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I have a circa 1989 Kubota B7200 HST with an "RC60-72H" 60-inch belly deck. I started using it April 2008 when it had 846 hours. In Sept. 2010, at 935 hours the middle blade broke. Then again at 956, 960, and 965 hours. So the middle blade has snapped four times - 3 x with old blades, once with a new blade.

The breaks always occur about two inches from the center mounting hole. They are clean breaks, and almost look at though they had been sawed off. Sometimes the blade breaks in one place, sometimes in two places (on either side of the center hole).

I see nothing that the blades could be hitting. The blades are clearing each other, unless a blade shaft is distorting a lot, and I detect no wobble in the shafts.
The middle blade is driven directly by a gearbox, while the outer blades are belt-driven. I wonder if vibration is fatiguing the metal blade. (The outer blades would be cushioned by the belt.) The U-joints from the PTO to the gearbox seem OK, but the U-joints aren't a really tight fit on the splined PTO shaft. I can see the PTO shaft wobbling, but perhaps that's normal due to the action of the U-joints turning. It's not a very long shaft.

Any ideas what's wrong? Anyone ever heard of this?

If you see the PTO shaft wobbling, then I'd argue (without even knowing the specific mechanicals on this) that you've got a basic problem. There's no way that's part of the design regardless of the u-joints or shaft lengths.

I think you're probably right about the blades wobbling and getting metal fatigue, and that the outer blades are cushioned by the belts.

I also wonder if the U-joints or PTO shaft wobble is triggering huge forces when the blades are at speed, which are hard to see when you move them by hand. A U joint that's gone bad (in cars at least but just in general I think) can cause enormous forces when a torque load is applied I believe.

It all makes sense to me.

I'm sure some others with more familiarity with this particular deck and drive will comment of course....

Steve

With the blade breaking about every 5 hrs., it's obvious you've got an issue here.:wink: Here's my idea; Buy yet another blade, install it, and with everything shut off (obviously!) measure the distance to the ground from the tip of one blade. Then rotate the blade 180* and take another measurement. If those two numbers don't match, you've probably got a bent quill. As that blade travels in an elliptical path, it is probably stressing the blade right at the failure point you've been seeing. Also obviously check for broken/missing bolts that hold all that jazz to the deck, and any cracks or damage in the mounting area on the deck.

Thanks for the Replys. I took a movie of the PTO shaft while rotating at idle, & posted it at Kubota pictures by Roger_deMO - Photobucket

The wobble I see doesn't seem any worse that I've seen on some tractors pulling implements, but maybe it is the culprit.

I guess I'll remove the deck this week & take another look at the situation.

Hard for me to understand how any blade would just break like you describe (I believe you, just don't understand it) . Those blades should be heat treated (tempered) to withstand quite a jolt and maybe bend, but not break.
Where do the pieces that break off go ? They could kill someone !

What blades are you using ? I have bent a couple Oregon Blades and they are like spring steel - they are nearly impossible to straighten because they spring back when you bend them.

Three of the broken blades were on the mower when I started using it.

Purchased new blades from a Case/IHC implement dealer that sells Kubotas.

9/24/2010 $31.30 for three mower blades. I asked for Kubota P/N 70722-3433-3. I've broken one of these; it lasted 21 hours.

7/29/2011 $54.27 for three blades - P/N 70000-25006 I haven't tried these blades yet.

Let's see if I can post some photos of the broken blades...

I don't see any big dents or chips in the blades and it doesn't show that the spindle bolts are moving (might not see that under the grass anyway). You have a mystery here, but breaking in the same place suggests a defect in the blade manufacture.

I'd probably take more good pictures and send the pictures to the blade manufacturer and Kubota. Keep the actual blades for evidence.
This is a dangerous situation and I would think that at least Kubota would want to help you solve the problem.

I have a HD 60-inch 7-Iron MMM on my JD and have been trying to think of a cause of your blades breaking since you began this thread....appears most others have ideas while I don't, but just curious...what rpm are you running the deck at? PTO speed, or higher, or lower? Might be a stupid question but I am wondering if you are running the mower spindles at a speed greater than the blades are rated for. My understanding is that walk behind push mowers have their engines limited to a certain rpm to the blades so not exceed a certain FPM speed.

