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I'm getting ready to work on the deck of my YT4500 (917.288580) lawn tractor, it's the 54" deck. This mower is 3 or 4 years old.
The left mandrel assembly (I think) has been making noise lately, and I figure it's time to look at the bearings.
There is a grease zerk on the end of the spindles, and I do give them a couple shots per the manual, but does that really do anything? I mean, looking at the parts online, it looks like both bearings are sealed, so where does that grease go? Am I missing something here?
I took the deck off yesterday to check things out in prep for removing the mandrel, and for the life of me, none of them feels any rougher than the others. The middle one spins the most freely; with blade and belt removed it will coast for several seconds when given a spin. The other two don't coast as long, but I don't feel any roughness or any end or side play in the bearings. I have not yet removed any of the mandrel assemblies from the deck due to bad weather rolling in later in the day.
Symptoms: When mowing everything will be fine, then suddenly it will make a... well, not a squealing noise but more like a loud buzzing noise... and vibrate for a few seconds, then that will stop and everything is normal until a couple minutes later when it happens again. I checked the other pulleys, those feel smooth and spin freely also. :confused2:
All in all, this has been a pretty good mower so far.

The bearings are sealed inside the spindle. they don't require greasing but they put grease zerks on them to make people feel good. Now with that said, Murray years ago took bearings with one color grease, and then put another color grease in the housing and then ran the spindle, which afterwards they disassembled the spindle to find that the grease that was pumped into the spindle housing had worked its way past the seals, and had mixed with the grease in the bearing. so some of that grease that you pump in does migrate into the bearing.

I suspect that you have a bearing going bad some place either an idler pulley or spindle bearings that causes a vibration from time to time. Had a MTD rider years ago that I have to replace all three spindles on under warranty to elimimate a vibration that would make your feet go numb after mowing for 20 minutes.

I have a complete mandrel / spindle assembly ordered, should be here in a couple of more days. It is frustrating that with the deck off and doing the hands-on feeling about, I can't tell which is the culprit. You make a good point that it could be one of the idlers, I checked those also. If I don't find anything else I'll go ahead and replace the left mandrel assembly and see if that was it. Then I'll take the old one apart and have a close look at the bearings. We'll see.

Even though I still couldn't feel anything wrong with the old bearings once I got the old mandrel off the deck, I went ahead and installed the new one; noise and vibration solved.
Now I'll see if I can get the old assembly apart with an eye towards rebuilding it for next time.
Thanks for your help.

It's certainly a judgement call at that point and it sounds as if you did good.

Something I've found over the years is a bearing that's near complete failure, but not there, can sound and feel different with and without belt and load tension. You probably saved the shaft and housing.