Repairs Kubota middle blade breaks

[B7200HST]

REMOVED THE DECK

I removed the mower deck, then the remnant of the middle blade.

The deck is missing a rear wheel and the guard chute.

The middle blade parts I removed seem to be what the manual indicates.

Note that the bottom of the middle shaft has a shoulder on it. The shoulder is the same diameter as the hole in the blade, and the blade slips onto that shoulder.

Now that I have a better view, all three mower pulleys still look to have the same diameter.

I don't feel any wobble in the shafts.

Next I'll put on a middle blade and see if I can detect any runout/wobble.

 

[benski]

REMOVED THE DECK

I removed the mower deck, then the remnant of the middle blade.

The deck is missing a rear wheel and the guard chute.

The middle blade parts I removed seem to be what the manual indicates.

Note that the bottom of the middle shaft has a shoulder on it. The shoulder is the same diameter as the hole in the blade, and the blade slips onto that shoulder.

Now that I have a better view, all three mower pulleys still look to have the same diameter.


I don't feel any wobble in the shafts.

Next I'll put on a middle blade and see if I can detect any runout/wobble.

"Um, "quills" for $400.00, Alex"

 

[BKBrown]

That center spindle does turn directly from the PTO gearbox and could be setting up a harmonic because of bad U-joints ??? or bad gears in that gearbox ??? or worn bearings in the spindle ???
The outer spindles would not have all that transfered though the belts.

Does the dealer have any similar experience ?

I'll be interested to know what else you find out !

 

[benski]

That center spindle does turn directly from the PTO gearbox and could be setting up a harmonic because of bad U-joints ??? or bad gears in that gearbox ??? or worn bearings in the spindle ???
The outer spindles would not have all that transfered though the belts.

Does the dealer have any similar experience ?

I'll be interested to know what else you find out !

Indeed! I think it has to be a harmonic issue, exacerbated by a bent quill, that is causing all this. Once again, not being a forensic engineer, it is just a SWAG.

 

[B7200HST]

UPDATE 10/27/2011

9/16/2011 Talked to mechanic at implement company that sells Kubotas. He was kind and helpful. Looking at my blades, he thought I was hitting things with them. I thought the nicks were from the breaking blades hitting the other blades. But maybe I've hit more junk than I thought; it wouldn't be the first time I let my ego lie to me!

He explained that the two cup washers used when mounting the blade are made to slip against each other, to absorb shocks to the blade. He said the cup washers can become squashed if the blade bolt is over-torqued.

9/23/2011 At implement company paid $33.23 for two cup washers for the Kubota mower deck (looks to me like they're worth about $3.00 each!), though I didn't think the old washers looked deformed.

I checked the Kubota mower middle blade for run-out the best I could. It seemed ok. The right blade not only had a big nick in it, but was bent down, 5/8" at one end and 1/8" at the other - obviously damage from being struck by a broken blade.

9/27/2011 Used wire brush to clean up one side of each of the four old cup washers that go on the outer blades, since I was told they're supposed to slip against each other.

I installed all three new (pink) mower blades. I installed the two new cup washers with the middle blade. I sprayed WD-40 on the mating surfaces of the two pair of old cup washers.

Because I bought my torque wrench 3/21/1979, and I know it's been used as a breaker bar, and having read that after the wrench has been stored it should be used mid-scale half a dozen times to exercise it, I attempted to increase its accuracy. I torqued the three blade bolts to 50, then 60, then 70, then 75 foot pounds. (i.e., maybe some of my problem has been due to inaccurate torque wrench.)

The rear deck wheel was set to around 1-1/2" high. I reset theat rear wheel to 3-1/2", to match the other settings.

Bottom line, I installed new cup washers on middle blade, and tried to be careful about torquing blade bolt.

I've mowed 2.8 hours since the last broken blade, without incident. I hope my posts haven't been too windy. I'll post an update next mowing season.

 

[jim j]

I have this exact same issue with my Kubota. Kubota F2400 with front mount rc-72 deck. Middle blade has broke numerous times just as described in this thread.
The only thing that I found on my mower deck was that there was not a friction washer as the manual says there should. I have replaced all blades and added the friction washer. So far, so good, but haven't run that long this spring.

 

[buckeyepost]

I've had the same problem on my Kubota B8200 with a 60" RCB60-1 belly mower and started a thread on another forum and someone posted a link to this topic. >>> Are my mower blades hitting each other? - MyTractorForum.com - The Friendliest Tractor Forum and Best Place for Tractor Information

I was tightening my blade down a lot too so I'll torque it properly and lube it when I go to my local Kubota dealer to get a new blade and give it a shot mowing tomorrow. Glad to hear that I wasn't alone but this doesn't appear to be very common as a lot of smart guys were stumped.

 

[buckeyepost]

Did the same solution as the OP did... 2 new (expensive!) Kubota slip washers lubed in PB Blaster and torqued to 75ftlb's..... mowed 2 acres of 1-2' grass and so far so good!

 

[B7200HST]

Blade Broke, 5th time

jim j & buckeyepost, thanks for sharing. There's some comfort in finally knowing I'm not the only one with this problem. And thanks for the link to the other forum.

It's happened again - the middle blade has broken. This is the fith time. There was not much wear on this blade. This new blade survived almost 24 hours of mowing.

I was again suspecting that the breaks may be due to the harmonics that other posters have mentioned, passing through from the PTO shaft (see the movies I posted 08-31-2011 12:52 PM). I see that buckeyepost has confirmed his problem as stemming from the PTO.

I am now posting some photos of my latest broken blade. I think that the damage on the far right of the photo probably occurred after the blade broke. One blade is bent down, but that doesn't always happen so I think that also happened after the blade broke.

While mowing, my engine runs at 2600 to 2700 RPM. I'm wondering if running at 2400 RPM (about 10% slower) would take the blade out of the destructive speed. A pipe dream, perhaps.

 

[buckeyepost]

Sorry, I've been busy and haven't replied in a while but I was still breaking blades after the slip washers so went ahead and dropped about $1k on the front pto shaft, pulley, bearings, clips, key.... pretty much everything except for that front pto housing (which they dont even make anymore). Anyway, after I swapped everything out and engaged the mower deck for the first time I knew exactly what it was. That whole front pto drive line was worn out reeeeally bad because the whole tractor used to shake when the mower was engaged and now it was soooo quiet, calm and smooth. More money than I wanted to spend but I've probably put 15 hrs on it since then and noooo problems what so ever. So yeah, the harmonics from the wicked vibration of a lot of worn out parts are most likely the cause for anyone having this issue.