BKBrown said:

I don't see any big dents or chips in the blades and it doesn't show that the spindle bolts are moving (might not see that under the grass anyway). You have a mystery here, but breaking in the same place suggests a defect in the blade manufacture.

I'd probably take more good pictures and send the pictures to the blade manufacturer and Kubota. Keep the actual blades for evidence.
This is a dangerous situation and I would think that at least Kubota would want to help you solve the problem.

Click to expand...

I agree it's a dangerous situation (VERY dangerous), but I don't see how blade defects alone would cause this type of failure unless he hit something vertically each time the blade breaks.

I'm much more suspicious of metal fatigue due to a vertical vibration at speed under load. It could be a mfr defect that's exacerbated by a vibration, but the loads of a spinning blade that's balanced are not going to break the way these are unless they're so freakin' weak or cracked to begin with! Without a vibration, the stresses are less vertical and more horizontal I believe.

I can't wait to hear what the real cause is.

One thing I was wondering: The bolt has a gap to the blades, and there's usually a "washer" that usually locks the blade in sealing the gap. Is that washer's "lock" feature possibly worn out allowing the blade to slide from side to side or front to back? That would cause enormous stresses I'd think.

In other words, is the lock mechanism that holds the blade in place (not just the bolt) worn out?

Steve

OK this topic was very interesting to me, and I took the opportunity to send it to a friend who is a mechanical and metallurgical genius and here's his reply which is VERY interesting:

1)
From: Rob
Sent: Thursday, September 01, 2011 6:21 AM
To: Ken
Subject: Ok, this needs Ken's diagnostic brain

http://www.lawnmowerforum.com/general-mower-discussion/5418-kubota-middle-blade-breaks.html

This is very intriguing to me and I'm at a loss except to think that the blades originally on the mower were inferior and had a design flaw. If anyone can pinpoint a theoretical cause, I think it's you.

So, what do you think? I think it has to be a blade problem but you see stuff like this frequently and might better understand the physics of what would do this.

This is pretty interesting.

Rob
2)
From: Ken
Sent: Thursday, September 01, 2011 11:06 AM
To: Rob
Subject: RE: Ok, this needs Ken's diagnostic brain

The only way it could break in that short of time is from fatigue due to harmonics. His gearbox bearings / gears will be next in line for catastrophic failure.

I am not a member so I can't look at the photos of the breaks.

There was a good suggestion for measuring the blade at 180 degree rotations to see if the quill was bent.

Also, I know this is counter intuitive, but if he ran with a lighter blade this would push the natural frequency up and likely out of the range of the rotational frequency of the mower.

At present, he either needs to slow the motor speed down or speed it up to avoid the critical frequency of the blade.

Ken

-------I copied and emailed him the pics you posted and here's his final reply-------------

3)
From: Rob
Sent: Thursday, September 01, 2011 8:54 AM
To: Ken
Subject: RE: Ok, this needs Ken's diagnostic brain

[photo of mandril and remaining blade center with bolt looking up]
This tells the story (the guy takes pictures of all the wrong angles!!)

This is a classic fatigue failure from bending UP and DOWN. The original crack started in the dark colored areas and the final failure was a complete fracture (light colored areas).

The original crack had time enough to rust and looks like the crack started on the top surface and propagated downward with would mean the bending was oriented downward at the tip of the blade.

[photo of center of blade and bolt looking down]
This photo shows fretting corrosion.

Not enough clamping force to deal with the vibration of the blade doing the harmonic dance . . .

Also the bolt hole is huge compared to the bolt. Presumably there is a bushing that centers the blade on the shaft / quill . . . if not, that would explain the problem right there.

Ken

Click to expand...

I know that my Land Pride mower uses bushings around the bolt to nest the blade perfectly centered so that its balanced. I assume that this one does too but maybe not and perhaps that IS the problem right there.

I'd get a bushing for this as per original spec, and I'd make darn sure you have that blade properly centered and tight in this nest.

