Gearing

1Q. What is one thing you can tell me about precision gearing, meaning high speed or high horsepower?

1A. Any wear on the face of the gear should be considered worthy of further analysis. Be careful when you are told it is "arrestive or self healing pitting".

2Q. We had a major gearbox failure with the gears and the bearings failing. The inside of the box is a pile of rubble. How can we determine what caused the failure or can we?

2A. Failures are always trying to tell you something, and usually experienced specialists can "read" the pieces to determine what is the "most probable cause". You may never know the exact cause but the idea is to address all the most probable causes. For example, you analyze the gear loads and bearing loads and find the bearings have been over loaded, but the overload wouldn't have failed the gears. You find pieces of gear teeth with the end broken off which could be an indication of edge loading. You could speculate that the bearing failed, caused misalignment, end loading on the gear tooth which broke it off. Imprints on the other teeth might indicate that the piece fell between the mesh and caused the catastrophic failure. So one of the actions you might want to take is to implement methods which prevent the bearing overload, or higher capacity bearings. In any case, even failures that appear to be "rubble" to some, may be just a good puzzle that needs to be assemble, to others.

3Q. We experience a banging noise in our ship gear box at a certain speed as we go through it. All other times except at this speed it is real smooth. What can it be and what should we do?

3A. This sounds like you may be experiencing a torsional vibration problem, which isn't a good thing. If it is, you need to avoid running in that speed range or evaluate your system and redesign it. Your coupling might be bad or your propeller damaged. Either could alter the torsional system. When the vibratory torque is greater than the drive torque the gears may "hammer". This means they separate from the normal contact face of the gear and impacts the back face which should be unloaded. If you look at the gear teeth and the front and back both are damaged this is a good indication. If this is a new installation have a torsional analysis done by your consultant if one was not provided. If it is an existing installation which never had such a problem before, check the coupling and propeller also.

4Q. At a recent inspection we noticed that one of the pinion gears in a large high horsepower gearbox had a crack at the root about 1/10 across the face width. Can we continue to operate for a couple of weeks.

4A. I recently had a similar question and went through some crack growth calculations which resulted in a definite No!. Since the crack will probably not run straight across the root but will break off a corner, its not really something one can calculate. When the life calculations show a few hours though, a couple of weeks would have been out of the question. My advice is any crack is a gamble. If it breaks off and falls in the mesh, which it probably will, you'll have a major wreak. So take the production hit or do a risk analysis on the cost of a downtime and a gear change-out or a downtime and a major costly repair and long downtime.

5Q. We noticed heavy spalling on one side of a pinion, which was caused by a bad bearing. We replaced the bearing and the gear looks OK. Can we just flip the pinion and use the other side of the teeth until we get another one?

5A. While it can be done on low power units, you should be cautioned that it can cause tooth bending failure if the tooth loads are high. You have essentially narrowed the tooth and put a stress raiser on the other side. By the way, why did the bearing fail? I wouldn't do it.