It is PM'ed every 90 days. with oil and oil filter changed.
The sweeper gets inspected every day.
The scratching and worn rings are likely due to a single event where we found the air cleaner cap had fallen off, or was deliberately taken off, allowing both air filters to drop out of place.
At that time, both air filter elements were replaced and the air ducting all the way to the mixer was cleaned out.
There was no apparent reason the air cleaner cap fell off and it has been suspected that a particular sweeper operator may have tampered with it for unknown reasons. There have been rumors from a few of his co-workers alluding to other acts of vandalism or damage to forklifts and other property. I recall installing a new seat on the sweeper once and the next day the seat vinyl of the back cushion had a slash all the way across it and it looked like the work of a utility knife or pocket knife. And the same operator was suspected of having done that, but no proof was to be found.
We have since been lacing a long plastic zip tie around the canister that keeps the latch buckles from being operated.
The sweeper could not have operated more than maybe 16 hours or so with the induction system unprotected (before being discovered and fixed), but considering the environment, that was likely enough to start the process. That happened about 3 years ago.
It's enough to have to deal with genuine mechanical defects/failures alone, but it seems we get a fair amount of employee inflicted vandalism occasionally too.