Building the Octagon Pt.61. Put two wheels on your hatch?


A CN forum member suggested inserting a pulley into the hatch lifting system. At first I could not see the advantage. The pulley would halve the force applied. The real trick was in grabbing the pulley idea and running with it. Am I allowed to say that at my age?

Why have bell-cranks when you can have two pulleys? Imagine a large, wooden pulley attached firmly to the hatch on each end of its hinge line. Now wrap two ropes around and anchor them to their own pulleys. Using only one pulley would tend to twist the hatch and cause all sorts of diagonal force problems. 

Two pulleys make a balanced system provided the wights are fairly equal. But it gets better! Now hang one rope loop over both pulleys and anchor each end to its own pulley. At the bottom of your loop you hang the hatch's lifting weight. The pulley and single hanging weight provide perfect balance between the two pulleys. All asymmetry of forces is vanquished.

Why have pulleys on the hatch instead of simple bell-cranks? Because the limited range of movement of the bell-cranks was causing considerable changes in lifting force as the hatch opened. Pulleys provide constant force because the lifting moment never changes. Moment = Mass x pulley Radius. Both remain unchanged.

The lift begins from fully closed and continues steadily throughout the hatch's total movement to fully open. 

Obviously the pulleys must be strongly attach to the heavy hatch. The ropes are easily anchored to wooden pulleys. You just drill down from the grooved rim to exit out the side. Or drill down straight into a pre-drilled and slightly larger cross hole. You push the end of the rope down the rim hole. PUll it out the side and tie your knot. Now you pull back on the rope at the rim and your rope is safely anchored. The knot can never escape unless you take the weight off the rope and pull it out of its cross hole.

The image shows a quadrant arrangement to help to lift the hatch. The quadrants start high and descend into slots between the boards of the floor behind the ladder. So the slots are never open. Each end of a cable or rope is anchored in each quadrant. The weight hangs from a single pulley to balance the forces on each quadrant and thus the hatch.  

The size of the pulley wants to be the largest possible which won't intrude beyond the handrails.  Why turn wooden pulleys when you have plastic or steel wheels with the tyres removed? Three laminations of plywood with a smaller central disk automatically makes a pulley. 

Time to start measuring how large a pair of pulleys I can have. But why make do with complete disks when you can have a larger quadrant? One which projects backwards from the ladder and out of the way? The quadrant just wants to have the same angle as the total hatch movement =120°.
Constant force x Constant radius = Smaller weight = lower wear.


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