Pre-Load Analysis Overview.docx

 Overview: After deflection calculations were completed on springs a 3in spring with k=3120 lb/in was chosen. The required distance to preload the spring to 1000lbf is 0.288in. Additionally analysis was completed on acme screws. After looking at 9 screws, a ? in diameter with 10 threads per inch was chosen. The full analysis in in the pre-load analysis spreadsheet. All screw options were “low” cost therefore none were ruled out until after analysis. The design below details the process of preloading. Design: The bearing housing will have an attached cylindrical ‘peg’ to align the screw dead center. The screw will then be lined up with a peg in the middle block also dead center. On the opposite side of the spring alignment block there will be a cylindrical hole to align the acme screw. the Acme screw with then be thread through the furthermost block. This block will be bolted to a lift that will hold the whole assembly in a location that will center the preload on the bearing block. The lift block will be added to the CAD model when it is on the full assembly. Because the right block will be bolted with 3 inches of the over all 6 inches of threads engaged it will also serve as a locking mechanism. However for safety it would be advised to simply clamp the acme screw with a standard c clamp as a fail safe. Additionally an acme wing nut will serve as the turning mechanism for the screw and the final failsafe if anything were to buckle. It also will be added to the CAD model. For reference it is detailed below. Analysis: It was first determined to use B7 4140 Alloy steel because of it was the lowest cost of the materials availableNo more than 12” was needed so all analysis was done with sizes available at 12” and smaller If length was reduces critical failure occurs at a higher force, therefore calculating at 12” ensured a factor of safety of about 2 (6” was the expected needed length)The basic depth thread was found for each possible screw from a Starett reference table Pitch diameter was assumed to be the outer diameter, although the nomenclature varies depending on literature usedRoot diameter was found by subtracting the 2 times the depth of the thread from the pitch diameterThe tensile stress area was calculated with the pitch diameter and the root diameter as detailed The torque required to move a load up the thread would be the case of load application it was determined by Likewise, the torque required to move the load down the thread was determined by a similar equation- it was expected to be very low because the screw would no longer be pushing a force L=pitch and f=friction Conservatively friction was determined to be 0.15, standard can be as low as 0.5 and as high as .30 depending on the material and the cleanliness of the screw. Therefore a conservative estimate of 0.15 was used. 4533900514350Normal and shear stress of the cross section were determined to provide a reference for the nominal stress, because a screw does not have an evenly distributed load on each thread this is just an estimation where T= Torque F= Force to be moved Failure due to yield was obtained with The nominal bearing stress was calculated to determine stressed on engaged threads, I am unsure what to compare this information to and will be looking into it further. It was determined using Additionally because shearing the screw is a very likely risk, the maximum shear stress was calculated using Lastly, the critical load of the screw was calculated to determine under what force would the screw ultimately failC=4, ti is determined by the end of the screw (fixed vs. not fixed)After this analysis a ?”-10 screw with 6 inches of length was chosen.It is a cheaper option Will withstand 3 times anticipated forces Has lower stresses than some other options Will not need to be cut, it will already be to the appropriate length A quick detail of the analysis is below, the specifics and the comparisons can be found in the excel document as mentioned above . ................
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