Hi all, I will preface this by saying that my technical understanding is extremely limited.I am considering a mechanism in a project I am working on. My description will make sense after viewing the attachment but probably not before. The mechanism is a plastic ring (imagine like a cut out section of a pipe) about 3 inches in diameter, with three threaded holes distributed evenly around the ring. Screws, with a rubber stopper attached to the end, are screwed into the holes and can be adjusted to hold another "pipe section" of variable diameter (the point of this mechanism is that it can adjust to hold things of slightly varying sizes). My drawing, which is very rough, only shows one of the hole/screw/stoppers, but imagine that in fact there are 3 or 4 or etc. The point of the rubber stopper is that you can turn the screw past the initial point where rubber meets inner cylinder and exert a whole lot of holding pressure on the inner part so hopefully it doesn't slip. Imagine that someone is holding the inner tube, and pressing it down on a surface with the outer ring actually being the thing that contacts the surface. This isn't the case but just to illustrate what I mean think of like a plunger where you're actually putting pressure on the handle, but the bottom rubber part is what is against the ground - just to give an idea of the way pressure is going to be exerted on this mechanism. Someone will be holding by the inner shaft, and the ring w/ screws will need to support itself and not loosen or slip. How might I go about, other than actually building a bunch of different test mechanisms, finding out how much force I could exert on the shaft without loosening/slipping? What I am thinking is like specifically, is there any existing information about screws and rubber stoppers and how much/how effective they are at this sort of thing. Or a way I could synthesize this knowledge from a few different sources (like screws and pressure, vs material specs about how much rubber will compress under how much pressure, how to combine these two things and consider multiples of this turned towards the center of a circle distributed throughout the circumference of the circle, and then like if that force is going inward and exerting on inner shaft which is trying to move in a direction at 90 degree angle to all of their pressure). I wish I could explain better but hopefully this is somewhat clear why I think this information might be attainable. I am limited on time and money and am wondering if I can save myself both by trying to learn something about this before I actually make test thing. I hope this is somewhat clear what I am getting at, thanks a lot.
John
Your inquiry requires the consideration of a whole lot of issues that are hard to address in reasonable time on this forum; but, I want to make you aware of an issue you did not cover in your inquiry and should consider before you start any prortype testing.
First, I would recommend, if possible, you use a rigid metal ring rather than plastic one. With the plastic ring there is an issue of the rings flexibility when trying to apply loading on the pipe with screws. The amount of loading you can apply to the pipe may be more controlled by the plastic ring deforming than the screws' loading limit. Also, if the pipe in your arrangement is also plastic then it can have the same limiting effect on the load the screws can apply. For example, if you use three loading screws then the plastic ring is going to try to reshape itself into a rounded triangle with a screw at each triangle point so it will act a bit like a circular spring to limit the screws' pressure on the pipe.
Another issue with using plastic for your outer ring is that if you intend to place tapped holes in the ring for the screws then the low shearing strength of the plastic is almost surely going to result in stripping the threads out of the holes well before you have to worry about the metal screws failing.
But even with a stiffer metal ring, still any chance of succeeding in holding the pipe from slipping with screws depends very much upon how much loading is going to be applied to slide the pipe and under what conditions i.e. dry or wet pipe. Without control of that factor, the chances of designing a screw loaded design that will insure no pipe movement is going to be difficult. In other words, who is going to be pushing on the pipe, a 150 lb weakling or a 250 lb gorilla; and, can you control which of these will be using your equipment. I come from an early oilfield and construction background, so I have seen what some field hands are capable of doing either intentionally or unintentionally.
I know tis all still leaves most of ypour questions unanswered but I hope it will give you some things to think about for your design.
Jack
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