Ian Rae
From:
Redditch, England
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Posted 27 Aug 2014 6:45 am |
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On another thread I'm documenting progress on a uni 12 I'm building, and I've got to the point of determining what helper springs to put on the raises. I don't have any on my D10 as no change pulls more than two strings. But even my fairly modest U12 copedent has 3- and 4-string changes, and these will be stiff without assistance.
What I've figured so far is that I need a low-rated spring so that the "help" diminishes as little as possible as you make the pull. If I regard the weight of an unassisted 2-string pull as acceptable, I guess I want the 3- and 4-string ones to be no heavier. It is difficult to find a spring with a low rate, a high enough load and a small enough diameter, but I'm lucky to live in a town where springs are a local industry.
Are there any general principles I should observe? (I prefer calculation to trial and error.)
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Make sleeping dogs tell the truth!
Homebuilt keyless U12 7x5, Excel keyless U12 8x8, Williams keyless U12 7x8, Telonics rack and 15" cabs |
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Ian Rae
From:
Redditch, England
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Posted 30 Aug 2014 1:12 pm
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I may have sorted this, at least in theory. In the spreadsheet below the rightmost column represents raise helper springs (H) adjustable in one-pound increments, achieved by hooks screwed into a matrix of holes. (I don't believe continuous adjusters will be necessary.) Columns P - U are the string tensions of each note. The mechanical advantage of the changer is exactly 2, and columns V & W show the initial and final pullrod tensions. Column X is the minimum return spring tension required to hold the raises. The actual return springs in use exert 8.8 lbs in the rest position, so column Y is the effort needed for the (first) lowers.
Entering different values in column Z alters the numbers in the coloured portion of the chart, and in turn the totals in row 40 which show the total maximum force for each pull, the idea being to fiddle about until everything is reasonably balanced. Of course there is still a choice of hole to be made at the puller. So now I need to get some suitable springs and see if all this comes anywhere close in practice 
_________________
Make sleeping dogs tell the truth!
Homebuilt keyless U12 7x5, Excel keyless U12 8x8, Williams keyless U12 7x8, Telonics rack and 15" cabs |
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