My mower blades are all hardened steel and if these are "el cheapo" aftermarket blades, then they might be prone to flex that could contribute too.

So #1, do you have that bushing spacer that goes around the shaft of the bolt to take up the space in the hole of the blade?
#2 - use original OEM Kubota blades, or a KNOWN safe equal. You hurl one of these babies off the mower and out the side chute when a car is driving by or a kid walking by and it's going to be a sad day.

This thread has been one of the most interesting I have ever followed during my membership with LMF. Been thinking, however, since it is the middle blade that breaks, odds are any broken sections will be directed down, and the broken section will be heavy enough so that it is unlikely it will be thrown out of the discharge chute with enough force to become airborne...this assumes the discharge chute is NOT propped up or missing. OP never told us what resulted when a blade broke, ie, if anything was thrown out.

I await with bated breath what gets discovered.:smile: A dial indicator on a magnetic base might help. Crumb, this is scary.

Here's what the washer and bushing combo looks like. Is this what you have, or do you have any bushing in there to center the bolt in the blade hole?

Interesting .... I should have noticed the blade hole being too big for the bolt.

I don't remember seeing an answer from the OP about him using a bushing or not.

If the blade was not centered and not mounted tight enough I could see that it would flex, but still believe the blades were not tempered correctly.

I guess (when you think about it) at speed, they would be flexing at quite a high frequency and would set up a harmonic that would eventually break the blade.

Hey B7200 - would you please let us know what's happening here?

I would also wonder if he's using fricton washers...russ

Any new news?

Thanks for the interest in my mower problem. I got sidetracked by other projects, seems like everything around here's breaking at once. I hadn't imagined I'd missed so many posts here. Sorry to keep you hanging.

Attached are items I scanned from the manual.

ABOUT BLADE SPEED
My mower has two mid-PTO speeds indicated on a decal, 1546 and 2456 RPM. I run it at 2456 RPM. I run the motor at full throttle, somewhere around 2500 RPM, I think.

My deck manual says "Place the PTO lever in the HIGH (2nd speed) position.". I push the PTO lever down toward the 2456 indication. I hope that's what they mean.

The middle blade pulley appears to be about the same size as the outer pulleys. I won't be able to measure it until I pull the deck from the tractor.

Roger

ABOUT BLADE QUALITY
I think of three sets of mower blades: the "old set" of blades (that were on the mower when I started using it), the "black set" (they were painted black when I bought them in July 2010), and the "pink set" (I painted them pink when I bought them in July 2011, to hopefully keep track of this mess).

When I paid $54 for the "pink set" in September 2011, after paying $31 for the "black set" in July 2010, and the retailer saying the part number had changed, it crossed my mind that there was a manufacturing defect in the "old set" and the "black set" that was being corrected with the "pink set".

Although there could be some truth to that, I have disregarded it because the mower has broken all three of the "old set" (all of which lasted more than 89 hours after I started using the mower, and one of the "black set" (which lasted 21 hours). And it was always the middle blade that broke. I have not yet installed any of the "pink set".

BTW, due to the hot dry spell here in Missouri, there may not be five hours of mowing left this season.

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.

B7200HST said:

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.

Click to expand...

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

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 !

BKBrown said:

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 !

Click to expand...

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.

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.

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.

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.

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!

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.

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.

B7200HST said:

I have a circa 1989 Kubota B7200 HST with an "RC60-72H" 60-inch belly deck. I started using it April 2008 when it had 846 hours. In Sept. 2010, at 935 hours the middle blade broke. Then again at 956, 960, and 965 hours. So the middle blade has snapped four times - 3 x with old blades, once with a new blade.

The breaks always occur about two inches from the center mounting hole. They are clean breaks, and almost look at though they had been sawed off. Sometimes the blade breaks in one place, sometimes in two places (on either side of the center hole).

