Weird Bush Hog oil leak

[jstreet]

I have a two-year-old Bush Hog SQ600 rotary cutter that I use for fairly light autumn mowing of grassy trails on our 100 acres. It has had only about 10-12 hours of use. This week as I got ready for this fall's mowing, I noticed a 2-to-3-foot-wide pool of seemingly fresh gear oil on the concrete floor under ther mower. The mower never has leaked before. I refilled it with 80-90 gear oil, ran it briefly, and then let it sit overnight. In the morning there was a little more oil on the floor -- not much, a pool about the size of the palm of my hand.
I decided to try some mowing, checking periodically for low oil. I mowed for about two hours, put the mower away for the night, and this morning the oil level was full and there was no evidence of any further leak.
My questions: Can such a leak cure itself? What would have caused the leak, and why did the leak seem to stop? Why would the leak have seemed to have started during the summer when the mower hadn't been used since last fall?
At my dealer's suggestion, I checked the vent plug on the mower gearcase, and it seemed to be OK. He said the seal might have "dried out" after months of disuse, but I find that hard to believe since the gearcase had oil in it.
Should I have the seal replaced, or just keep watching the oil level and hope for the best?
I'd be happy to hear any theories, or learn of anyone else who's had this problem.

 

[normde2001]

Your dealer is right about the seals drying out from non-use. The worst thing you can do to a piece of gear is let it sit for long periods of time. It needs the heat buildup from the turning shaft to stay pliable. I don't know if you can get access to it or not, but spraying with silicon before storage would probably help a lot. Maybe some transmission "stopleak" wouldn't hurt in this case since there are no internal valves to mess up.

 

[Justaplain]

I had the whole blade assembly drop this summer due to the big cotter pin coming out and letting the main nut run off. I had to replace the bottom seal. It is a regular seal with a cover cap. You might be able to squirt some oil up in there but it would be hit or miss. bcs

 

[djradz]

Other opinions may differ, but my experience is that the seals on new pieces of equipment such as this don't just "dry out". Back when leather was used as the primary sealing component, that was the case. Now seals are made from polymers that should last 10 to 20 years without replacement, unless something gets at them and messes them up. I surmise either the seal was mis-installed or something is wrong with it or with the shaft. If the vent was not working, you might have lost some fluid, but it sounds like you've lost a bunch. It could also be possible if the bearing preload is too much, (too tight) there is a lot of overheating of the gear box and the fluid is getting pressurized beyond what is normal, but then I'd expect fluid would have also been pressed out the front seal also. With the PTO shaft disconnected, can you freely turn the blades?

 

[JimMc]

Sometimes, a piece of debris jams its way between the seal and shaft, allowing leakage. Later, when running the cutter, the debris is worked or worn out and the seal returns to contact the shaft. In severe cases, the debris will accumulate, wrapping around the shaft and turn with the shaft, cutting out the seal completely. Many manufacturers place a piece of metal near the shaft to keep this from happening.

 

[brantley]

I had the exact same thing happen to me earlier in the year. My brush hog is two years old as well. I just added more gear oil, ran it and watched it. There have been no more leaks.