What will damage a Kirby lexan fan?
Age.
This particular plastic deteriorates from age, becomes brittle, and looses its strength. Regular use of the vacuum will cause the fan to start coming apart on its own. Picking up any hard debris may cause additional breakage or catastrophic failure to them at any time.
Normally you will first see tiny hairline cracks at the insides of the blades near the hub. They will get bigger over time.
In most cases, the cracks will grow gradually until centrifugal force will cause parts of the fan to bend and stretch until the plastic contacts the fan case, which when running, resulting in a bad noise and the smell of burning plastic (from plastic-metal friction). At that point do not use the machine until it is fixed.
After then the fan may eventually explode or come apart in a way that the motor is bogged down or eventually stalls, both causing motor damage if not switched off immediately.
I was given a free Legend II where the latter occurred. The idiot user kept trying to vacuum with a locked motor and burned it up completely. The smell was horrendous. I did not keep that machine due to the smell.
There are also times where the fan looks okay at a glance, the vacuum works fine, and then - SNAP!!! It just explodes. I caught one of those on camera last year when testing the loads of machines from the closed Plano shop. It sounded fine, then it blew. The emtor was not attached and pieces of the fan were found throughout the back yard for the next several months, some as far as 40 feet from the vacuum was.
If the emtor and bag is attached, they will safely catch the fragments.
Moral of the story is to replace any grey fans with either white plastic ones or metal ones. I have not seen age affect either of these, but the metal ones could break if a large enough hard object is ingested.