tackle, and then adjust it for friction losses • Effect of 10% of the weight per sheave on the total effective load
• First, determine how many parts of the fall (rope) are actually supporting the weight for a threefold purchase rove to advantage. What is the ideal MA from that? • Next, calculate the total friction as a percentage of the load by counting the number of sheaves in the system and applying 10% per sheave. How does that change the effective load the hauling part must move? • Once you know the effective load and the mechanical advantage, how do you relate those to the tension in the hauling part?
• Be sure you have the correct number of supporting parts for a threefold purchase rove to advantage (don’t mix it up with rove to disadvantage). • Carefully count the total number of sheaves in both the moving and fixed blocks when applying the 10% per sheave friction allowance. • Before picking an answer, confirm your final hauling part tension is in pounds and is consistent with the order of magnitude you’d expect from the MA (it should be significantly less than 4,200 lbs, but not unrealistically small).
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