I was told one time that hydrostatic transmissions can start going out about 500 hours. I don't know how true it is but I have a Cub Cadet (around 500 hours at the time) that didn't have a way to drain and replace the oil so I hung the mower up and turned it upside down to drain it. The oil was not black or brown but it was silver so.... The mower still runs today about 700 hours I think, still pulls strong. The only thing I've done to it is adjust the valves and put steering knuckles on it and put an electric clutch on it which is $$$$$.
I supposed the newer ones that have a way to drain the oil last alot longer.
I looked into the transmission issue once. There is a famous thread on the TuffTorq 46. Several hundred pages as I recall. In short, it uses a brutally simple system of rotating a series of oil filled tubes past a plate with a tiny hole. The tubes have a spring loaded piston, and sit in the oil reserve. It looks like a cylinder of a cowboy pistol. As the motor spins the transmission, the pistons full of oil are pushed down by a triangle shaped ramp on the back side which creates pressure through the tiny hole. Like a spray bottle idea. It is called a squash plate transmission in the famous thread. Cheap, and designed to wear during use. The tolerances are tight in thousandths so any significant wear loosens the gaps and pressure is lost. But 18 years is still pretty good so far.
I might replace it one day, but that replacement will be the K66, the tractor tyranny, which has 300+ ft-lbs of torque instead of the 150 or so the k46 in it has currently. My truck V8 makes 400ish ft-lbs. The K66 is the first ground breaking capable tyranny offered in the direct replacements from that company. I park a 13 foot trailer with it now, sometimes loaded, but could not get it to back up my driveway hill the other day, though it pulled it forward uphill successfully. If 500 hours is close to lifespan then we have about 3 more years together!
