Power screw design for tensile specimen

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Saladsamurai
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I was reading through some examples in my Design text when I came across this one.
Picture7-3.png
This was their solution:

Picture8-1.png


I am not sure that I completely follow it. Can someone help shed some light on what's going on here?

I am assuming that the specimen to be put under tension goes where the "B" dimension is right?
 

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Yes, the tensile specimen goes where the B dimension is. Both screws must have the same hand. My impression is, their explanation is very poorly written. Here is the answer I arrived at before reading their solution. The machine is missing thrust bearings at the top of the screws. Therefore, the machine should not be used for a compression test, because galling of the upper channel web material might occur. Plus, the diagram shows only a small-diameter shank inside the upper channel.