At 04:38 PM 3/22/2000 -0800, you wrote: >Hi Brett -- > >Nice to hear from you again. You too, Karl, it's been a while. >I have constructed a 1:1 model of the STS-1 leaf spring, based on the >sketch in the paper referenced by Sean-Thomas. By the way, I can send >electronic copies of the article to anyone that wants them. I have done >testing with several variations in dimensions and don't see any significant >"zero length" effect (which, incidentally, I think might be more properly >termed infinite length). In looking at the first results of the numbers, it became apparent what they were doing. The major spring effect was from the bending effect at the ends, not the end forces. It's working more like the mainspring of a clock than like a coil spring. Imagine a leaf spring with each of its ends attached to the side of a round stick, so that by twisting the two sticks, you could make the spring form into a curve. If you only twist and don't push or pull on the ends, the curve will be an arc of a circle. Now, by just twisting, formin the spring into a half circle. Then start pulling to the sides to make the ends of the spring separate more. To keep the ends vertical (parallel), you will have to twist harder. Pull apart more, twist more. That's what seems to be happening to the spring in the STS-1. The end *force* exerted by the spring is actually trying to make the boom rotate downward. However the twist (the end moment) is a substantially greater effect, and is working in the direction to lift the mass. In that configuration, as the beam tips up and down, the end moment changes very little. With everything balanced, beam tilt has very little effect on the spring moment which equals very little change on the force supporting the mass which means long period. If you look at pictures of the STS-1, you can see the flattened curve the spring takes when the end forces applied to the spring are directed outwards, rather than inwards, as you might expect to have. To get that in your prototype, you need to be sure the spring is the length given in the article, so that it has to flatten out somewhat. Also, it appears that the STS-1 must be using fairly thin spring material. With something like 0.008" for a single spring, the model started looking fairly reasonable. 0.018" material bent that sharply would take a very permanent set. Over a +/- 5 degree boom motion, the lifting force looked to be changing by less than 0.6% Don't know what that implies for natural period, but pretty long for a first try. Maybe too good; I need to go over the numbers more closely to be sure they are all making sense. Still need much more playing to understand what's happening, particularly to find a configuration where the beam will sit stable at zero without help. As you can tell, this is still all real preliminary. Regards, Brett __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>