You are lifting a 3 ton weight with a single whip rove on a swinging boom set at an angle of 60° to the horizontal. Use the formula for the size of a shackle with a safe working load and determine the minimum size shackle that should be used to secure the head block to the boom.
• Mechanical advantage and line tension in a single whip (no purchase) • How boom angle changes the load in the topping/boom support and head block compared to the weight being lifted • Rule-of-thumb formula relating shackle size (diameter) to safe working load (SWL) in tons
• First, determine the actual line pull in the fall of the whip when lifting a 3 ton load with no mechanical advantage. How does that compare to 3 tons? • Then, consider how a 60° boom angle to the horizontal affects the force in the boom/head block compared to the vertical load (think about the vertical vs. horizontal components). Is the force in the head block greater than, equal to, or less than 3 tons? • Once you have the estimated load on the head block, apply the standard shackle sizing formula used in cargo gear tables to find the required shackle diameter, then compare that to the answer choices.
• Be sure you are using the correct angle reference (60° to the horizontal, not vertical) when resolving forces. • Confirm that your shackle SWL formula is in the correct units (tons vs. inches) before plugging in numbers. • After calculating the required shackle diameter, round up to the next available standard shackle size from the choices, not down.
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