Man and Machine

I have always been fascinated by fracture and failure analysis since the course that i had taken during my masters program. Like these articles, I had to work on actual case studies and write technical reports on the why's and how's of the failure. Getting back to the article in discussion here, a gear pump pumped a solution of ethylene glycol and water to cool a laser in a space application.The gear pump failed before its intended duration.

Mr. Russell explains the basics of the gear pump and makes an important point that designing two parts to rub against each other without galling which is excessive wear is a daunting task and also the fact that one should never rub two soft materials against each other. The author of the article does not elaborate on this to substantiate the case. The said gears were made of medium hardness stainless steel. The surface of the gears were implanted with nitrogen which when combined with chromium additive in the steel forms a very hard nitride layer which reduces galling. But the same design modification creates another issue. This cross linked design obstacle reminds one of the"axiomatic design principles" wherein the functional requirement has to match up with the design criteria and form a matrix wherein one design does not affect the functioning of the other. Lets talk about the axiomatic design aspect in another blog and get back to this case.

One might think that if one gets rid of the chromium and carry out another effective method of galling.Unfortunately, stainless steel is stainless mainly because of the chromium. It adds the sufficient strength to the steel which is required for this kind of application. Another solution could be to isolate the coolant from the pumps as corrosion does not take place as long as pumps are in motion and therefore are only subject to limited exposure increasing functioning time. Mr. Russell completely rules out this option although it is not clear why. Maybe the application produced so much heat that it was necessary for the gears to be exposed to coolant when not in motion too. Mr. Russell opines that the stainless steel should be replaced with a steel of a higher carbon content. I agree with this view as pitting of stainless steel can be avoided by addition of a small percentage of molybdenum.

However, unlike normal gears that roll with each other, these gears are rubbing against each other. Even if the steel is replaced with a higher carbon content steel, the continued mettalic contact will prove detrimental to the continued performance. If the application requires the gears to work only for a limited amount of time then the pump would be more economical if the gears were replaced periodically than invest in new and much more expensive steel material.

The final call though has to be taken by the original designers considering the effects their change in design has on important criteria like size, power consumption, maximum pressure difference, and output.

Sherlock Ohms saves the day. Case solved!