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Mechanical Engineering

Abdulrahman-1

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  1. Pablo, you should not think one-dimensionally. I touched upon several errors in your expectations, but you only think of one of them. This is not correct, a mechanical engineer or Polytechnic graduate must analyze all issues, i suggest to read my answer again.
  2. Actually for 4140 Alloy, the hardness achieved by heat treatment in that chart was excessive, and it is far too brittle. The values you see in graph are suitable for AISI-4340 but not 4140. It will crack in service. But 4340 has more alloying elements allowing the hardness values to be achieved without cracking. The reason that the bar at 3 inches has lower tensile strength is that the official figures for tensile strength of an alloy are normally based upon a 1 inch test bar, however, as is well known, tensile strength drops gradually with diameter increase of test bar, this is a well known phenomena not related to quality of manufacturing. The cause of this is more errors that occur at the grain boundaries, so the larger the size of test bar or actual part, the larger the amount of microscopic defects, thus lower tensile yield and ULT. Which tensile strength to use: First of all, for machinery design, must be using Yield strength, not ULT. Second of all, use tensile strength corrected to size of actual shaft, for example 6 inch shaft has lower strength compared to 3 inch test specimen. Finally, you can use the internal hardness as the indicator of shaft strength, not the outer hardness. Abdulrahman Alkhowaiter
  3. Hi, From the pictures: 1- This gear coupling has very high teeth wear, far beyond allowable. This is named a gradual deterioration failure mode. 2- Such gear couplings should be manufactured with gear tooth surface hardness of no less than Rockwell C-45, i advise making a hardness confirmation test using a local lab.Even a university mechanical engineering dept can assist you.take hardness looking axial at top of tooth in a non worn location. 3- The high wear at center of tooth length definitely shows high misalignment of drive shaft coming from main rotor drive, to the tail rotor. Misalignment throws high loads causing fretting wear. Contact a local machinery alignment company, and request them to perform laser Alignment for you..This is the most superior technique. 4- Be careful of grease types selection; For this application, there are special shaft coupling greases on the market, specially made for gear couplings; just search them on Google: "Polyurea shaft coupling grease". 5- Very Important: Contact the helicopter manufacturer and give them the part identification name and even part number and ask them to provide all aviation bulletins associated with the tail rotor coupling that were published in past 20 years.Ask them to include your E-mail in all future service bulletins. 6- Check all shafts connected to coupling for cracks using Dye check. This coupling was basically stuck and was not acting in a flexible manner, so any shafts coupled to it were subjected to bending fatigue, not only torsion.
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