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Old 12-11-2007, 10:41 AM   #18
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Good that you've pointed it out.
I've never really considered it in detail, since the tension bar itself applies a constant torque, which means the scale bar is moved up uniformly, a correction for the manufacturers to make.

Originally Posted by nousername View Post
first, it's amazing how a touch of a 1st yr engineering course can impress so many!!! =) ... nevertheless, your overall analysis looks good (although i didn't read the details). BUT the above highlighted comment is not right.

the effect of the tension rod weight is not negligible. when i got my drop-weight, being the nerdy engineer i am, i had to verify that the tension scale on the rod was correct. i took all the above measurements and to come up with my own solution to where the weight should be for a desired tension. i KEPT coming up answers different than the tension scale on the machine. i kept thinking about what horrible engineers must have designed my machine. THEN i realize i forgot about the moment produced by the weight of the tension rod. abra-cada-bra all the numbers mached! and thus, the tension scale on my machine WAS correct! it turns out the tension rod (on my machine) contributed about 5-6 lbs of tension! ... definitely, not to be neglected. this is a bias, so no matter where the weight is along the rod, the tension rod itself always adds about 5-6 lbs.

in addition, you need to be very careful when tweaking the mass of your dropweight to account for other error sources. your should NOT do this to account for the tension rod weight. this is b/c the tension rod mass adds a constant bias to the pulled tension, but the mass of the rod affects the tension via a scale-factor (i.e. the distance R). the bias is a result of the fact that the tension rod mass is fixed, where as the scale factor arises b/c the drop-weight mass moves. different error sources, don't mix 'em.
"Physics is like sex: sure, it may give some practical results, but that's not why we do it." -- Richard P. Feynman
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