Question 6.CS.6: Redesign of a Failed Laybar for a Water-Jet Power Loom Probl...
Redesign of a Failed Laybar for a Water-Jet Power Loom
Problem The laybars in a number of water-jet looms have begun to fail in fatigue. The owner of the weaving works had increased the speed of the looms to boost production. The original design of a painted steel laybar had lasted with no failures for 5 years of 3-shift operation at the lower speed but began failing within months of the speed increase. The owner had a local machine shop make painted steel replacements similar to the original and these failed in six months of use. The owner substituted an aluminum replacement laybar of his own design, which lasted 3 months. He then sought engineering assistance. Analyze the failures of the three existing designs and redesign the part to last for an additional 5 years at the higher speed.
Given The laybar is 54 in long and is carried between the rockers of two identical Grashof crank-rocker fourbar linkages that are driven synchronous and in-phase by gear trains connected through a 54-in-long transmission shaft. The loom arrangement is shown in Figure 6-50 and the linkage is shown in Figure 6-51. Details of its operation are discussed below. Cross sections of the failed designs are shown in Figure 6-53 and photographs in Figure 6-54. The new design cannot be any wider than the widest existing one (2.5 in). The original loom speed was 400 rpm and the new speed is 500 rpm. The cost of a new design should be competitive with the cost of current (failed) designs (about $300 each in lots of 50).
Assumptions The major fluctuating loading on the part is inertial and occurs because its own mass plus that of the reed carried on it are being accelerated and decelerated by the linkage motion. There is also a “beat-up” force on the reed when it strikes the cloth to push the latest weave thread into place. This force causes a repeated torque on the laybar that may or may not be significant in the failure. The magnitude of the beat-up force is not accurately known and will vary with the weight of the cloth being woven. It is estimated to be 10 lb/in of cloth width (540-lb total). The environment is wet with fresh water and all failed specimens show evidence of corrosion.
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