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Stone Shifting 2
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Steve Gray
Steve Gray
931 posts

Re: Stone Shifting 2
Aug 29, 2003, 14:53
> You can achieve a good result many ways by different combinations of variables – height, offset etc so I’m a bit puzzled about how to focus. Would it be right that the height variable should be pinned down to “as low as practicable”, and then the other variables can be tweaked? Or are you and Gordon undecided about height?

Have you got today's update? The model has changed a bit. I saw the video of Gordon's 4 ton effort, yesterday and he basically used a very small offset so as to place the minimum load on the A frame. The tower was only as high as necessary to hit the hole and the hole was fairly close to the tower - all pretty sound thinking. The latest model has default values that use the same principles on the 40 ton stone.

> Secondly, you can get the stone to just stand up, but a very marginal alteration to the parameters will take it too far and it will pitch forward or not far enough and it will fall back. So I’m wondering about margins of error – A.) should the adopted computer model be one where X% variation in the variables won’t matter, as it will still work, and B.) should the real-world arrangement include “emergency buttressing” for if the stone pitches forward unpredictably?

OK, I have not yet modelled the effects of the hole. The model just stops if the angular speed is less than an arbitrary 5.7 degrees/second (0.1 rad/s) and the block is within 5.7 deg of vertical. When I do the modelling of the hole I will calculate the forces that the stone exerts on the ground due to its rotation. It should be farily easy to find out what sort of limits we can work within by waggling a small post in a hole to see how much force is needed to seriously deform the hole (we may have to repeat this at the actual site later on).

Having a short slope on the right hand side (furthest from the tower) of the hole would allow the block room to pivot into the hole and make a decent area of contact with the left face of the hole. This will stop the bottom end from rotating and the momentum will carry the top of the block up towards vertical. As it reaches (or passes) the vertical its angular speed is at a minimum and by that time gravity should be pulling the stone well down into the hole. As long as we can stay within the limits of strength of the ground, as mentioned before, the block should not topple over. I don't envisage more than about one third of the hole being sloped.
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Re: Stone Shifting 2 (nigelswift)
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