WOW! You are pushing it through some mighty big stuff. If you find that 'just-over-the-limit' size that it won't cut, are the parts available to repair it? Or is this cutter on the endangered list? /forums/images/graemlins/wink.gif
I had an old 4' Sunbeam rotary cutter that was tough as nails, and only when I loaned it out, did it get such rough treatment that the seals blew out. After that, I put grease in the gear box, and it worked okay for many years until I sold it. Still working for the buyer, and he still apparently is able to get parts where it was made in Nebraska (Ollala?), I believe.
My Deere isn't as tough but now I don't try to take on small trees anymore. Also I beefed up the rear shroud on the Deere 513 so it is a lot tougher.