Why do complex machines require more maintenance and break down more often than a simpler machines.

Question

I naturally understand why this is. We always say “more moving parts makes there more chance for failure”. But why is this? Is it just how the universe has to be due to the laws of physics?

For instance an airplane is pretty complex but the aluminum that makes up the structure of the plane rarely needs to have complete overhauls while something like the engine is constantly being serviced and kept up to standard.

What is it about spinning parts, heat, and lots of piping that make something constantly be trending towards breakdown?

In: Engineering

Answer 1

There are a lot of parts to your question so I am going to focus on the purely engineering part. Why do you need to overhaul the engine while the airframe can go on much longer without the same level of maintenance. The answer is: material science. Metals have a lattice structure and mechanical properties given to them by the alloy composition and grain boundary area through heat treatments etc… when you take a piece of metal and expose it to load cycles (thing bending a piece of metal back and forth), even if you don’t break it in one go, you develop microscopic cracks in the structure. These cracks will propagate over a certain number of load cycles. If the load is very large, it will break after few cycles. If the load is light, it will go on for more cycles. Other factors play also like temperature. An engine part experiences huge temperature swings which change the chemical and mechanical properties of the parts over time. They also typically experience huge loads and a lot of vibration. Therefore they develop cracks regularly and need to be replaced. There are entire field of engineering created to predict when this happens. An airframe on the other hand is designed with a pretty big safety factor in mind and does not experience the same level of stress. Mainly the wings and fuselage flex during flight. Now there are many cases where the airframe had to go heavy maintenance, or micro cracks were found in the wing structure. But over all the conditions make it less likely to fail over time because of the nature of the loading cycle.