I see nothing that the blades could be hitting. The blades are clearing each other, unless a blade shaft is distorting a lot, and I detect no wobble in the shafts.
The middle blade is driven directly by a gearbox, while the outer blades are belt-driven. I wonder if vibration is fatiguing the metal blade. (The outer blades would be cushioned by the belt.) The U-joints from the PTO to the gearbox seem OK, but the U-joints aren't a really tight fit on the splined PTO shaft. I can see the PTO shaft wobbling, but perhaps that's normal due to the action of the U-joints turning. It's not a very long shaft.

Any ideas what's wrong? Anyone ever heard of this?

Click to expand...

Mine's done exactly the same thing. It's caused by the gearbox bearings and PTO shaft getting worn, developing play, and vibrating back-and-forth. That shakes the vertical quill to the centre blade, which then fatigues, and breaks.

Just replacing the gearbox bearings and seals will correct SOME of the out-of-balance play and vibration; but not all of it. The gearbox input shaft is very poorly-designed: supported by two 6205 [25x52x15] ball bearings, that are mounted virtually back-to-back. There is only 9/16" separation between them; so their resistance to sideways forces is minimal.

And those sideways forces can be considerable, since the PTO shaft typically operated at an angle about 15 degrees, in the case of my mower. So the bearings wear out quickly, and the resultant play wrecks the seals. Then the oil runs out; and the gearbox seizes. LOVELY . . . .

UPGRADES
1: Make sure the bearings you fit are close-tolerance: at least ABEC-3; preferably ABEC-5. And close fit as well: C2 preferably, or C0, CM or CN, rather than the usual C3 [loose fit]. That will help - for a time.
2: Replace the ball bearings with cylindrical roller bearings: flanged NJ205 bearings are 25x52x15, so will fit right in. Limited resistance to axial thrust; but much higher sideways resistance.
3: Do away with the oil seal, and replace it with ANOTHER bearing, with its own oil seals. That's a 6205 2RS. That means leaving out the internal circlip, drilling and tapping 5/16" holes around the end face of the gearbox housing casting [I drill 6; you could go to 8]. Making up a clamp ring-plate to bolt round the shaft and onto the casting end. Using that to clamp and hold in the outer of the TWO bearings. If you want, you could then make the inner of the two bearings an NJ205 roller, for more strength - since the two ball bearings will take care of end thrust.
4: Carry out the same gearbox housing modification as [3] above. But, in place of an extra bearing replace BOTH ball bearings with 30205 Taper Roller bearings. They are 25x52x16.25 so, to get the gear wheel properly positioned, you have to machine 1.25mm off the inner and outer shells of the inside bearing. If you take a wider 25x52x22 taper roller for the outer bearing, and add a 10mm oilseal, that will protrude from the housing, and you use the ring clamp frame to apply the necessary preload - through the oilseal - for the taper rollers.

Four solutions, in rough order of effort needed, and resulting strength.

But the bearings may not be the whole story. There can also be play BETWEEN the bearing outer races and the gearbox casting. Mine certainly did; I had to use one thou brass shim, plus Bearing Mount, to get the bearings properly located. Not sure whether the bearing races had been fretting material way from the cast iron housing, or whether the housing was machined 'loose' from the factory. But loose they definitely were.

The original source of the vibration is the PTO shaft, which gets worn and starts flopping around. A new one from Kubota will run you $600-plus. But you can make one up using parts from Weasler or Neapco, and with standard Domestic Series 6 UJs [in place of the special - and weaker - Kubota ones] for about $200-250.

Finally, the deck belt also causes vibration, if it is unevenly worn. A new belt will be much more even, and run smoother.

But God knows why Kubota made such a crappy design in the first place . . . .

Kubota makes some good stuff, but all manufactures want to go as cheap as possible, and with that quality suffers.

Put the the blade and the camera some sort of a stand.
Take a few clear photos on the highest optical magnification at the highest resolution of the fracture surface.
Better if you can sit the two fracture surfaces one on top of the other.
Fatigue, biaxially loaded stress , impact, tortional & brittle failure all have charasteric appearences.
Once the failure mode is determined then we can start postulating how it occurred.
These need to be fairly high resolution so please use a real camera ( not your smart phone ).
To be of any use they will need to be too big to post on the forum but pop them on photobucket, click, dropbox or similar and post the link.